5.0 HORMONAL CONTROL OF CARBOHYDRATE METABOLISM
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1 5.0 HORMONAL CONTROL OF CARBOHYDRATE METABOLISM Introduction: Variety of hormones and other molecules regulate the carbohydrates metabolism. Some of these have already been cited in previous sections. The appropriate functions of the body are dependent on precise control of the glucose concentration in the blood mg/100 mlisthe normal fasting level of glucose in the blood. A condition called hyperglycemia results if the concentration of glucose in blood is too high (above 120 mg/100 ml). Hyperglycemia may temporarily exist as a result of eating a meal rich in carbohydrates.when the concentration of glucose is too low (below 70 mg/100 ml) the condition is called as hypoglycemia. Hypoglycemia is characterized by general weakness, trembling, headache,profuse perspiration, drowsiness, rapid heartbeat, and possible loss of consciousness. The liver and skeletal muscle self-regulate carbohydrate metabolism in basically the same way as do other cells. Yet, these cell types are also required to respond to external signals by altering their carbohydrate metabolism. These two tissues, along with adipose tissue (which is primarily involved in lipid metabolism), act as the major regulators of nutrient levels in circulation during most metabolic conditions. OBJECTIVES To understand the significance of regulation of carbohydrate metabolism by hormones to maintain the body homeostasis To appreciate the regulatory mechanisms that play an important role in maintaining the blood glucose level To understand the responses of the body to different energy utilisation conditions. The regulation has two goals: 1) maintenance of normal circulating glucose levels in the face of changing conditions, and (when necessary) 2) support of physical activity. These tissues are tightly controlled by external signals: the levels of the pancreatic hormones insulin and glucagon, the adrenal hormones epinephrine and cortisol. In the case of skeletal muscle, the neuronal signals govern muscle contraction. In general, glucagon and epinephrine result in phosphorylation of regulatory enzymes, whereas insulin results in removal of the phosphate; calcium usually increases phosphorylation (one major exception is the
2 mitochondrial enzyme pyruvate dehydrogenase, wherein calcium stimulates phosphate removal). Regulation of glycolysis by pyruvate kinase Some of the hormones, especially cortisol and insulin, and to a lesser extent glucagon, alter the amounts of the enzymes present in the cell. The phosphorylation and dephosphorylation events occur quickly, though effects on enzyme concentration are relatively slow processes. The cells of the liver and muscle must also use the same feedback regulatory metabolites as do normal cells ; these effects interact with the hormonal signals to result in the overall metabolic changes that occur within these cells. The two diagrams below summarize the control of the various pathways by metabolic and hormonal effects in liver and in skeletal muscle.
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4 Lets now see the regulation at various conditions of the body 5.1 Fed state During absorption of nutrients from the digestive tract, circulating glucose levels typically become somewhat elevated. This results in decreased glucagon levels and increased insulin. High insulin levels result in increased glucose uptake in skeletal muscle and adipose tissue (as a direct result of increased glucose transport due to the movement of the transporters to the membrane). The muscle is capable of dramatic increases in glucose uptake, a specialization that is related to its increased glucose requirements during muscle contraction; the organism uses this ability to rapidly deal with sudden increases in plasma glucose. The glycogen storage in liver and muscle is induced by insulin. It also increases glycolysis in liver. The increase in liver glycolysis has two purposes: 1) it decreases glucose availability, and 2) it increases the amount of intermediates that are used for biosynthetic reactions. Hexose monophosphate pathway activity is also increased by insulin by increasing the concentration of glucose-6-phosphate dehydrogenase; this also increases both glucose breakdown and the cellular capacity for biosynthetic processes. The net effect of these processes is the removal of glucose from circulation to be stored, used for energy, or used to produce other biologically relevant compounds. The effect of insulin on the various metabolic pathways operating is shown in the figure
5 5. 2 Exercise During exercise, the liver and muscle must respond somewhat differently to the same external signals. The responses are facilitated by three physiological signals: 1) increased epinephrine, 2) increased calcium in muscle, and 3) decreased insulin levels. In both muscle and liver there is an increase in glycogen breakdown. This is the result of phosphorylation of both glycogen synthase and glycogen phosphorylase in both tissues; phosphorylase is stimulated by phosphorylation, while glycogen synthase is inhibited. The glucose is released into the circulation by the liver; the muscle takes up glucose from circulation, and also uses the glucose released from its own glycogen stores. One method of simultaneously altering both gluconeogenesis and glycolysis is to alter fructose-2,6-bisphosphate levels. The fructose-2,6- bisphosphate stimulates phosphofructokinase and inhibits fructosebisphosphatase; fructose-2,6-bisphosphate therefore simultaneously stimulates glycolysis and inhibits gluconeogenesis. The exercising muscle must be able to do glycolysis; in contrast, during exercise, the liver must turn off glycolysis and turn on gluconeogenesis. These contrasting events are mediated by different isozymes of the PFK-2/FBPase-2 protein. Phosphorylation of liver PFK-2/FBPase-2 results in decreased fructose- 2,6-bisphosphate levels (because it alters the enzyme conformation to favor the phosphatase activity). In contrast, phosphorylation of the muscle PFK-2/FBPase-2 isozyme results in an increase in the fructose- 2,6-bisphosphate levels, and thus in an increase in the muscle capacity for glycolysis. As with other biochemical pathways, hormones affect gluconeogenesis by varying the concentrations of allosteric effectors and the key rate-determining enzymes. As stated previously, glucagon depresses the synthesis of fructose- 2,6-bisphosphate, which releases the inhibition of fructose-1,6-bisphosphatase, and thereby inactivates the glycolytic enzyme pyruvate kinase. Hormones also affect gluconeogenesis by altering enzyme synthesis. For example, cortisol, a steroid hormone produced in the cortex of the adrenal gland that facilitates the body s adaptation to stressful situations stimulates the synthesis of gluconeogenic enzymes. In conclusion, insulin action leads to the synthesis of new molecules of glucokinase, PFK-1 (SREBP1c-induced), and PFK-2 (glycolysis favoured). Insulin also depresses the synthesis (also via SREBP1c) of PEP carboxykinase, fructose- 1,6-bisphosphatase, and glucose-6-phosphatase. The action of glucagon leads to the synthesis of additional molecules of PEP carboxykinase, fructose-1,6- bisphosphatase, and glucose-6-phosphatase (gluconeogenesis favoured). The hormones that regulate glycolysis and gluconeogenesis alter the phosphorylation
6 state of certain target proteins in the liver cell, which in turn modifies gene expression. The vital point to remember is that insulin and glucagon have opposing effects on carbohydrate metabolism. The direction of metabolite flux, (i.e., whether either glycolysis or gluconeogenesis is active) is largely determined by the ratio of insulin to glucagon. The insulin/glucagon ratio is high after a carbohydrate meal and glycolysis in the liver predominates over gluconeogenesis. After a period of fasting or following a high-fat, low-carbohydrate meal, the insulin/glucagon ratio is low and gluconeogenesis in the liver predominates over glycolysis. The availability of ATP is the second significant regulator in the reciprocal control of glycolysis and gluconeogenesis in that high levels of AMP, the low-energy hydrolysis product of ATP, increase the flux through glycolysis at the expense of gluconeogenesis, and low levels of AMP increase the flux through gluconeogenesis at the expense of glycolysis. Even though control at the PFK-1/fructose- 1,6-bisphosphatase cycle would appear to be sufficient for this pathway, control at the pyruvate kinase step is key because it permits the maximal retention of PEP, a molecule with a very high phosphate transfer potential. The liver also contains an isozyme of pyruvate kinase that is inhibited by phosphorylation. The phosphorylation does not affect muscle pyruvate kinase. Exercising muscle tends to release some lactate into circulation; the liver uses this as substrate for gluconeogenesis. This exchange of materials between the liver and the muscle is known as the Cori cycle. The net effect of these metabolic changes is increased glucose release by
7 the liver and uptake and utilization by the muscle to support generation of mechanical energy by the muscle. Cori Cycle: During muscle contractions, ATP is constantly being used to supply energy and more ATP is produced to replenish supplies.at first glycolysis produces pyruvic acid which is then converted into acetyl CoA and is metabolized in the citric acid cycle to make ATP using the electron transport chain.if muscular activity continues, the availability of oxygen for use at the end of the electron transport chain becomes the limiting factor and the cells soon exhaust their supplies of oxygen. When this occurs, the citric acid cycle is inhibited and causes pyruvic acid to accumulate.however, glycolysis continues even under anaerobic conditions even though the citric acid cycle works only under aerobic conditions. Figure below shows that epinephrine at (1) stimulates the enzymes to work on glycogen as discussed in the above panel. Glycogenolysis at (2) is stimulated to make more glucose-6-phosphate.when the cells become anaerobic, glycolysis (3) continues if pyruvic acid is converted to lactic acid (4). Remember that the synthesis of lactic acid requires NADH from Step 5 in glycolysis and produces NAD + so that Step 5 can continue.the formation of lactic acid buys time and shifts part of the metabolic burden to the liver.even though not as much ATP can be furnished by glycolysis alone, it is an important source of ATP when muscular activity continues for any extent of time. The concluding limiting factor in continued muscular activity is the build up of lactic acid. The lactic acid ultimately produces muscular pain and cramps which force discontinuation of activity. Generally before this happens and after activity has finished, lactic acid diffuses out of the muscle cells and into the blood where it enters the liver. The body is very efficient in that lactic acid is sent in the blood (5) to the liver which can convert it back to pyruvic acid (6) and then to glucose through gluconeogenesis (8). The glucose can enter the blood (9) and be carried to muscles and instantly used. If by this time the muscles have stopped activity, the glucose can be used to reconstruct supplies of glycogen through glycogenesis (10). This recycling of lactic acid is referred to as the Cori Cycle. This Cori cycle also functions more efficiently when the muscular activity has ceased. At this time the oxygen debt can be made up so that the citric cycle and electron transport chain also initiate to function again. In order for most of the lactic acid to be converted to glucose, some must be converted to pyruvic acid and then to acetyl CoA (7). The citric acid cycle and electron transport chain must provide ATP to "fuel" the gluconeogenesis of the remainder of the lactic acid to glucose.
8 5.3 Starvation During starvation, insulin levels decrease whereas glucagon and cortisol levels increase resulting in a variety of metabolic changes. One major effect is the breakdown of muscle protein to provide amino acids for gluconeogenesis. The liver responds by increasing glycogen breakdown and by increasing glucose release. It also responds by greatly increasing gluconeogenesis, using amino acids released by the muscle as substrates.the net effect is the release of glucose from the liver for the use by other tissues, with breakdown of muscle fat to supply the requisite energy and intermediates.
9 5. 4. Insulin deprivation Diabetes mellitus is a relatively common disorder resulting either from destruction of the cells that release insulin or from lack of response by the tissues to insulin. The absence of insulin action results in a condition that is similar to starvation; tissues are broken down to provide substrates for liver gluconeogenesis. The liver then works very hard to produce glucose to release into circulation, raising plasma glucose levels to concentrations that have deleterious effects. This effect is exacerbated by the fact that the individual typically continues to eat, and therefore absorb nutrients, and by the fact that the skeletal muscle and adipose tissue require insulin for uptake of glucose. The net effect is a dramatic increase in plasma glucose concentration, because the liver is releasing glucose while the other tissues are not taking up glucose and are performing reduced levels of glycolysis. Conclusion Most cells are capable of regulating their carbohydrate metabolism based on their current energy necessities. This is attained by feedback inhibition or stimulation of regulatory enzymes by various metabolites. The important metabolites involved in regulation of carbohydrate metabolism include ATP, NADH, glucose-6- phosphate, citrate, and fructose-2,6-
10 bisphosphate. Many cell types respond to hormonal and neuronal signals that allow the coordination of metabolism at the level of the entire organism. For carbohydrate metabolism, the liver and skeletal muscles have the most important roles. The liver either takes up glucose for storage in the form of glycogen, or releases glucose for use by other tissues. The muscle takes up glucose for storage or for conversion to mechanical energy; it can release free amino acids derived from protein breakdown to act as substrates for liver gluconeogenesis. Both synthesis and degradation are controlled through a complex mechanism involving insulin, glucagon, and epinephrine, as well as allosteric regulators. Glucagon is released from the pancreas when blood glucose levels drop in the hours after a meal. It binds to receptors on hepatocytes and initiates a signal transduction process that elevates intracellular camp levels. camp increases the original glucagon signal and initiates a phosphorylation cascade that leads to the activation of glycogen phosphorylase along with a number of other proteins. Within seconds, glycogenolysis leads to the release of glucose into the bloodstream. When occupied, the insulin receptor becomes an active tyrosine kinase enzyme that causes a phosphorylation cascade that ultimately has the opposite effect of the glucagon/camp system: the enzymes of glycogenolysis are inhibited and the enzymes of glycogenesis are activated. Insulin also increases the rate of glucose uptake into several types of target cells, but not liver or brain cells. Emotional or physical stress releases the hormone epinephrine from the adrenal medulla. Epinephrine stimulates glycogenolysis and inhibits glycogenesis. In emergency conditions, when epinephrine is released in relatively large quantities, enormous production of glucose provides the energy required to manage the situation.
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