SA Glycolysis Flashcards
Define “fermentation” and explain, by describing relevant reactions, how it differs from glycolysis. Your explanation should include a discussion of the role of NADH in the reaction(s).
Fermentation is the operation of the glycolytic pathway under anaerobic conditions.
Under aerobic conditions, the pyruvate produced by glycolysis is oxidized to acetyl-CoA, which passes through the citric acid cycle. NADH produced in the oxidations passes electrons to O2, and is thus recycled to NAD+ allowing the continuation of the glycolytic reactions.
When no O2 is available to reoxidize the NADH produced by the glyceraldehyde 3-phosphate dehydrogenase reaction, electrons from NADH must be passed to one of the products of glycolysis, such as pyruvate or acetaldehyde, forming lactate or ethanol.
Briefly describe the possible metabolic fates of pyruvate produced by glycolysis in humans, and explain the circumstances that favor each.
Under aerobic conditions, pyruvate is oxidized to acetyl-CoA and passes through the citric acid cycle. Under anaerobic conditions, pyruvate is reduced to lactate to recycle NADH to NAD+, allowing the continuation of glycolysis.
Show how NADH is recycled to NAD+ under aerobic conditions and under anaerobic conditions. Why is it important to recycle NADH produced during glycolysis to NAD+?
Cells contain a limited supply of NAD+ and NADH. The oxidation of glyceraldehyde 3- phosphate requires NAD+ as as electron acceptor—it converts NAD+ to NADH. Unless this NADH is recycled to NAD+, oxidative metabolism in this cell will cease for lack of an electron acceptor. Under aerobic conditions, NADH passes electrons to O2; under anaerobic conditions, NADH reducespyruvate to lactate, and is thereby recycled to NAD+.
Explain with words, diagrams, or structures why lactate accumulates in the blood during bursts of very vigorous exercise (such as a 100-meter dash).
uring vigorous exercise, the cardiovascular system cannot deliver O2 to the muscle tissue fast enough to maintain aerobic conditions.
As glycolysis proceeds under anaerobic conditions, NAD+ is converted to NADH (during the glyceraldehyde 3-phosphate dehydrogenase reaction), but the muscle tissue has no O2 to which NADH can pass electrons.
To recycle NADH to NAD+, which is essential for continuing glycolysis, electrons from NADH are used to reduce pyruvate to lactate.
Describe the fate of pyruvate, formed by glycolysis in animal skeletal muscle, under two conditions: (a) at rest, and (b) during an all-out sprint. Show enough detail in your answer to explain why pyruvate metabolism is different in these two cases.
At rest, plenty of O2 is being delivered to the muscle, and pyruvate formed during glycolysis is oxidized to acetyl-CoA by the pyruvate dehydrogenase complex. Acetyl groups then enter the citric acid cycle and are oxidized to CO2 (b) Under the conditions of all-out exertion, skeletal muscle cannot be supplied with enough O2 to keep metabolism completely aerobic; under these conditions, muscle tissue must function anaerobically. Pyruvate is reduced to lactate to recycle NADH, formed by glycolysis, to NAD+, so that glycolysis can continue.
During strenuous activity, muscle tissue demands large quantities of ATP, compared with resting muscle. In white skeletal muscle (in contrast with red muscle), ATP is produced almost exclusively by fermentation of glucose to lactate. If a person had white muscle tissue devoid of the enzyme lactate dehydrogenase, how would this affect his or her metabolism at rest and during strenuous exercise?
Lactate dehydrogenase allows cells to pass electrons from NADH to pyruvate, thus regenerating NAD+ for continued glycolysis under anaerobic conditions. The lack of this enzyme would cause no significant problems at rest because aerobic red muscle tissue would function well. During strenuous exercise, however, the absence of lactate dehydrogenase would severely reduce the ability of muscle to perform anaerobically.
Describe the part of the glycolytic pathway from fructose 6-phosphate to glyceraldehyde 3-phosphate. Show structures of intermediates, enzyme names, and indicate where any cofactors participate.
: This part of the pathway involves the reactions catalyzed by phosphofructokinase-1, aldolase, and triose phosphate isomerase. (See the figures from pp. 533-534.)
Describe the glycolytic pathway from fructose 1,6-bisphosphate to 1,3-bisphospho-glycerate, showing structures of intermediates and names of enzymes. Indicate where any cofactors participate.
The answer should show the reactions catalyzed by aldolase, triose phosphate isomerase, and glyceraldehyde 3-phosphate dehydrogenase. (See figures from pp. 533-536.)
Explain why Pi (inorganic phosphate) is absolutely required for glycolysis to proceed.
Inorganic phosphate (Pi) is an essential substrate in the reaction catalyzed by glyceraldehyde 3- phosphate dehydrogenase.
If brewer’s yeast is mixed with pure sugar (glucose) in the absence of phosphate (Pi), no ethanol is
produced. With the addition of a little Pi, ethanol production soon begins. Explain this observation in 25 words or less.
The reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase requires Pi as a substrate. Without Pi, glycolysis ceases, and no ethanol is produced.
Two reactions in glycolysis produce ATP. For each of these, show the name and structure of reactant and product, indicate which cofactors participate and where, and name the enzymes.
The two reactions are those catalyzed by phosphoglycerate kinase and pyruvate kinase. (See reactions on pp. 537 [top] and 539 [bottom].)
Explain why the phosphorolysis of glycogen is more efficient than the hydrolysis of glycogen in mobilizing glucose for the glycolytic pathway.
Phosphorolysis yields glucose 1-phosphate, which can be converted into glucose 6-phosphate without the investment of energy from ATP. Hydrolysis of glycogen yields free glucose, which must be converted into glucose 6-phosphate (at the expense of ATP) before it can enter glycolysis.
Describe the process of glycogen breakdown in muscle. Include a description of the structure of glycogen, the nature of the breakdown reaction and the breakdown product, and the required enzyme(s).
Muscle glycogen consists of linear polymers of α(1 → 4)-linked D-glucose, with many branches formed by α(1 → 6) glycosidic linkages to D-glucose.
Glycogen phosphorylase in muscle catalyzes phosphorolytic cleavage of the terminal residue at the nonreducing ends, producing glucose 1-phosphate.
When phosphorylase approaches α(1 → 6) branch points, a second enzyme (the “debranching enzyme”) removes the four glucose residues nearest the branch point and reattaches them in α(1 → 4) linkage at a nonreducing end.
Now phosphorylase can continue to degrade the molecule.
Explain the biochemical basis of the human metabolic disorder called lactose intolerance.
In lactose intolerance, the enzyme lactase, found in the surface of intestinal epithelial cells in children, has been lost in adulthood. Without this enzyme, the individual cannot hydrolyze lactose in the small intestine and take up the resulting monosaccharides; instead, lactose passes into the large intestine, where it is metabolized by bacteria, producing gastric distress.
