Lesson XIII: Metabolism

Transcription

1 Lesson XIII: Metabolism Metabolism is a term that refers to the sum of all chemical reactions in the body, produced by a delicate balance of catabolic and anabolic activities, Catabolism is the process of breaking complex organic molecules in simpler molecules, releasing the energy contained between chemical bonds and transferring it to ATP, which is then used to fuel a variety of physiological processes. Anabolism refers to a group of chemical reactions in which simple molecules are combined to form larger organic molecules with the energy stored in ATP. ATP is obviously an important component in metabolism, which essentially functions as a kind of currency in the economy of the body. Like cash however, ATP is made to be spent, and is not used as a long term storage for energy. Adenosine triphosphate (ATP) consists of an adenine molecule, a ribose molecule, and three phosphate groups. When the terminal phosphate group is split from ATP, the result is a high energy phosphate group (P) that is used to fuel metabolic processes, and adenosine diphosphate (ADP). ATP (adenosine triphosphate) ADP (adenosine diphosphate) + (P) (phosphate group) + E (energy) This reaction is a reversible reaction, and energy that is made available from catabolic processes can attach a phosphate group to ADP to form ATP: ADP (adenosine diphosphate) + (P) (phosphate group) + E (energy) ATP (adenosine triphosphate) Energy transfer One of the features of higher plants and animals is that they are dependent upon the presence of oxygen to produce energy, called aerobic respiration. This is not to say that anaerobic processes are not also used, but they are nowhere near as efficient as aerobic processes and could not supply our bodies with the energy needed to fuel the needs of our body. Aerobic respiration describes a process by which energy-rich carbohydrates and fats are oxidized is a stepwise manner within the cell to release energy, some of it used to fuel the cell s activities, and the remainder released 1

2 as heat. The processes of aerobic respiration are characterized by oxidation-reduction reactions. Oxidation is the removal of electrons from a molecule, resulting in a decrease in its energy content. Reduction is the opposite of oxidation, referring to the addition of electrons to a molecule, thereby increasing its energy content. The term reduction may seem to be counterintuitive when it describes a process in which a molecule is added to, but this term specifically means a reduction of its oxidation number, a form of book-keeping that chemists use to keep track of which molecules have been oxidized and which have been reduced. Within biological systems, oxidation and reduction reactions are always coupled. Thus, whenever a substance is oxidized, another is simultaneously reduced. The coupling of these two reactions is referred to as redox reactions, and ultimately, all of the energy needed to fuel biological systems comes from the energy of these reactions. Human bodies, just as all other living organisms, are reduced relative to the oxygenated atmosphere in which we live. This means that organic molecules contain far more hydrogen atoms than they do oxygen molecules. The large energy difference between the reduced molecules of life that contain many hydrogen atoms, such as carbohydrates, fats and proteins, and the oxidizing atmosphere, provides far more chemical energy than processes that don t take advantage of oxygen, such as anaerobic respiration. When oxidation involves the loss of hydrogen atoms, which is almost always the case in biological systems, this is called dehydrogentation. For example, when lactic acid is oxidized into pyruvic acid, two hydrogen atoms are removed from lactic acid as one hydrogen ion (H + ) and one hydride ion (H - ). Conversely, when pyruvic acid is reduced to lactic acid, one hydrogen ion (H + ) and one hydride ion (H - ) are added. When a substance is oxidized the liberated hydrogen atoms are immediately transferred by co-enzymes to other compounds. The two enzymes most commonly utilized for this purpose in animal cells are nicotinamide adenine dinucleotide (NAD + ), a derivative of the B vitamin niacin, and flavin adenine dinucleotide (FAD), derived from the B vitamin riboflavin. When NAD + is reduced, one hydride ion (H - ) are added neutralizing the charge on NAD + to produce NADH. When FAD is reduced, both a hydrogen 2

3 ion (H + ) and a hydride ion (H - ) are added, resulting in FADH 2. As stated previously, oxidation and reduction reactions are always coupled. Thus, when lactic acid is oxidized to form pyruvic acid, the hydrogen ion (H + ) and hydride ion (H - ) liberated in this process are used to reduced NAD + into NADH, plus a remaining hydrogen ion that is free in the cytosol (i.e. NADH + H + ) ATP generation The process by which ATP is generated involves the addition of high energy phosphate groups to an adenine/ribose molecule, called phosphorylation. The addition of a phosphate group to a molecule increases its energy potential. The human body uses two primary phosphorylating mechanisms to generate ATP. 1. In substrate level phosphorylation ATP is generated when a high energy phosphate group is transferred directly from an intermediary phosphorylated compound to ADP. In human cells this process occurs in the cytosol. 2. In oxidative phosphorylation electrons are removed from organic compounds and passed through a series of electron receptors to molecules of O 2 or other inorganic molecules. This process occurs within the inner membrane of the mitochondria. The series of electron receptors used in oxidative phosphorylation is called the electron transport chain. Carbohydrate metabolism One of the reasons why catabolic reactions in the body are so complex is to limit the amount of energy that is released in each reaction, thereby ensuring a greater efficiency in the storage of that energy. If the process of nutrient catabolism were carried out in a single reaction it would generate an intense burst of heat, similar to that of lighting a piece of paper on fire. During digestion, complex carbohydrates such as polysaccharides (e.g. starch), disaccharides, and monosaccharides are converted by the epithelial cells of the small intestine and the liver into glucose. Glucose is the 3

4 only monosaccharide present in the blood in any significant volume, and is thus known as blood sugar. The highly reduced property of glucose is then harnessed to fuel the metabolic activities of the body. Once absorbed in bloodstream, the fate of glucose depends upon the energy needs of the body. Glucose may be used to produce ATP, form amino acids, replenish the body s supply of glycogen, synthesize triglycerides, or be excreted in the urine. The entry of glucose into most body cells occurs by facilitated diffusion, a process that is enhanced by insulin. Once glucose enters the cell it is phosphorylated by a kinase enzyme, combining it with a phosphate group from the breakdown of ATP to form glucose 6-phosphate. The phosphorylation of glucose essentially traps the glucose molecule inside so it cannot diffuse back out of the cell. Some cells such as liver cells, kidney tubules, and intestinal epithelial cells contain the enzyme phosphatase that can remove the phosphate group from the glucose molecule and enable it to move back into the blood stream There are three major pathways of glucose metabolism: 1. Glycolysis, which splits a glucose molecule into two molecules of pyruvic acid. 2. Anaerobic fermentation, which occurs in the absence of oxygen and reduces pyruvic acid into lactic acid. 3. Aerobic respiration, which occurs in the presence of oxygen and oxidizes pyruvic acid into carbon dioxide and water Glycolysis Upon entering the cell glucose undergoes a series of reactions called glycolysis. After being acted upon by the enzyme hexokinase to phosphorylate (add a phosphate group to) the glucose molecule, glucose 6-phosphate (G6P) may be shunted to different pathways dependent upon the metabolic need of the body. For example, G6P may be converted into glycogen, triglycerides or amino acids. If G6P is going to be used to replenish the body s supply of ATP however, it is broken down from a six carbon glucose molecule into two three-carbon sugars of pyruvic acid (pyruvate). 4

5 After phosphorylation, the second and third steps of glycolysis are called priming. G6P is rearranged (isomerized) to form fructose 6-phosphate, which is then phosphorylated again to form fructose 1,6-biphosphate. By this activity, the activation energy of the original glucose molecule becomes enhanced. Please note that so far two molecules of ATP have been used to generate this reaction, an investment that will later pay a dividend. The fourth step of glycolysis is called cleavage, in which fructose 1,6-biphosphate is split into two three-carbon molecules of glyceraldehyde 3-phosphate (G3P), each of which has one phosphate group. Each molecule is then oxidized in the fifth step of glycolysis, in which one pair of hydrogen atoms are removed. NAD + picks up two electrons and one proton from each G3P molecule, and the remaining hydrogen proton is released into the cytosol, the two G3P molecules being oxidized to yield 2NADH + 2H +. Immediately after oxidation, a phosphate group is added to each of the two remaining 3-carbon fragments, yielding two molecules of 1,3-biphosphoglyceric acid. Phosphorylation occurs however not from ATP, but from the cell s pool of free phosphate ions. Recall that each G3P fragment already contained a phosphate group, so that the addition of another phosphate group results in a molecule of 1,3-biphosphoglyceric acid. In steps six and seven of glycolysis the phosphate groups are removed from the two molecules of 1,3- biphosphoglyceric acid and transferred to ADP to yield four molecules of ATP. This reaction also results in the conversion of 1,3-biphosphoglyceric acid into two pyruvate molecules. Although glycolysis produces a total of four ATP molecules, two molecules of ATP were required to generate these reactions. Thus glycolysis yields a net gain of two ATP molecules. The end products of glycolysis then, can be expressed as thus: 2 pyruvic acid + 2 NADH + 2H ATP Anaerobic fermentation At this point, the fate of pyruvate depends upon the availability of oxygen. If oxygen is scare (anaerobic conditions), as is the case in skeletal muscles during strenuous exercise, pyruvic acid will be reduced by adding two hydrogen atoms to form lactic acid, donated by NADH 5

6 (which in turn is converted back to NAD + ). This process is referred to as anaerobic fermentation. Lactic acid may then enter the blood stream where it travels to the liver to be converted back into pyruvic acid, or remain in the cells until aerobic conditions return and is converted back into pyruvic acid. Once contained in the liver, lactic acid may be converted back into glucose 6-phosphate, and is either polymerized to produce glycogen, or has its phosphate group removed and the free glucose molecule is then released into the blood. Although anaerobic fermentation can keep glycolysis going a little longer, it has two important drawbacks. Firstly, anaerobic fermentation is inefficient because most of the energy of the glucose is still bound up in the lactic acid molecule. The other drawback is that lactic acid is somewhat toxic and contributes to muscle fatigue. Some cells, such as those within muscle tissue are relatively tolerant of anaerobic conditions, and have biological adaptations such as the presence of creatine phosphate that can replenish ATP, and significant stores of glycogen that can provide an ongoing source of glucose. Cardiac muscle is relatively intolerant of anaerobic conditions however, and brain tissue cannot survive without oxygen. During labour, when an infant s blood supply is cut off, almost every system of the body switches to anaerobic fermentation, thereby allocating most of the oxygen to the brain. Aerobic respiration When oxygen is plentiful (aerobic conditions) pyruvic acid is converted into acetyl coenzyme A. This molecule links glycolysis, which occurs within the cytosol, with the citric acid cycle (or Kreb s cycle) of aerobic respiration, which occurs within the matrix of the mitochondria. Before entering into the citric acid cycle, pyruvic acid is converted into a two-carbon fragment through a process called decarboxylation (the loss of a molecule of CO 2 ). The resulting two carbon fragment, called an acetyl group, attaches to coenzyme A (derived from the B vitamin pantothenic acid) to form acetyl coenzyme A (acetyl CoA). Remember, when a molecule is oxidized is loses hydrogen atoms, and when a molecule is reduced it gains hydrogen atoms. In the conversion of pyruvic acid into 6

7 acetyl CoA each pyruvic acid loses one hydrogen atom. The hydrogen atoms released by the oxidation of the two pyruvic acid molecules are picked up by the coenzyme NAD + to form NADH + H +. Now that pyruvate has been converted into acetyl coenzyme A, it is ready to enter into the citric acid cycle that occurs within the matrix of the mitochondria. In the first step of the citric acid cycle CoA passes off the acetyl group to a four carbon compound called oxaloacetic acid, to form citric acid, the first molecule that is formed when the acetyl group joins the cycle, and for which the citric cycle is named. CoA is then available to another pyruvate molecule that becomes available through glycolysis. In the second step of the citric acid cycle water (H 2 O) is removed from citric acid, and although the compound retains its six-carbon structure, it is reorganized into aconitic acid. In the third step aconitic acid is converted into isocitric acid through the activities of the enzyme aconitase. In the fourth step, a hydrogen atom is removed from isocitric acid followed by the removal of a CO 2 molecule. The hydrogen atom is accepted by NAD +, and with the removal of CO 2 the compound is oxidized into a five carbon compound called α-ketoglutaric acid. This process is repeated again in step five, with NAD + accepting another hydrogen atom, and the loss of another CO 2 molecule. No further carbon atoms are removed beyond this point, and the substrate ends up as oxaloacetic acid, as it was at the beginning of the citric acid cycle. The three carbon atoms of pyruvic acid have been removed during this cycle, and the result of these decarboxylation reactions are the source of most of the CO 2 that is exhaled. During the oxidation of α-ketoglutaric acid, CoA enters to combine with the product of this reaction to form succinyl CoA. This is an energy rich compound that when acted upon by succinyl kinase, releases the CoA molecule and takes some of the energy within the substrate and gives it to guanosine diphosphate (GDP), which is then converted into guanosine triphosphate (GTP). GTP then transfers this high energy phosphate group to ADP to form a molecule of ATP. As a result of these reactions, the substrate is converted into succinic acid. 7

8 Succinic acid is then oxidized into fumaric acid, and the coenzyme that picks up the hydrogen atom is FAD. Fumaric acid is then hydrated with a molecule of water (H 2 O) to form malic acid, which is then oxidized to oxaloacetic acid, with NAD + accepting the hydrogen atom. Thus the cycle ends with the oxidation of malic acid to form oxaloacetic acid, only to be continued by the introduction of a new molecule of acetyl coenzyme A. It is important to remember that for every glucose molecule that entered glycolysis, all of these matrix reactions occur twice (once for each pyruvic acid). If we look at the citric acid cycle as a whole we can see that for every two acetyl CoA molecules that enter into it, four molecules of CO 2 are liberated by decarboxylation, and 6 NADH + 6H + and 2 FADH 2 are produced by oxidation-reduction reactions. Although two molecules of GTP are also produced, the vast majority of the energy is now contained within the reduced coenzymes, i.e. NADH + H + and FADH. Thus these enzymes must be oxidized in order to extract the energy from them. In the next stage of aerobic respiration a series redox reactions transfers the energy stored in the coenzymes to ADP to form ATP. These reactions involve the electron transport chain and occur on the cristae of the inner mitochondrial membrane. Electron Transport chain The electron transport chain involves a sequence of electron carrier molecules on the inner mitochondrial membrane that are capable of oxidation-reduction reactions. As electrons pass through this chain there is a step-wise release of energy to generate ATP. In aerobic cellular respiration the last electron acceptor is molecular oxygen (O 2 ). The energy is released as electrons are passed from one carrier to the next, and is used to pump H + (protons) from the mitochondrial matrix into the space between the inner and outer membranes. This method of ATP generation is called chemiosmosis. There are variety of electron carriers found in the inner mitochondrial membrane that are alternatively oxidized and reduced through the process of chemiosmosis. 1. Flavin mononucleotide (FMN), similar to FAD, FMN is derived from dietary riboflavin. It is bound to membrane proteins and accepts electrons from NADH. 8

9 2. Cytochromes are proteins with an iron-containing group (heme) capable of existing alternately in a reduced (Fe 2+ ) and oxidized (Fe 3+ ) state. In order of participation within the chain they are cytochromes b (cyt b), c 1, (cyt c 1 ), c (cyt c), a (cyt a) and a 3 (cyt a 3 ). 3. Iron-sulfur centers are complexes of iron and sulfur atoms bound to membrane proteins. 4. Copper ions bound to membrane proteins. 5. Ubiquinones are non-protein carriers of low molecular weight and are mobile in the phospholipid bilayer of the membrane. Also called Coenzyme Q, it accepts electrons from FADH 2. The accompanying textbook displays graphics that show the order in which electrons are passed along the chain. The first step is the transfer of high energy electrons from NADH + H + to FMN, the first carrier in the chain. Two NADH + 2H + are generated from glycolysis, two from formation of acetyl CoA, and six from the citric acid cycle. In this transfer, a hydrogen atom with two electrons passes to FMN, which then picks up an additional H + from the surrounding aqueous medium. As a result, NADH + H + is oxidized to NAD + and FMN is reduced to FMNH 2. The second step of the electron transport occurs when FMNH 2 passes electrons (hydrogen atoms with 2 electrons) to several iron-sulfur containing centers and then to coenzyme Q, which picks up another H + from the surrounding aqueous medium. As a result FMNH 2 is oxidized to FMN. The following sequences of the electron transport chain involve cytochromes, iron-sulfur (Fe-S) centers, and copper atoms located between coenzyme Q and molecular oxygen. Electrons are passed successively from coenzyme Q to cytochrome B (cyt b), to Fe-S centers, to cytochrome c 1 (cyt c 1 ), to cytochrome c (cyt c), to copper (Cu), to cytochrome a (cyt a), and finally to cytochrome a 3 (cyt a 3 ). Each carrier in the chain is reduced as it picks up electrons and is oxidized as it gives up electrons. The last cytochrome (Cyt a 3 ) passes its electrons to one-half of a molecule of oxygen, which becomes negatively charged and picks up 2H + from the surrounding aqueous medium to 9

10 form H 2 O. This process is much like a row of people passing along a hot potato by the time the potato reaches the last member of the chain the potato is relatively cool its energy has been used to generate ATP. Of primary importance is what happens to the energy released from the hot potato as it is passed along from person to person. Some of the energy in the electron transport chain is lost as heat, but the remaining energy drives proton pumps, that expels H + from the mitochondrial matrix helping to create an electrochemical gradient of H +. The carriers in the electron transport chain are clustered into three groups called the respiratory enzyme complexes. The first complex includes FMN and five or more Fe-S centers; the second complex includes cytochromes b and c 1 and an Fe-S center; and the third complex includes two copper centers and cytochromes a and a 3. Each complex collectively acts as a proton pump that removes H + from the mitochondrial matrix and pumps it into the space between the inner and outer mitochondrial membranes. Coenzyme Q acts as a shuttle that transfers electrons from the first pump to the second, and cytochrome c shuttles electrons from the second pump to the third. These proton pumps create a very high H + concentration (low ph) and a positive charge between the inner and outer membranes, compared to a low H + concentration and negative charge in the mitochondrial matrix. This creates a steep electrochemical gradient across the inner mitochondrial membrane. If the inner membrane were completely permeable to H +, these ions would have a strong tendency to diffuse down across the concentration gradient back into the mitochondrial matrix. The inner membrane of the mitochondria however is permeable to H + only through specific channel proteins called ATP synthase. As H + flows through these channels it creates an electrical current. ATP synthase harnesses this energy to synthesize ATP from ADP and a phosphate group. This process is called chemiosmosis, and is responsible for generating most of the ATP during cellular respiration. From the point at which NADH passes its pair of electrons to FMN, to the end of the electron transport chain, three 10

11 ATP molecules are synthesized. FADH 2 however releases its pair of electron further down the chain to coenzyme Q (bypassing the first enzyme complex) and is responsible for generating two molecules of ATP. During the various electron transfers in the electron transport chain, between 28 and 30 molecules of ATP are generated from the ten molecules of NADH + H +, and two molecules of ATP from each of two molecules of FADH 2 (to total four). Add to this the four molecules of ATP generated during glycolysis and the citric acid cycle, the theoretical maximum for ATP generation during aerobic respiration is between 36 and 38 molecules of ATP. The overall reaction for aerobic respiration can be expressed as thus: C 6 H 12 O 6 + 6O or 38 ADPs + 36 or 38 [P] Glucose Oxygen 6CO 2 + 6H 2 O + 36 or 38 ATPs Carbon dioxide Water Got it? Probably not: most students have a hard time keeping track of all the different events that occur during aerobic respiration. Please review this section and that within the accompanying text until you understand what is actually happening. Glycogenesis, Glycogenolysis and Glyconeogenesis When glucose is not immediately needed for ATP production it is polymerized to form a long chain polysaccharide called glycogen. This process is called glycogenesis, and occurs under the influence of insulin. This typically occurs just after a meal rich in carbohydrates. The primary sites of glycogenesis are the liver (25%) and skeletal muscles (75%). To manufacture glycogen, glucose enters liver or skeletal muscle cells and is phosphorylated to G6P. This is then converted to glucose 1-phosphate, then to uridine diphosphate glucose, and then into glycogen. When the body requires energy and the supply of blood glucose is low, glycogen is broken down into glucose and released into the blood stream. This process is called glycogenolysis, and typically occurs between meals. The process of glycogenolysis is not exactly the reverse of that 11

12 for glycogenesis. Under the influence of glucagon from the endocrine pancreas and/or epinepherine from the adrenal medullae, glucose molecules are broken off from glycogen and are phosphorylated. In liver cells, the enzyme phosphorylase converts glycogen into molecules of G6P, but in skeletal muscle glycogen is broken down into molecules of glucose 1-phosphate. In the liver, G6P is then converted into glucose under the influence of glucagon. In muscle cells glucose 1-phosphate is catalyzed into pyruvic acid and enters the citric acid cycle. Under anaerobic conditions however, pyruvate will be converted to lactic acid, where it may be released into the blood and converted by the liver back into glucose. When blood sugar levels diminish and the supply of glycogen is depleted, the body begins to look for a supply of nutrients to replenish ATP. If food is not eaten, the liver will then begin to catalyze nutrients such as fats and proteins into glucose. Such a process, in which new glucose is manufactured from non-carbohydrate sources, is called glyconeogenesis. Amino acids such as alanine, cysteine, glycine, glycine, serine and threonine can be converted into pyruvic acid, which can then enter the citric acid cycle or be further converted to glucose. Glycerol can be split from triglycerides and be converted into glyceraldehyde 3-phosphate directly, which may then be converted into pyruvate, or further synthesized into glucose. Glyconeogenesis is stimulated by cortisol secretions from the adrenal cortex, and by triiodothyronine (T 3 ) from the thyroid gland. Cortisol mobilizes proteins from body cells and breaks them down into amino acids, providing fodder for glycogenolysis. T 3 too mobilizes proteins for glyconeogenesis, and also mobilzes trigylcerides from adipose tissue, making glycerol available for glyconeogenesis. Other hormones such as epinepherine, glucagon and human growth hormone (hgh) also stimulate glyconeogenesis. Lipid metabolism As dietary fats are emulsified by the bile acids produced by the liver, pancreatic lipase breaks down these fats into fatty acids and monoglycerides. Short-chain fatty acids then 12

13 diffuse into epithelial cells of the intestinal villi and then into capillaries. Long-chain fatty acids and monoglycerides are carried in micelles to the intestinal villi and are further broken down into simpler fatty acids and glycerol, and are then recombined to form triglycerides. These triglycerides exit the intestinal cells as chylomicrons and enter into lymphatic circulation, and then into systemic circulation. Chylomicrons are essential for the transport of non-polar fat molecules in the aqueous medium of the blood. When it enters into circulation, the triglyceride component of the chylomicron is cleaved by lipoprotein lipase, and some of the triglycerides are taken up by fat and muscle cells. This leaves a cholesterol-rich lipoprotein remnant that is taken up by the liver and excreted back into the intestine as bile salts, or repacked with triglycerides into very low-density lipoproteins (VLDL), where it then reenters into circulation. Once again VLDL is acted upon by lipoprotein lipase, which removes triglycerides from VLDL, forming intermediate-density lipoproteins (IDL) that are eventually converted into cholesterol-rich lowdensity lipoproteins (LDL), which are then taken up and processed by a variety of cells, leading to the accumulation of cholesterol within these cells. A third lipoprotein, which unlike VLDL and LDL that ultimately function to transport cholesterol to peripheral cells, high-density lipoproteins (HDL) functions primarily to scavenge cholesterol from peripheral cells and return it to the liver for excretion. This is why elevated levels of serum VLDL and LDL have been associated with a greater risk of cardiovascular disease (CVD) because they deposit cholesterol into peripheral cells, whereas elevated levels of HDL is correlated with a lower risk because it removes cholesterol from cells. This assumption is based on the theory that it is cholesterol that causes CVD in the first place, which is discussed and debated in the Wild Rose pathology course. Similar to carbohydrates, fats can be used by the body to produce ATP, but if the energy requirements of the body are already met by the consumption of carbohydrates, most dietary fat will be stored in adipose tissue and in the liver for later usage. Beyond their use as a source of energy, fats have many functions in the body, including the formation of the cell membrane (e.g. phospholipids), as structural models to build molecules (e.g. cholesterol in the formation of steroidal hormones), or as transport molecules (e.g. 13

14 lipoproteins). A very few fats are called essential fatty acids, and must be obtained in the diet, including linoleic acid, linolenic acid, and arachidonic acid, all of which are essential in the proper growth and maintenance of the body. The primary function of adipose tissue is to store triglycerides until they are needed as an energy source, but adipose tissue also serves as a form of protection and insulation. About 50% of adipose tissue is located subcutaneously, the remainder is found around the kidneys (12%), in the omenta (10-15%), in the genitalia (15%), in between skeletal muscles (5-8%), and the remaining 5% found behind the eyes, in the sulci of the heart, and attached to the outside of the colon. Lipolysis Muscle, liver and adipose tissue routinely oxidize fatty acids from stored triglycerides to produce ATP in a process called lipolysis. This involves the break down of triglycerides into glycerol and fatty acids by enzymes called lipases. This action is facilitated by the sympathetic stimulation and the activities of the hormones epinephrine and norepinepherine. It has become popular of late to use the chemically similar ephedrine alkaloids in the herb Ephedra sinica (Ma Huang) to promote weight loss, an approach however that is both dangerous and disrespectful to traditional Chinese medicine, which reserves this useful herb for conditions such as asthma. Other hormones involved in lipolysis include cortisol, thyroid hormones, insulin-like growth factors; the hormone insulin however inhibit lipolysis, which is why a diet high in carbohydrates is not all that effective to lipolysis. The glycerol and fatty acids that result from lipolysis are catabolized by different pathways. Glycerol can be converted into glyceraldehyde 3-phosphate (G3P), and if ATP supply is low, is further metabolized to form pyruvic acid; if however ATP supply is high, G3P will be converted into glucose. Fatty acids are metabolized differently and yield more ATP. The first stage in fatty acid metabolism is beta oxidation, which occurs in the mitochondrial matrix, enzymes removing two carbon atoms at a time from the carbon chain that makes up a fatty acid, and attaching these to CoA, forming acetyl CoA, which then enters the citric 14

15 acid cycle. The rich availability of carbon atoms in long chain fats such as the 16 carbon palmitic acid can yield as many as 129 molecules of ATP through aerobic respiration. Hepatocytes have a special ability to take two acetyl CoA molecules at a time and condense them into acetoacetic acid, some of which is converted into β-hydroxybutyric acid and acetone. This process is called ketogenesis, and collectively, these three molecules are called ketone bodies. Ketone bodies are fat soluble and freely diffuse out of the hepatocytes into circulation. Peripheral cells then take up these four carbon molecules and attach it to a CoA, forming a acetyl CoA molecule that can then enter the citric acid cycle. Some tissues such as the heart muscle and the cortex of the kidneys actually prefer to use ketones to manufacture ATP. In insulin-dependent diabetics however, the lack of insulin promotes lipolysis, and combined with the exclusive usage of fats as an energy source, the production of ketone bodies soon outstrips that body s ability to utilize them. The rise in the ketone concentration of the blood makes the blood more acidic, using up buffering agents and decreasing the ph, which can lead to coma. Lipogenesis Liver cells and adipose cells have the ability to synthesize lipids from glucose and amino acids through lipogenesis, which is stimulated by insulin. Lipogenesis occurs when the body s requirements for basic nutrition is met, and excess carbohydrates or proteins are stored as fat for later use. This feature is an evolutionary adaptation, which ensured that humans could survive periods of famine by eating in excess of their nutritional needs during periods of plenty, and storing this excess in adipose cells. Protein metabolism Unlike carbohydrates and fats, proteins are not stored for later usage. Once they are broken down into amino acids, they can be oxidized to produce ATP or used to synthesize new proteins to support bodily growth and repair. Excess amino acids are not excreted but are converted into glucose or triglycerides for storage. Apart from the diet, about 50% of proteins absorbed from the small intestine are actually 15

16 obtained from the body itself, in the form of dead epithelial cells from the intestinal mucosa, or from enzymes. Protein catabolism The amino acids contained in the slurry of nutrients obtained from the digestive tract are acted upon by insulinlike growth factor and insulin, which promotes their uptake by hepatocytes and other body cells. Once inside a cell, these amino acids can be used as a source of fuel, or converted to fats or glucose. Three processes guides these activities: 1. deamination: the removal of an amino group (NH 2 ) 2. amination: the addition of an amino group 3. transamination: the transfer of an amino group from one molecule to another When amino acids are going to be used as a fuel they first undergo the process of deamination, removing the NH 2 group, which leaves a molecule called a keto acid. Depending on which amino acid it is, this keto acid may be converted to pyruvic acid, acetyl CoA, or another of the acids in the citric acid cycle. When an amino acid is deaminated, the amino group is transferred to the citric acid cycle intermediate α- ketoglutaric acid, converting it to glutamic acid. Once glutamic acid is produced, it travels back to the liver where it is deaminated back into α-ketoglutaric acid, which renders ammonia (NH 3 ), which is highly toxic and immediately converted by liver cells into urea, in a metabolic pathway called the ornithine cycle. Protein anabolism The formation of new peptide bonds between individual amino acids to form proteins is carried out on the ribosomes in almost every cell, directed by DNA and RNA. The process of protein anabolism is stimulated by human growth hormone, thyroid hormones, and insulin. Out of the 20 amino acids found in the human body, ten of these cannot be synthesized or are not synthesized in adequate amounts, and are thus called essential amino acids. Nonessential amino acids can be synthesized from essential amino acids through the process of transamination, the transfer of an amino group from an amino acid to pyruvic acid or to an acid in the citric acid cycle. 16

17 Absorptive and post-absorptive states The regulation of metabolic activities depend in part upon how much time has passed since the last meal. During the absorptive state, the slurry of nutrients enter into the blood stream and are directed to the liver, where nutrients are made available as glucose or ketones for ATP production. Glucose derived from dietary carbohydrates are transported to the liver via the hepatic portal, where most of this passes into systemic circulation to be made available to cells everywhere in the body. When glucose is absorbed in excess of energy requirements, it is absorbed by the liver and converted to glycogen, or if glycogen needs are met, as fat. Fats in enter into systemic circulation via the lymphatic system, allowing the chylomicrons to by-pass the liver. The chylomicrons are instead acted upon by lipoprotein lipase in the blood, which removes some portion of the fats in the chylomicrons, which are then taken up by muscle and adipose cells. This leaves a cholesterol-rich lipoprotein remnant that is taken up by the liver and excreted back into the intestine as bile salts, or repacked with triglycerides into very low-density lipoproteins (VLDL), where it then reenters into circulation, and the process is initiated again. Like glucose, amino acids are directed to the liver via the hepatic portal vein, passing onwards to be available by other peripheral cells for protein synthesis. The liver however does remove some of the amino acids, to be used in protein synthesis, to be deaminated and used as a fuel for ATP synthesis, or to be deaminated and used for fatty acid synthesis. The regulation of the absorptive state is performed largely regulated by insulin, which is secreted in response to elevated blood glucose and amino acid levels in the blood. Insulin dramatically increases the cellular uptake of glucose by as much as 20 times, except in neurons, where the rate of glucose absorption functions independently of insulin. Insulin is primarily an anabolic hormone, stimulating the oxidation of glucose, glycogenesis and lipogenesis. Insulin inhibits gluconeogenesis, and the active transport of amino acids into cells. One marked difference however between a high carbohydrate-low protein meal and a high protein-low carbohydrate one is that the latter also stimulates the secretion of glucagon, which prevents the hypoglycemia (low blood sugar) that would occur as the result of insulin secretion and low blood sugar levels. The regulation of the 17

18 absorptive state is also influenced by digestive hormones including gastrin, secretin and cholcystokinnin. The post-absorptive state is when the absorption of nutrients from the GIT is complete, and the energy needs of body must be met by the food energy already stored in the body. It takes about four hours to completely assimilate all the nutrients from each meal, and given that most people eat three meals a day, as well as snacks, the only period of post-absorption is during sleep. It is therefore not surprising that there is such a problem with obesity in our culture, simply because the body has limited opportunities to move into the post-absorptive state and utilize the stored food energy, usually as fat. During the post-absorptive state glucose is made available into the blood stream from the body s glycogen reserves, or is synthesized via gluconeogenesis. There is usually about four hours worth of glycogen in the liver to support post-absorptive needs, after which the body relies upon gluconeogenesis. Adipocytes and hepatocytes hydrolyze fats and convert glycerol to glucose. The free fatty acids can then be oxidized by the liver to produce ketone bodies which can then be used to produced ATP, allowing glucose to be saved to supply the brain (called a glucose-sparing effect), which cannot use ketones as efficiently. When glycogen and fat stores are depleted, as in during starvation, the body will begin to use proteins in muscles to supply the body with a source of energy, which is characteristic of the wasting that is seen during long periods of fasting. The regulation of the post absorptive state is performed by a combination of the sympathetic nervous system and by glucagon. Under sympathetic stimulus, the hormones released by the adrenal medulla promote glycogenolysis and lipolysis. Glucagon promotes glycogenolysis and gluconeogenesis, which raises blood sugar and promotes lipolysis and a rise in free fatty acids in the blood, making more glucose and fats available for ATP synthesis. Growth hormone is also secreted in response to rapid drops in blood glucose level and in prolonged fasting, opposing the effects of insulin and raising the blood glucose concentration. Metabolic rate The metabolic rate is a term used to describe the amount of energy that is liberated by the body per unit of time, 18

19 expressed in either kilocalories per hour (kcal/hr) or kcal/day. A calorie (cal) is defined as the amount of heat required to raise the temperature of one gram water by one degree; a kilocalorie (or Calorie with a capital c ) equals 1000 calories. The metabolic rate can be measured by putting a person in a closed chamber called a calorimeter that has water filled walls that absorb the heat given off by the body. It can also be measured with a spirometer, which measures the amount of oxygen consumed. It is inferred that every person utilizes about 4.82 kilocalories from organic nutrients for every liter of oxygen consumed: it is however an imprecise measurement that doesn t take into account the types of nutrients that are being oxidized at the time of measurement. The method for determining the calorie content in food is based upon the amount of heat energy that is released from a given food item as it is combusted in a laboratory. Unfortunately, it is an imprecise technique that doesn t take into account how the different nutrients are metabolized in a biological system such as the human body, and apart from general observations, isn t a practical method to determine how much and what kinds of food an individual can eat to optimize their metabolic activities. Scientists understand that several factors can affect the metabolic rate, and thus use a standard called the basal metabolic rate (BMR) that is a reflection of metabolic function when the body is in a quiet, resting state. The BMR is usually determined by measuring the amount of oxygen that is used per calorie of food that is metabolized. Generally speaking, the BMR is between kcal/day in adults, or about 24 kcal/kg of body mass in adult males and 22 kcal/kg in adult females. The added calories needed to maintain activities over and above the BMR varies considerably, depending on the individual and the type of activities performed. Thus for someone who is relatively sedentary, perhaps only 500 kcal/day would be needed in excess of that required to maintain the BMR, whereas someone exercising vigorously or engaging in hard labor may require up to extra kcal/day. It is deceptive however to make such generalizations, because some people, such as those that fidget or engage in other nervous habits but remain sedentary, can burn just as much energy as someone else might burn by jogging several kilometers a day. The importance of taking into account 19

20 individual factors in determining diet cannot be over-stated. Individuals can be broadly classified according to the predominance of muscle (brown or white) and fat in their body. Thus someone who has a thin build and a higher proportion of brown to white muscle, with little fat, may require many more calories than someone who has more white muscle (which is less metabolically active) and/or fat. Ayurvedic medicine classifies these types according to the predominance of the humors of the body: 1. Vata (Wind) asthenic build, nervous behaviours 2. Pitta (Bile) mildly sthenic build, physically active 3. Kapha (Phlegm) sthenic build, physically inactive Thus the diet is based upon the predominance of Vata, Pitta or Kapha, or an mixture of these types, in the body. Vata individuals require a greater emphasis upon proteins and fatty foods. Pitta individuals require an equal balance between carbohydrates, proteins and fats. Kapha individuals benefit from a diet that is lower in fat and carbohydrates. Regulation of body temperature In order for all the metabolic reactions in the body to occur with maximum efficiency homeostatic mechanisms exist to maintain an optimal body temperature. Heat is a natural by-product of nutrient catabolism and digestion, and this heat is harnessed and circulated through the body by the blood to maintain an optimal temperature. Heat is also produced by physical exercise, which increases the catabolism of nutrients, as do activities of the sympathetic nervous system and influence of the environment. As we age, our metabolic function generally declines, since a substantial amount of heat is generated simply through the processes of growth and development during childhood. Perhaps this is one reason why some kids refuse to wear the proper jackets, hats and gloves during the cold season when they run outside and play, despite the admonishment of the parents to the contrary, whose metabolic activity is lower and therefore feel the cold more acutely. The regulation of body temperature is maintained by receptors in the skin and hypothalamus and is exerted by several effectors: 20

21 1. Peripheral vasoconstriction (in response to cold) and vasodilation (in response heat, leading to perspiration) 2. The stimulation (in response to cold) or inhibition (in response heat) of sympathetic activity 3. Skeletal muscle contraction (in response to cold) or relaxation (in response heat) 4. Stimulation (in response to cold) or inhibition (in response heat) of thyroid activity The core temperature is the temperature in body structures deep to the skin and subcutaneous layer, whereas the shell temperature is the temperature located on the body surface, in the skin and subcutaneous layer. Depending on environmental conditions the shell temperature is 1-6 C lower than the core body temperature, which is maintained at a fairly even 37 C, except during fever. 21