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medical sciences
biochemistry

Questions and Answers of Biochemistry

NADH is an important coenzyme in catabolic processes, whereas NADPH appears in anabolic processes. Explain how an exchange of the two can be effected.
How many enzymes are involved in mammalian pyruvate dehydrogenase? What are their functions?
Recall Briefly summarize the steps in the electron transport chain from NADH to oxygen.
Calculate E°' for the following reaction:NADH + H+ + Pyruvate ( NAD+ + Lactate
Calculate E°' for the following reaction:Succinate + 1/2 O2 ( Fumarate + H2O
For the following reaction, identify the electron donor and the electron acceptor and calculate E°'.FAD + 2 Cyt c (Fe2+) + 2H+ ( FADH2 + 2 Cyt c (Fe3+)
Which is more favorable energetically, the oxidation of succinate to fumarate by NAD+ or by FAD? Give the reason for your answer.
Comment on the fact that the reduction of pyruvate to lactate, catalyzed by lactate dehydrogenase, is strongly exergonic (recall this from Chapter 15), even though the standard free-energy change for
Recall Are electron transport and oxidative phosphorylation the same process? Why or why not?
Two biochemistry students are about to use mitochondria isolated from rat liver for an experiment on oxidative phosphorylation. The directions for the experiment specify addition of purified
Reflect and apply What are two advantages of the components of the electron transport chain being embedded in the inner mitochondrial membrane?
Reflect on the evolutionary implications of the structural similarities and functional differences of cytochromes on the one hand and hemoglobin and myoglobin on the other.
Reflect and apply Experimental evidence strongly suggests that the protein portions of cytochromes have evolved more slowly (as judged by the number of changes in amino acids per million years) than
Reflect and apply List the reactions of electron transport that liberate enough energy to drive the phosphorylation of ADP.
Define P/O ratio and indicate why it is important.
Why is it difficult to determine an exact number for P/O ratios?
Reflect and apply What are some of the difficulties in determining the exact number of protons pumped across the inner mitochondrial membrane by the respiratory complexes?
Why is it difficult to determine an exact number for P/O ratios? Discuss.
Reflect and apply Show how the reactions of the electron trans port chain differ from those in Question 3 when FADH2 is the start ing point for electron transport. Show how the reactions that liber
Can an uncoupler of oxidative phosphorylation inhibit electron transport from one component of the electron transport chain to another? Why or why not?
How does the yield of ATP from complete oxidation of one molecule of glucose in muscle and brain differ from that in liver, heart, and kidney? What is the underlying reason for this difference?
What yield of ATP can be expected from complete oxidation of each of the following substrates by the reactions of glycolysis, the citric acid cycle, and oxidative phosphorylation? (a)
Reflect and apply How does mitochondrial structure contribute to aerobic metabolism, particularly to the integration of the citric acid cycle and electron transport?
The free-energy change (∆G°') for the oxidation of the cytochrome aa3 complex by molecular oxygen is – 102.3 kJ = – 224.5 kcal for each mole of electron pairs transferred. What is the maximum
Calculate E°' for the following reaction:NADH + H+ + 1/2 O2 ( NAD+ + H2O
(a) The major energy storage compound of animals is fats (except in muscles). Why would this be advantageous? (b) Why don’t plants use fats/oils as their major energy storage compound?
Calculate the ATP yield for the complete oxidation of one molecule of palmitic acid (16 carbons). How does this figure differ from that obtained for stearic acid (18 carbons)? Consider the
Describe briefly how b-oxidation of an odd-chain fatty acid is different from that for an even-chain fatty acid.
Calculate the net ATP yield from the complete processing of a saturated fatty acid containing 17 carbons. Consider the b-oxidation steps, processing of acetyl-CoA through the citric acid cycle, and
Calculate the net ATP yield from linoleic acid (18:2 D9,12). For this calculation, assume that the loss of an NADPH is the same as the loss of an NADH.
It has been stated many times that fatty acids cannot yield a net gain in carbohydrates. Why can odd-chain fatty acids be thought to break this rule to a small extent?
RECALL Under what conditions are ketone bodies produced?
A friend who is trying to lose weight complains about the odd taste in his mouth in the mornings. He says it seems like a filling has broken loose, and the metallic sensation is bothersome. What
Compare and contrast the pathways of fatty-acid breakdown and biosynthesis. What features do these two pathways have in common? How do they differ?
Outline the steps involved in the production of malonyl-CoA from acetyl-CoA.
How are the two redox reactions of b-oxidation different from their counterparts in fatty-acid synthesis?
In fatty-acid synthesis, malonyl-CoA, rather than acetyl-CoA, is used as a “condensing group.” Suggest a reason for this.
(a) Where in an earlier chapter have we encountered something comparable to the action of the acyl carrier protein (ACP) of fatty-acid synthesis? (b) What is a critical feature of the action of the
How are isoprene units important in cholesterol biosynthesis and other biochemical pathways?
Draw a six-carbon saturated fatty acid and show where the double bond is created during the first step of b-oxidation. What is the orientation of this bond?
What are the two places where light energy is required in the light reaction of photosynthesis? Why must energy be supplied at precisely these points?
Describe some similarities between the electron transport chains in chloroplasts and in mitochondria.
Uncouplers of oxidative phosphorylation in mitochondria also uncouple photoelectron transport and ATP synthesis in chloroplasts. Give an explanation for this observation.
A larger proton gradient is required to form a single ATP in chloroplasts than in mitochondria. Suggest a reason why. Hint: Ions can move across the thylakoid mem-brane more easily than across the
Albert Szent-Gyorgi, a pioneer in early photosynthesis research, stated, “What drives life is a little electric current, kept up by the sunshine.” What did he mean by this?
Antimycin A (an inhibitor of electron transport) is an inhibitor of photosynthesis in chloroplasts. Why is this likely to be so?
Would you expect H2O or CO2 to be the source of the oxygen produced in photosynthesis? Give the reason for your answer.
Why do we describe the path of electrons in photosynthesis as starting in photosystem II and ending in photosystem I? In other words, why is the nomenclature “backward”?
The oxidation of water requires four electrons, but chlorophyll molecules can transfer only one electron at a time. Describe how these two statements can be reconciled.
Is water the only possible electron donor in photosynthesis? Why or why not?
Using information from Sections 15-3 and 15-6, show how the ∆G°' of 478 kJ (114 kcal) is obtained for each mole of CO2 fixed in photosynthesis. The reaction in question is 6CO2 + 6H2O ( Glucose +
Rubisco has a very low turnover number, about 3CO2 per second. What might this low number tell us about the evolution of rubisco?
Biochemical connections What key aspects of chloroplasts (and mitochondria) are consistent with the theory that they may have once been bacteria? List three specific features.
Reflect and apply Suggest a reason why the evolution of the pathway for the regeneration of ribulose-1,5-bisphosphate from glyceraldehyde-3-phosphate was “no big deal.”
How does photosynthesis in C4 plants differ from the process in C3 plants?
Why is it advantageous to tropical plants to use the C4 rather than the C3 fixation pathway?
List three ways in which the structure of chlorophyll differs from that of heme.
What would be the effect on plants if photorespiration did not exist?
Outline the events that take place at the photosynthetic reaction center in Rhodopseudomonas.
Draw a transamination reaction between a-ketoglutarate and alanine.
Diagram the reactions involving glutamate dehydrogenase and glutamine synthetase that produce glutamine from ammonia and a-ketoglutarate.
What is the difference between glutamine synthetase and glutaminase?
How is nitrogen fixed (converted from N2 to NH4+)? How is it subsequently assimilated into organic compounds?
Write an equation for the net reaction of the urea cycle. Show how the urea cycle is linked to the citric acid cycle.
Which amino acids in the urea cycle are the links to the citric acid cycle? Show how these links occur
Will an amino acid be glucogenic or ketogenic if it is catabolized to the following molecules? (a) Phosphoenolpyruvate (b) a-Ketoglutarate (c) Succinyl-CoA (d) Acetyl-CoA (e) Oxaloacetate (f)
Would you expect an ostrich to excrete excess nitrogen as uric acid, urea, or ammonia? Make an argument for your answer.
Argue logically that the urea cycle should not have evolved. Then, logically counter your argument.
Biochemical coNNectioNs How is the importance of folic acid related to chemotherapy?
Write the overall reaction for the fixation of nitrogen via the nitrogenase complex.
What are the sources of the carbons and nitrogens in the purine bases?
Does the conversion of IMP to GMP use or produce ATP either directly or indirectly? Justify your answer.
Discuss the role of feedback inhibition in the anabolism of purine-containing nucleotides.
Compare the fates of the products of purine and pyrimidine catabolism.
What roles do thioredoxin and thioredoxin reductase play in the metabolism of nucleotides?
Describe the nitrogenase complex. How is the enzyme organized? What are its unique components?
What is the difference between the old food pyramid and the new version?
What do we mean when we say that there is no storage form for protein? How is this different from fats and carbohydrates?
People who are both alcoholic and exposed to halogen compounds often die of liver failure. Why might this be a logical ultimate result?
A cat named Lucullus is so spoiled that he will eat nothing but freshly opened canned tuna. Another cat, Griselda, is given only dry cat food by her far less indulgent owner. Canned tuna is
Why are amino acids such as arginine and histidine required in relatively large amounts by children but in smaller amounts by adults? The adult human is not able to make these amino acids.
During colonial times, irondeficiency anemia was almost unknown in America. Why?
People on high-fiber diets often have less cancer (especially of the colon) and lower blood-cholesterol levels than people on low-fiber diets, even though fiber is not digestible. Suggest reasons for
Most calcium supplements have calcium carbonate as the main ingredient. Other supplements that have calcium citrate as the main ingredient are advertised as being more easily absorbed. Do you
Alcoholics tend to be malnourished, with thiamine deficiency being a particularly severe problem. Suggest a reason why this is so.
An athletic friend is preparing to run a marathon and intends to load glycogen before the race. Someone told your friend that the way to load more glycogen is by exercising excessively for two days
Many components of the glycolytic pathway and the citric acid cycle are direct exit or entry points to metabolic pathways of other substances. Indicate another pathway available to the following
How is hormone production affected by damage to the pituitary gland? To the adrenal cortex?
What is the difference between a G protein and a receptor tyrosine kinase? Give an example of a hormone that uses each.
What stops a hormone response when a G protein is involved?
Briefly describe the series of events that takes place when cAMP acts as a second messenger.
Is it likely that any metabolic pathway can exist without control mechanisms?
The epinephrine-mediated “amplification cascade” of Figure 24.14 has six steps, all of which are catalytic with one exception. This cascade leads to the activation of glycogen phosphorylase. This
Explain what insulin and low-carbohydrate diets have to do with one another.
The functioning of a particular pathway often depends not only on control enzymes in that pathway but also on control enzymes of other pathways. What happens in the following pathways under the
What is insulin’s effect on the following?(a) Glycogen breakdown(b) Glycogen synthesis(c) Glycolysis(d) Fatty-acid synthesis(e) Fatty-acid storage
How is it possible that both insulin and epinephrine stimulate muscle glycolysis?
Why would a runner who has a 5-km race to run at 9 am be concerned about insulin?
Why do some people call GLUT4 the training glucose transporter?
How are insulin, GLUT4, obesity, and type II diabetes related?

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