Deck 27: Energy Metabolism: Integration and Organ Specialization

Question 1

The energetics of opposing metabolic sequences
(Integrates with Chapters 3, 18, and 22.) The conversion of PEP to pyruvate by pyruvate kinase (glycolysis) and the reverse reaction to form PEP from pyruvate by pyruvate carboxylase and PEP carboxykinase (gluconeogenesis) represents a so -called substrate cycle. The direction of net conversion is determined by the relative concentrations of allosteric regulators that exert kinetic control over pyruvate kinase, pyruvate carboxylase, and PEP carboxykinase. Recall that the last step in glycolysis is catalyzed by pyruvate kinase:
PEP + ADP pyruvate + ATP
The standard free energy change is -31.7 kJ/mol.
a. Calculate the equilibrium constant for this reaction.
b. If [ATP] = [ADP], by what factor must [pyruvate] exceed [PEP] for this reaction to proceed in the reverse direction
The reversal of this reaction in eukaryotic cells is essential to gluconeogenesis and proceeds in two steps, each requiring an equivalent of nucleoside triphosphate energy:
Pyruvate carboxylase
Pyruvate + CO₂ + ATP oxaloacetate + ADP + Pi
PEP carboxykinase
Oxaloacetate + GTP PEP + CO₂ + GDP
Net: Pyruvate + ATP + GTP PEP + ADP + GDP + Pi
c. The Gº' for the overall reaction is +0.8 kJ/mol. What is the value of Keq
d. Assuming [ATP] = [ADP], [GTP] = [GDP], and Pi = 1 mM when this reaction reaches equilibrium, what is the ration of [PEP]/[pyruvate]
e. Are both directions in the substrate cycle likely to be strongly favored under physiological conditions

Answer 1

a. We can determine the equilibrium constant as follows
b. The reaction will proceed in the reverse direction when G 0. The natural log term must be greater than 1.
We are told that [ATP] = [ADP]. Using this information and the value obtained in part a gives:
c. Using the formula determined in part a, K eq can be calculated.
d. Given that [ATP] = [ADP], [GTP] = [GDP], and [P i ] = 1 m M , we can figure out the ratio of [PEP] to [pyruvate].
e. Using the knowledge we gained from part b and part d of this question, the reaction will be favorable in both directions under the following conditions.

Question 2

Calculating energy charge and phosphorylation potential
(Integrates with Chapter 3.) Assume the following intracellular concentrations in muscle tissue: ATP = 8 mM, ADP = 0.9 mM, AMP = 0.04 mM, Pi = 8 mM. What is the energy charge in muscle What is the phosphorylation potential

Answer 2

We can determine energy charge with the following formula.
We can determine the phosphorylation potential as follows:

Question 3

How long do ATP and phosphocreatine supplies last during muscle contraction
Strenuous muscle exertion (as in the 100-meter dash) rapidly depletes ATP levels. How long will 8 mM ATP last if 1 gram of muscle consumes 300 µmol of ATP per minute (Assume muscle is 70% water). Muscle contains phosphocreatine as a reserve of phosphorylation potential. Assuming [phosphocreatine] = 40 mM, [creatine] = 4 mM, and G°' (phosphocreatine + H₂ O * creatine + Pi) = - 43.3 kJ/mol, how low must [ATP] become before it can be replenished by the reaction phosphocreatine + ADP * ATP + creatine. [Remember, G°' (ATP hydrolysis) = -30.5 kJ/mol.]

Answer 3

Muscle is 70% water. This means that for every gram of muscle, there is 0.7 g of water. Knowing the density of water, this is equal to 0.7 mL. [ATP] = 8 m M. We can determine the number of moles of ATP.
Assuming 1 gram of muscle consumes 300 M of ATP per minute, we can determine how long the ATP will last.
Phosphocreatine and ATP react as follows with a G °' = -12.8 kJ/mol.
For this reaction to be favorable, G must be less than 0.
Rearranging the equation and solving for [ATP] gives:
The reaction will proceed in the forward direction and ADP will be phosphorylated when the ratio of [ATP] to [ADP] is less than 1,753 if, [Cr-P] = 40 m M , and [Cr] = 4 m M.

Question 4

What is the value of NADPH in ATP equivalents
(Integrates with Chapter 20.) The standard reduction potentials for the (NAD+/NADH) and the (NADP+/NADPH) couples are identical, namely -320 mV. Assuming the in vivo concentration ratios NAD+/NADH = 20 and NADP+/NADPH = 0.1, what is G for the following reaction
NADPH + NAD+ NADP+ + NADH
Calculate how many ATP equivalents can be formed from ADP + Pi by the energy released in this reaction.

Question 5

Adenylate kinase is an amplifier
(Integrates with Chapter 3.) Assume the total intracellular pool of adenylates (ATP + ADP + AMP) = 8 mM, 90% of which is ATP. What are [ADP] and [AMP] if the adenylate kinase reaction is at equilibrium Suppose [ATP] drops suddenly by 10%. What are the concentrations now for ADP and AMP, assuming adenylate kinase reaction is at equilibrium By what factor has the AMP concentration changed

Question 6

Regulating the flux through metabolic pathways
(Integrates with Chapters 18 and 22.) The reactions catalyzed by PFK and FBPase constitute another substrate cycle. PFK is AMP-activated; FBPase is AMP-inhibited. In muscle, the maximal activity of PFK (µmoles of substrate transformed per minute) is ten times greater than FBPase activity. If the increase in [AMP] described in Problem 5 raised PFK activity from 10% to 90% of its maximal value but lowered FBPase activity from 90% to 10% of its maximal value, by what factor is the flux of fructose-6-P through the glycolytic pathway changed (Hint: Let PFK maximal activity = 10, FBPase maximal activity = 1; calculate the relative activities of the two enzymes at low [AMP] and at high [AMP]; let J, the flux of F-6-P through the substrate cycle under any condition, equal the velocity of PFK reaction minus the velocity of the FBPase reaction.)

Question 7

The effects of leptin on fat metabolism
(Integrates with Chapters 23 and 24.) Leptin not only induces synthesis of fatty acid oxidation enzymes and uncoupling protein-2 in adipocytes, but it also causes inhibition of acetyl-CoA carboxylase, resulting in a decline in fatty acid biosynthesis. This effect on acetyl-CoA carboxylase, as an additional consequence, enhances fatty acid oxidation. Explain how leptin-induced inhibition of acetyl-CoA carboxylase might promote fatty acid oxidation.

Question 8

Ethanol as a source of metabolic energy
(Integrates with Chapters 19 and 20.) Acetate produced in ethanol metabolism can be transformed into acetyl-CoA by the acetyl thiokinase reaction:
Acetate + ATP + CoASH acetyl-CoA + AMP + PPi
Acetyl-CoA then can enter the citric acid cycle and undergo oxidation tO₂ CO₂. How many ATP equivalents can be generated in a liver cell from the oxidation of one molecule of ethanol tO₂ CO₂ by this route, assuming oxidative phosphorylation is part of the process (Assume all reactions prior to acetyl-CoA entering the citric acid cycle occur outside the mitochondrion.) Per carbon atom, which is a better metabolic fuel, ethanol or glucose That is, how many ATP equivalents per carbon atom are generated by combustion of glucose versus ethanol to CO₂

Question 9

ATP coupling coefficients and the thermodynamics of opposing metabolic sequences I
(Integrates with Chapter 19.) Assuming each NADH is wortH₃ ATP, each FADH₂ is wortH₂ ATP, and each NADPH is worth 4 ATP: How many ATP equivalents are produced when one molecule of palmitoyl-CoA is oxidized to 8 molecules of acetyl-CoA by the fatty acid -oxidation pathway How many ATP equivalents are consumed when 8 molecules of acetyl-CoA are transformed into one molecule of palmitoyl-CoA by the fatty acid biosynthetic pathway Can both of these metabolic sequences be metabolically favorable at the same time if G for ATP synthesis is +50 kJ/mol

Question 10

ATP coupling coefficients and the thermodynamics of opposing metabolic sequences II
(Integrates with Chapters18-21.) If each NADH is wortH₃ ATP, each FADH₂ is wortH₂ ATP, and each NADPH is worth 4 ATP, calculate the equilibrium constant for cellular respiration, assuming synthesis of each ATP costs 50 kJ/mol of energy. Calculate the equilibrium constant for CO₂ fixation under the same conditions, except here ATP will hydrolyze to ADP + Pi with the release of 50 kJ/mol. Comment on whether these reactions are thermodynamically favorable under such conditions.

Question 11

Metabolic inhibitors as a diabetes treatment strategy
(Integrates with Chapters 22.) In type 2 diabetes, glucose production in the liver is not appropriately regulated, so glucose is over produced. One strategy to treat this disease focuses on the development of drugs targeted against regulated steps in glycogenolysis and gluconeogenesis, the pathways by which liver produces glucose for release into the blood. Which enzymes would you select for as potential targets for such drugs

Question 12

Drug targets to regulate eating behavior
As chief scientist for drug development at PhatFarmaceuticals, Inc., you want to create a series of new diet drugs. You have a grand plan to design drugs that might limit production of some hormones or promote the production of others. Which hormones are on your "limit production" list and which are on your "raise levels" list

Question 13

Leptin injections as an obesity therapy
The existence of leptin was revealed when the ob/ob genetically obese strain of mice was discovered. These mice have a defect leptin gene. Predict the effects of daily leptin injections on ob/ob mice on food intake, fatty acid oxidation, and body weight. Similar clinical trials have been conducted on humans, with limited success. Suggest a reason why this therapy might not be a miracle cure for overweight individuals.

Question 14

Hormones that regulate eating behavior
Would it be appropriate to call neuropeptide Y (NPY) the obesity-promoting hormone What would be the phenotype of a mouse whose melanocortin-producing neurons failed to produce melanocortin What would be the phenotype of a mouse lacking a functional MC3R gene What would be the phenotype of a mouse lacking a functional leptin receptor gene

Question 15

Consequences of alcohol consumption on the NAD+/NAHD ratio
The Human Biochemistry box The Metabolic Effects of Alcohol Consumption, points out that ethanol is metabolized to acetate in the liver by alcohol dehydrogenase and aldehyde dehydrogenase:
CH₃ CH₂ OH + NAD + CH₃ CHO + NADH + H +
CH₃ CHO + NAD + CH₃ COO- + NADH + 2 H +
These reactions alter the NAD + /NADH ratio in liver cells. From your knowledge of glycolysis, gluconeogenesis, and fatty acid oxidation, what might be the effect of an altered NAD + /NADH ratio on these pathways What is the basis of this effect

Question 16

The phenotype of a T172D mutation in AMPK
A T172D mutant of the AMPK is locked in a permanently active state. Explain.

Question 17

Malonyl-CoA as a key indicator of nutrient availability
a. Some scientists support the "malonyl-CoA hypothesis," which suggests that malonyl-CoA is a key indicator of nutrient availability and the b in uses its abundance to assess whole-body energy homeostasis. Others have pointed out that malonyl-CoA is a significant inhibitor of carnitine acylt nsfe se-1 (see Figure 24.16). Thus, malonyl-CoA may be influencing the levels of another metabolite whose concent tion is more important as a signal of energy status. What metabolite might that be
b. Another test of the malonyl-CoA hypothesis was conducted through the creation of a transgenic strain of mice that lacked functional hypothalamic fatty acid synthase (see Chapter 24). Predict the effect of this genetic modification on cellular malonyl-CoA levels in the hypothalamus, the eating behavior of these transgenic mice, their body fat content, and their physical activity levels. Defend your predictions.

Question 18

Exploring leptin's mechanism of action
a. Leptin was discovered when a congenitally obese strain of mice (ob/ob mice) was found to lack both copies of a gene encoding a peptide hormone produced mainly by adipose tissue. The peptide hormone was named leptin. Leptin is an anorexic (appetite suppressing) agent; its absence leads to obesity. Propose an experiment to test these ideas.
b. A second strain of obese mice (db/db mice) produces leptin in abundance but fails to respond to it. Assuming the db mutation leads to loss of function in a protein, what protein is likely to be nonfunctional or absent How might you test your idea

Question 19

Consult Figure 27.7 and answer the following questions: Which organs use both fatty acids and glucose as a fuel in the well-fed state, which rely mostly on glucose, which rely mostly on fatty acids, which one never uses fatty acids, and which one produces lactate.

Question 20

Figure 27.3 illustrates the response of R (ATP-regenerating) and U (ATP-utilizing) enzymes to energy charge.
a. Would hexokinase be an R enzyme or a U enzyme Would glutamine:PRPP amidotransferase, the second enzyme in purine biosynthesis, be an R enzyme or a U enzyme
b. If energy charge = 0.5: Is the activity of hexokinase high or low Is ribose-5-P pyrophosphokinase activity high or low
c. If energy charge = 0.95: Is the activity of hexokinase high or low Is ribose-5-P pyrophosphokinase activity high or low