Gross versus net. The gross yield of ATP from the metabolism of glucose to two molecules of pyruvate is four molecules of ATP. However, the net yield is only two molecules of ATP. Why are the gross and net values different?

Go together like the owl and the pussycat. Match each term with its description.

Who takes? Who gives? Lactic acid fermentation and alcoholic fermentation are oxidation–reduction reactions. Identify the ultimate electron donor and electron acceptor.

ATP yield. Each of the following molecules is processed by glycolysis to lactate. How much ATP is generated from each molecule?

Enzyme redundancy? Why is it advantageous for the liver to have both hexokinase and glucokinase to phosphorylate glucose?

Magic? The interconverison of DHAP and GAP greatly favors the formation of DHAP at equilibirum. Yet the conversion of DHAP by triose phosphate isomerase proceeds readily. Why?

Between two extremes. What is the role of a thioester in the formation of ATP in glycolysis?

Corporate sponsors. Some of the early research on glycolysis was supported by the brewing industry. Why would the brewing industry be interested in glycolysis?

Recommended daily allowance. The recommended daily allowance for the vitamin niacin is 15 mg per day. How would glycolysis be affected by niacin deficiency?

Who’s on first? Although both hexokinase and phosphofructokinase catalyze irreversible steps in glycolysis and the hexokinase-catalyzed step is first, phosphofructokinase is nonetheless the pacemaker of glycolysis. What does this information tell you about the fate of the glucose 6-phosphate formed by hexokinase?

The tortoise and the hare. Why is the regulation of phosphofructokinase by energy charge not as important in the liver as it is in muscle?

Like Minneapolis and St. Paul. Match each term with its description.

Running in reverse. Why can’t the reactions of the glycolytic pathway simply be run in reverse to synthesize glucose?

Road blocks. What reactions of glycolysis are not readily reversible under intracellular conditions?

No pickling. Why is it in the muscle’s best interest to export lactic acid into the blood during intense exercise?

Après vous. Why is it physiologically advantageous for the pancreas to use GLUT2, with a high K_M, as the transporter that allows glucose entry into β cells?

Bypass. In the liver, fructose can be converted into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate without passing through the phosphofructokinase-regulated reaction. Show the reactions that make this conversion possible. Why might ingesting high levels of fructose have deleterious physiological effects?

Match ’em 1. The following sequence is a part of the sequence of reactions in gluconeogenesis.

Match the capital letters representing the reaction in the gluconeogenic pathway with parts a, b, c, etc.

Like Batman and Robin. Match parts a through j with parts 1 through 10.

Trouble ahead. Suppose that a microorganism that was an obligate anaerobe suffered a mutation that resulted in the loss of triose phosphate isomerase activity. How would this loss affect the ATP yield of fermentation? Could such an organism survive?

Kitchen chemistry. Sucrose is commonly used to preserve fruits. Why is glucose not suitable for preserving foods?

Tracing carbon atoms 1. Glucose labeled with ¹⁴C at C-1 is incubated with the glycolytic enzymes and necessary cofactors.

Lactic acid fermentation.

High potential. What is the equilibrium ratio of phosphoenolpyruvate to pyruvate under standard conditions when [ATP]/[ADP] = 10?

Hexose–triose equilibrium. What are the equilibrium concentrations of fructose 1,6-bisphosphate, dihydroxyacetone phosphate, and glyceraldehyde 3-phosphate when 1 mM fructose 1,6-bisphosphate is incubated with aldolase under standard conditions?

Double labeling. 3-Phosphoglycerate labeled uniformly with¹⁴C is incubated with 1,3-BPG labeled with³² P at C-1. What is the radioisotope distribution of the 2,3-BPG that is formed on addition of BPG mutase?

An informative analog. Xylose has the same structure as that of glucose except that it has a hydrogen atom at G-5 in place of a hydroxymethyl group. The rate of ATP hydrolysis by hexokinase is markedly enhanced by the addition of xylose. Why?

Distinctive sugars. The intravenous infusion of fructose into healthy volunteers leads to a two- to fivefold increase in the level of lactate in the blood, a far greater increase than that observed after the infusion of the same amount of glucose.

It is not hard to meet expenses. They are everywhere. What energetic barrier prevents glycolysis from simply running in reverse to synthesis glucose? What is the energetic cost to overcome this barrier?

Match ’em 2. Indicate which of the conditions listed in the right-hand column increase the activity of the glycolytic and gluconeogenic pathways.

Waste not, want not. Why is the conversion of lactic acid from the blood into glucose in the liver in an organism’s best interest?

Road blocks bypassed. How are the irreversible reactions of glycolysis bypassed in gluconeogenesis?

Pointlessness averted. What are the regulatory means that prevent high levels of activity in glycolysis and gluconeogenesis simultaneously?

Different needs. Liver is primarily a gluconeogenic tissue, whereas muscle is primarily glycolytic. Why does this division of labor make good physiological sense?

Metabolic mutants. What would be the effect on an organism’s ability to use glucose as an energy source if a mutation inactivated glucose 6-phosphatase in the liver?

Never let me go. Why does the lack of glucose 6-phosphatase activity in the brain and muscle make good physiological sense?

Counting high-energy compounds 1. How many NTP molecules are required for the synthesis of one molecule of glucose from two molecules of pyruvate? How many NADH molecules?

Counting high-energy compounds 2. How many NTP molecules are required to synthesize glucose from each of the following compounds?

Lending a hand. How might enzymes that remove amino groups from alanine and aspartate contribute to gluconeogenesis?

More metabolic mutants. Predict the effect of each of the following mutations on the pace of glycolysis in liver cells:

Yet another metabolic mutant. What are the likely consequences of a genetic disorder rendering fructose 1,6-bisphosphatase in the liver less sensitive to regulation by fructose 2,6-bisphosphate?

Biotin snatcher. Avidin, a 70-kDa protein in egg white, has very high affinity for biotin. In fact, it is a highly specific inhibitor of biotin enzymes. Which of the following conversions would be blocked by the addition of avidin to a cell homogenate?

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Tracing carbon atoms 2. If cells synthesizing glucose from lactate are exposed to CO₂ labeled with ¹⁴C, what will be the distribution of label in the newly synthesized glucose?

Arsenate poisoning. Arsenate. closely resembles P_i in structure and reactivity. In the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase, arsenate can replace phosphate in attacking the energy-rich thioester intermediate. The product of this reaction, 1-arseno-3-phosphoglycerate, is unstable. It and other acyl arsenates are rapidly and spontaneously hydrolyzed. What is the effect of arsenate on energy generation in a cell?

Reduce, reuse, recycle. In the conversion of glucose into two molecules of lactate, the NADH generated earlier in the pathway is oxidized to NAD⁺. Why is it not to the cell’s advantage to simply make more NAD⁺ so that the regeneration would not be necessary? After all, the cell would save much energy because it would no longer need to synthesize lactic acid dehydrogenase.

Adenylate kinase again. Adenylate kinase, an enzyme considered in great detail in Chapter 9, is responsible for interconverting the adenylate nucleotide pool:

The equilibrium constant for this reaction is close to 1, inasmuch as the number of phosphoanhydride bonds is the same on each side of the equation. Using the equation for the equilibrium constant for this reaction, show why changes in [AMP] are a more effective indicator of the adenylate pool than [ATP].

Working at cross-purposes? Gluconeogenesis takes place during intense exercise, which seems counterintuitive. Why would an organism synthesize glucose and at the same time use glucose to generate energy?

Powering pathways. Compare the stoichiometries of glycolysis and gluconeogenesis. Recall that the input of one ATP equivalent changes the equilibrium constant of a reaction by a factor of about 10⁸ (Section 15.2). By what factor do the additional high-phosphoryl-transfer compounds alter the equilibrium constant of gluconeogenesis?

Argument by analogy. Propose a mechanism for the conversion of glucose 6-phosphate into fructose 6-phosphate by phosphoglucose isomerase based on the mechanism of triose phosphate isomerase.

Not just for energy. People with galactosemia display central nervous system abnormalities even if galactose is eliminated from the diet. The precise reason for it is not known. Suggest a plausible explanation.

Power to the rbc! Hexokinase in red blood cells has a K_M of approximately 50 μM. Because life is hard enough as it is, let‘s assume that the hexokinase displays Michaelis–Menten kinetics. What concentration of blood glucose would yield v_o equal to 90% V_max? What does this result tell you if normal blood glucose levels range between approximately 3.6 and 6.1 mM?

State function. Fructose 2,6-bisphosphate is a potent stimulator of phosphofructokinase. Explain how fructose 2,6-bisphosphate might function in the concerted model for allosteric enzymes.

Now, that’s unusual. Phosphofructokinase has recently been isolated from the hyperthermophilic archaeon Pyrococcus furiosus. It was subjected to standard biochemical analysis to determine basic catalytic parameters. The processes under study were of the form

The assay measured the increase in fructose 1,6-bisphosphate. Selected results are shown in the adjoining graph.

Cool bees. In principle, a futile cycle that includes phos-phofructokinase and fructose 2,6-bisphosphatase could be used to generate heat. The heat could be used to warm tissues. For instance, certain bumblebees have been reported to use such a futile cycle to warm their flight muscles on cool mornings. Scientists undertook a series of experiments to determine if a number of species of bumblebee use this futile cycle. Their approach was to measure the activity of PFK and F-1,6-BPase in flight muscle.

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Confused? The adjoining graph shows muscle phosphofructokinase activity as a function of ATP concentration in the presence of a constant concentration of fructose 6-phosphate. Explain these results and discuss how they relate to the role of phosphofructokinase in glycolysis.