number Done by Corrected by Doctor Nayef Karadsheh

Transcription

1 number 11 Done by حسام أبو عوض Corrected by Moayyad Al-Shafei Doctor Nayef Karadsheh 1 P a g e

2 General Regulatory Aspects in Metabolism: We can divide all pathways in metabolism to catabolicand anabolic. In catabolic pathways dietary molecules are broken down to produce energy-rich molecules (ATP) and also generate NADH (mainly). Then in anabolic pathways this energy is used to build upour macromolecules (proteins, fats, etc.). -Catabolic reactions end by producing energy-poor products (CO2, H2O, NH3). - NADH is produced in catabolic reactions and is later used in the electron transport chain to obtain energy by converting NADH back to NAD+,whereas the role of NADPH is mostly anabolic reactions, where NADPH is needed as a reducing agent. Cells only use the required amounts of resources (no excess). To do such a job, enzymes with regulatory subunits (in addition to the catalytic subunit) are needed. Signals can regulate the activity of such enzymes. 2 P a g e

3 The mechanisms of regulation can vary, they can be :. - Signals from within the cell :.Intracellular communications typically lead to rapid responses. -Having cells exchange substances with each other via cell junctions :. Gap junctions allow direct communication between adjacent cells (see introduction to histology). -Using the second messenger system. The two most common second messenger systems are the calcium phosphatidylinositol system and the adenylyl cyclase system (the latter used in carbohydrates and fats metabolism). [Example on adenylyl cyclase system] A hormonal signal (e.g glucagon) binds to the extracellular part of a receptor on the membrane of the cell. This binding is transmitted across a domain within the cell membrane and then to the intracellular part of the receptor. The receptor would be coupled to a G-protein. The signal could stimulate or inhibit the G- protein. The G- protein consists of 3 subunits, α, β and ϒ. Before the signal a GDP molecule would be bound to the α subunit. Upon binding, a GTP molecule would replace the GDP molecule which activates the α subunit letting it dissociate from the α, β and ϒ complex. The α subunit then interacts with another membrane bound enzyme, adenylyl cyclase. Adenylyl cyclase begins converting ATP into cyclic AMP and this continues as long as the first signaling molecule remains attached to the receptor. When the signaling molecule leaves,the GTPase enzyme present in the α subunit changes 3 P a g e

4 GTP to GDP,sodeactivating the α subunit and reuniting it with the β and ϒ subunits. The produced camp (cyclic AMP) binds to camp dependentprotein kinase enzyme in the cytosol. The protein kinase is formed from four subunits (2 regulatory and 2 catalytic). Upon camp binding, the regulatory subunits dissociate and the enzyme becomes active. This enzyme phosphorylates many other enzymes in the cell and this phosphorylation activates some enzymes and inhibit others (also can work on ion channels and DNA promoter regions [increases transcription]). These changes that occurred in the cell must be reversed after some time for the cell to maintain its physiological state. Protein phosphatase enzyme is activated and removes the phosphates from the other enzymes returning the cell to its original status. Phosphodiesterase enzyme converts camp to 5 -AMP (stopping the action of camp) 4 P a g e Glycolysis It is a universal pathway used by all cell types and by all organisms (aerobic and anaerobic) in order to generate ATP.In addition to that, it provides the

5 precursors for anabolic pathways; for example, some amino acids are produced from its intermediates, in addition to fats. -Glycolysis can be divided into 2 main phases 1- Preparative phase (energy-investment phase):. This phase consumes 2 ATP molecules in order to phosphorylate glucose (energizing it). 2- ATP generating phase:. This phase includes 2 NADH molecules and 4 ATP molecules production. So glucose will be converted to 2 molecules of pyruvate by the end of this phase. -Under anaerobic state only 2 ATP are formed at substrate level with conversion of glucose to lactate. -Under aerobic state glucose converted to CO2 (oxidized completely)and approximately 38 ATP molecules. At the end of the process (10 steps) pyruvate is produced. This can then go to an anaerobic pathway (2 ATP produced) or an aerobic pathway (36-38 ATP produced). 5 P a g e

6 Some tissues have an absolute requirement of glucose, this includes: brain, RBC s, eyes (cornea lens and retina), kidney medulla, testes, leukocytes and white muscle fibers (in excessive exercise it will undergo both aerobic and anaerobic respiration). Step 1 Steps of Glycolysis Glucose enters the cell and is immediately phosphorylatedto glucose-6- phosphate, this reaction requires the use of an ATP molecule. The reaction is catalyzed by two isoenzymes (according to position), glucokinase (Liver and beta cells) and hexokinase (most of the cells). -This reactionis also the first one in glycogenesis andpentosephosphatepathway. -Glucokinase has a higher Km and a higher Vmax (refer to summer biochemistry course) than hexokinase. This means that glucokinase has a lower affinity to glucose but can take more of it. This is completely suitable with its role in the liver, for when glucose concentration in the blood increases (above 100mg/100mL) the glucokinase enzyme will operate and phosphorylate glucose for storage (and if the storage gets full, glucose is changed to fat), but if the glucose concentration drops glucokinase will not function. Hexokinase, on the other hand, will always operate whether the concentration of glucose in the blood is high or low, this is due to its high affinity to glucose and this suits its role in supplying cells with glucose for energy production, cells must always get their energy requirement regardless of the situation. -Both hexokinase and glucokinase work best with glucose but can also work on other sugars like fructose, mannose and galactose. -Glucokinase s action is induced by insulin in hyperglycemia, hexokinase s is never induced. 6 P a g e

7 -During fasting glucose concentration in blood drops and the liver does not take any glucose, in fact it releases glucose to the blood stream from its storage. -This reaction is essentially irreversible (exergonic) and hexokinase is one of the regulated enzymes in glycolysis. Step 2 Phosphoglucoseisomerase (aldose-ketose isomerization) enzyme changes glucose-6-phosphate to fructose-6-phosphate. This is a reversible reaction. Step3 7 P a g e

8 Phosphofructokinase enzymecatalyzes the use of ATP to convert fructose-6- phosphate to fructose-1,6-bisphosphate. This enzyme is the most important enzyme in the regulation of glycolysis. Step 4 Fructose-1,6-bisphosphate is cleavedinto the two trioses dihydroxyacetonephosphate (a keto sugar) and glyceraldehyde-3-phosphate (an aldo-sugar). This reaction is catalyzed by the enzyme aldolase. Step 5 The two trioses produced are interconverted by the enzyme triosephosphate isomerase. But, given that glyceraldehyde-3-phosphate is the one that continues the pathway, overall, dihydroxyacetone-3-phosphate is the one being changed to glyceraladehyde-3-phosphate. So, we get two glyceraldehyde-3-phosphate molecules from each glucose molecule. (from step 6 to 10 are the oxidative steps ) Step 6 8 P a g e

9 Glyceraldehyde-3-phosphate is converted to 1,3-bisphosphoglycerate via the enzyme glyceraldehyde-3-phosphate dehydrogenase(ga3pd). The reaction produces an NADH molecule. Via a cysteine group in the active site, the enzyme forms a linkage with the substrate. The enzyme has an NAD+ molecule close to the active site. Then the substrate is oxidized forming a thioester linkage and NAD+ is changed to NADH. NADH has a low affinity to the enzyme so it leaves and is replaced by a new NAD+ molecule. An inorganic phosphate then attacks the thioester linkage releasing the substrate as the product, 1,3-bisphosphoglycerate, and returning the enzyme to its original form. Step 7 9 P a g e

10 Phosphoglycerate kinase enzyme changes 1,3-bisphosphoglycerate to3- phosphoglycerateand an ATP molecule. (Remember that from step 5 two molecules form from each product, so we actually get two ATP molecules here). Step 8 Phosphoglycerate mutase enzyme (mutases change the position of a functional group from one carbon to another) change3-phosphoglycerate to 2- phosphoglycerate. The molecule 2,3-bisphosphoglycerate is needed in addition to the enzyme for this reaction to occur (this is the only reaction in the cell in which this molecule is used, so it is only seen in small amounts here. This molecule is seen in large amounts in the blood where it regulates the red blood cells). Step 9 Enolase enzyme changes 2-phosphoglycerate to the high energy molecule phosphoenolpyruvate. This happens through a dehydration (H₂O is lost) reaction and a rearrangement reaction. Step P a g e

11 Phosphoenolpyruvate is converted into pyruvate using pyruvate kinaseenzyme. An ATP molecule is produced in the process. (Remember it is actually two ATP molecules produced by one molecule of glucose). This reaction is irreversible and this enzyme is among the ones that are regulated. (Notice that all the three regulated enzymes are kinases). The production of ATP in glycolysis is called as substrate level phosphorylation (like Krebs cycle). Anaerobic Respiration Mammals Metabolic Fates of Pyruvate Pyruvate is changed to lactate using the lactate dehydrogenase enzyme which allows us to reconvert NADH to NAD+ for it to be used in converting more glucose to pyruvate and producing more energy. This is seen in muscle fibers 11 P a g e

12 during intense exercise and in the red blood cells even with no exercise simply because there is limited oxygen compared to the amounts of pyruvate. This causes accumulation of lactate, so this lactate moves through the blood to the liver where it is changed back to pyruvate and back to glucose to be used again by the muscles to produce more energy. The liver oxidizes fat in order to get energy for formation of glucose from lactate. (The build-up of lactate causes fatigue). Yeast (and Some Bacteria) These follow the alcoholic system. Here pyruvate is changed via pyruvate decarboxylase (Thiamine pyrophosphate (TPP) is needed) into acetaldehyde and CO₂ (which is excreted). Acetaldehyde is changed to alcohol via alcohol dehydrogenase enzyme and in the process NADH is converted back to NAD+ to be used again in glycolysis. (No alcohol is produced in humans because they do not have pyruvate decarboxylase). When ethanol reaches 10-11% in concentration it begins affecting the efficiency of the bacteria, just like lactate causes fatigue to us. (Alcohol companies have different methods to increase the concentration of alcohol). Alcoholic fermentation is also used in home. In baking bread when yeast is added to the dough partial alcoholic fermentation occurs and CO₂ forms raising the dough. When the dough is placed in the oven the baking removes all the alcohol (evaporates it) so the smell of bread is actually that of the evaporated alcohol. 12 P a g e

13 Aerobic Respiration Pyruvate is changed to acetyl CoA via the pyruvate dehydrogenase complex. When acetyl CoA is in excess of the requirement of Krebs cycle the excess will be changed to fat. When fasting amino acids can be used to make pyruvate (due to shortage of sugar) then pyruvate is changed in the liver to glucose to maintain blood sugar level. This glucose is NOT used in glycolysis. Instead, the required acetyl CoA is produced from fat hydrolysis. Then this acetyl CoA will inhibit pyruvate dehydrogenase complex to prevent wasting pyruvate in formation of acetyl CoA and get pyruvate to be changed to oxaloacetate in order to be used in both Krebs cycle and glucose synthesis. 13 P a g e