Ch. 12 Carbohydrate Metabolism: Glycolysis, Gluconeogenesis, Glycogen Metabolism and the Pentose Phosphate Pathway

How many NADH are produced by glycolysis?

2

In glycolysis, ATP molecules are produced by _____.

substrate-level phosphorylation

Which of these is NOT a product of glycolysis?

FADH₂

In glycolysis, what starts the process of glucose oxidation?

ATP

In glycolysis there is a net gain of _____ ATP.

2

One or more of the following molecules are substrates or products in the glycolytic pathway. Identify them.

glucose

pyruvate

ATP

NADH

Among the products of glycolysis, which compounds contain energy that can be used by other biological reactions?

pyruvate, ATP, and NADH

Keto-enol tautomerization is an important mechanism in glycolysis. What the mechanism for keto-enol conversion involve?

[protonation] of a keto group first, followed by movement of electrons from a carbon-oxygen double bond to the oxygen atom to form the hydroxyl group

How many molecules of ATP are used in the reaction, and how many are directly made?

Two are used to phosphorylate the glucose, [and four] are directly made from 1,3-bisphosphoglycerate and phosphoenolpyruvate.

In the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate (Gly-6), NADH is made and phosphate is attached. What drives this reaction?

[that 1,3-bisphosphoglycerate] is much lower in energy (G°′) than glyceraldehyde-3-phosphate

Which of the following is true of Gly-10 (pyruvate kinase), which converts phosphoenol pyruvate to pyruvate?

It is [an example] of substrate-level phosphorylation, producing ATP.

The fourth reaction involving Gly-4 ( aldolase) is an especially important reaction in glycolysis. Why is this?

It [results] in a splitting of a 6-carbon molecule into two 3-carbon molecules.

Hexokinase IV has a Km value that is substantially higher than the Km for other hexokinases. What is the significance of this fact?

The higher Km reflects [lower] affinity for its substrate, and therefore this kinase is only operational when glucose levels are [high].

Which of the following are products of glycolysis?

1) ATP

2) pyruvate

3) NAD+

4) NADP+

Only 1 and 2 are products of glycolysis.

What reaction does glucose-6-phosphatase catalyze?

the conversion of glucose-6-phosphate to [glucose]

What enzyme catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate?

phosphofructokinase

Place the following sequence of events for the reaction catalyzed by fructose-1,6-phosphate aldolase in the correct order:

1) deprotonation and cleavage to release GAP and enamine products

2) protonation of the enamine

3) conversion of the ketone to the imine derived from lysine

4) hydrolysis to release DHAP

3-1-2-4

Which statement about the interconversion of DHAP to GAP is NOT true?

The reaction is [an oxidation] catalyzed by triose phosphate isomerase.

Place the following sequence of events for the conversion of GAP to 1,3-bisphosphoglycerate in the correct order:

1) attack by Pi

2) release of cysteine as a free thiol

3) transfer of hydride to NAD+

4) attack by cysteine at the carbonyl carbon of GAP

4-3-1-2

Which of the following statements about the reaction catalyzed by phosphoglycerate kinase is NOT true?

The [phosphate] group from C-3 of 1,3-BPG is transferred to ADP.

Which of the following is NOT a fate for pyruvate?

the conversion to [malate]

Which of the following enzymes is NOT part of gluconeogenesis?

phosphofructokinase

If KM for NADH is 2×10⁻⁵ M, then which of the following is the most appropriate NADH concentration to use when determining KM for pyruvate: 10⁻⁷ M, 10⁻⁶ M, 10⁻⁵ M, 10⁻⁴ M, or 10⁻³ M?

10⁻³ M

Suppose that you made some wine whose alcohol content was 10%w/v (i.e., 10 g of ethanol per 100 mL of wine).

The initial fermentation mixture would have had to contain what molar concentration of glucose or its equivalent to generate this much ethanol?

Concentration = 1.1 M

Is it likely that an initial fermentation mixture would contain that much glucose?

no

In what other forms might the fermentable carbon appear?

sucrose

starch

In different organisms sucrose can be cleaved either by hydrolysis or by phosphorolysis.

Calculate the ATP yield per mole of sucrose metabolized by anaerobic glycolysis starting with hydrolytic cleavage.

Yield = 4 mol

Calculate the ATP yield per mole of sucrose metabolized by anaerobic glycolysis starting with phosphorolytic cleavage.

Yield= 5 mol

The standard free energy change of the glycolytic pathway to pyruvate is -79.9 kJ/mol, while the standard free energy change associated with gluconeogenesis from pyruvate is -42.7 kJ/mol. What would the standard free energy change be for a direct reversal of the glycolytic pathway.

+79.9kJ/mol

How many high-energy phosphates are generated or consumed in converting 1 mole of glucose to lactate and 2 moles of lactate to glucose?

Is ATP generated or consumed in converting 1 mole of glucose to lactate?

generated

How many high-energy phosphates are generated in converting 1 mole of glucose to lactate?

2 ATP

Is ATP generated or consumed in converting 2 moles of lactate to glucose?

consumed

How many high-energy phosphates are consumed in converting 2 moles of lactate to glucose?

6 phosphates

Hexokinase, phosphofructokinase and pyruvate kinase are each subject to ________ control.

allosteric

How many ATP equivalents are consumed in the conversion of each of the following to a glucosyl residue in glycogen?

Dihydroxyacetone phosphate.

1 ATP equivalent

Fructose-1,6-bisphosphate.

1 ATP equivalent

Pyruvate

7 ATP equivalents

Glucose-6-phosphate.

1 ATP equivalent

Enolase has a strict requirement for two Mg2+ ions in its active site. Propose a role for these ions in the catalytic mechanism of the enzyme.

The two Mg2+ ions [neutralize] the negative charges on the substrate and [lower] the pKa of the C−2 proton, making it easier to abstract.

Because of the position of arsenic in the periodic table, arsenate (AsO43−) is chemically similar to inorganic phosphate and is used by phosphate-requiring enzymes as an alternative substrate. Organic arsenates are quite unstable, however, and spontaneously hydrolyze. Arsenate is known to inhibit ATP production in glycolysis.

Identify the target enzyme.

Glyceraldehyde-3-phosphate dehydrogenase

Explain the mechanism of inhibition.

The [acyl arsenate] analog of 1,3-bisphosphoglycerate spontaneously hydrolyzes.

Intracellular concentrations in resting muscle are as follows: fructose-6-phosphate, 1.0 mM; fructose-1,6-bisphosphate, 10 mM; AMP, 0.1 mM; ADP, 0.5 mM; ATP, 5 mM; and Pi, 10 mM.

neither more nor less exergonic

Which of the following types of reaction does NOT occur in glycolysis?

Aldol condensation

Suppose it were possible to label glucose with 14C at any position or combination of positions.

For yeast fermenting glucose to ethanol, which form or forms of labeled glucose would give the most radioactivity in CO2 and the least in ethanol?

C−3

C−4

As early as the 1930s, it was known that frog muscles could still contract when glycolysis was inhibited.

Where did the ATP come from to drive these contractions?

From the [creatine phosphate] stores in the muscle.

Anaerobic glycolysis can produce ATP at a much faster rate than aerobic oxidative phosphorylation.

True

Anaerobic glycolysis of 1 mole of sucrose, cleaved initially by sucrose phosphorylase.

Sucrose+[5Pi]+5ADP→4lactate+5ATP+4H₂O+4H⁺

Aerobic glycolysis of 1 mole of maltose.

Maltose+4Pi+4ADP+[4NAD⁺]→4pyruvate+4ATP+3H₂O+4NADH+4H⁺

Fermentation of one glucose residue in starch to ethanol, with the initial cleavage involving α-amylase.

(Glucose)residue+2Pi+2ADP→2ethanol+2ATP+H2O+2CO2+2H+ (the shortest one)

Assume that glyceraldehyde-3-phosphate dehydrogenase was inhibited with iodoacetate, which reacts with its active site cysteine sulfhydryl group. Which glycolytic intermediate would you expect to accumulate most rapidly?

Fructose-1,6-bisphosphate (FBP)

Why?

[As triose] phosphate began to accumulate, the reverse reaction may drive both DHAP and GAP back to FBP.

¹⁴CO₂ was bubbled through a suspension of liver cells that was undergoing gluconeogenesis from lactate to glucose.

Which carbons in the glucose molecule would become radioactive?

none

The flux rate through the gluconeogenic pathway is directly proportional to the amount of carbohydrate in the diet.

False

Which of the following cannot be used as a precursor for gluconeogenesis?

Leucine

The primary gluconeogenic organ in animals is:

liver

Why does it make good metabolic sense for phosphoenolpyruvate carboxykinase, rather than pyruvate carboxylase, to be the primary target for the regulation of gluconeogenesis at the level of control of enzyme synthesis?

Because [pyruvate carboxylase] has two metabolic roles: replenishment of citric acid cycle intermediates and initiation of gluconeogenesis. PEPCK catalyzes the first reaction committed to gluconeogenesis.

The reaction catalyzed by PFK−2.

fructose−6−phosphate+[ATP]→fructose−2,6−bisphosphate+ADP

The conversion of 2 moles of oxaloacetate to glucose.

2oxaloacetate+2ATP+2GTP+[2NADH]+2H++4H2O→glucose+2CO2+2NAD++2ADP+2GDP+4Pi

The conversion of glucose to UDP−Glc.

glucose+[ATP+UTP]→UDP−Glc+PPi+ADP

The conversion of 2 moles of glycerol to glucose.

2glycerol+2ATP+2NAD⁺+2H₂O→glucose+2ADP+2NADH+2H⁺+2Pi

The conversion of 2 moles of malate to glucose-6-phosphate.

2malate+2ATP+2GTP+3H₂O→glucose−6−phosphate+2CO₂+2ADP+2GDP+3Pi

Glycolysis is regulated primarily by:

[three strongly exergonic], nonequilibrium reactions.

Which of the following statements about regulation of phosphofructokinase is FALSE?

[ATP] decreases the apparent Km for fructose-6-phosphate.

Liver pyruvate kinase is activated by dephosphorylation in response to glucagon.

False

All of the reactions of both glycolysis and gluconeogenesis occur in the cytosol.

False

Much of the regulation of gluconeogenesis is a result of the inhibition of glycolysis.

True

The Pasteur effect describes the observation that yeast-metabolizing glucose anaerobically will dramatically increase their rate of glucose utilization when provided with air.

False

Both glycolysis and gluconeogenesis are controlled by fructose 2,6-bisphosphate in response to hormones.

True

Choose a pathway leading from glucose to lactose in mammary gland.

Glc→[G6P]→G1P→UDP−Glc→UDP−Gal+Glc→lactose (the shorter one)

Choose a balanced equation for the overall pathway.

2glucose+[ATP]+UTP→lactose+ADP+UDP+[PPi]

Glycogen is a major energy source for skeletal muscle contraction.

True

Which of the following is NOT involved in glycogen synthesis?

Glycogen phosphorylase

Which statement about control of glycogen metabolism is INCORRECT?

Glycogen synthase is [phosphorylated] at only one site.

Avidin is a protein that binds extremely tightly to biotin, so avidin is a potent inhibitor of biotin-requiring enzyme reactions.

Consider glucose biosynthesis from each of the following substrates and predict which of these pathways would be inhibited by avidin.

lactate

In liver, glucagon stimulates glycogen breakdown via cAMP. Although you might expect glucagon to stimulate catabolism of the glucose formed as well, glucagon inhibits glycolysis and stimulates gluconeogenesis in liver.

The function of glucagon is to [increase] blood glucose [concentration]. All the mentioned processes are consistent with this function.

An individual with a glucose-6-phosphatase deficiency suffers from chronic hypoglycemia.

The glucose-6-phosphatase deficiency would interfere with [release of glucose] from the liver for export to other tissues.

The presence in liver of glucose-6-phosphatase is essential to the function of the liver in synthesizing glucose for use by other tissues.

Glucose-6-phosphate [must be hydrolyzed] in order for glucose to exit the liver cell and be exported to other tissues.

Phosphorylases and phosphatases catalyze the same reaction, the removal of a phosphate group.

False

[1−14C]Ribose-5-phosphate is incubated with a mixture of purified transketolase, transaldolase, phosphopentose isomerase, phosphopentose epimerase, and glyceraldehyde-3-phosphate.

Predict the distribution of radioactivity in the erythrose-4-phosphate that is formed in this mixture.

none

Predict the distribution of radioactivity in the fructose-6-phosphate that is formed in this mixture.

C-1

C-3

Xylulose-5-phosphate is an intermediate in the pentose phosphate pathway.

As xylulose-5-phosphate levels rise in response to excess glucose shunting through the pentose phosphate pathway, does flux through glycolysis increase or decrease?

increase

Flux through the pentose phosphate pathway is controlled mainly by the NADP+/NADPH ratio in the cell.

True

FILL IN THE BLANK 12.2

Ethanol will generate ___, through the action of alcohol dehydrogenase, and this will ___ formaldehyde back to ___, also through alcohol dehydrogenase.

The ___ formed can be metabolized further to acetate. Ethanol may also compete with methanol for binding to alcohol dehydrogenase.

NADH

reduce

methanol

acetaldehyde

DIAGRAM 12.15 top to bottom

Left: C, A, [NADH]

Right: B, A, D, [pyruvate]

FILL IN THE BLANK 12.25

1. The liver is the ___ active gluconeogenic tissue. Since alanine is an important gluconeogenic ___, its accumulation in liver is a signal that gluconeogenesis should be ___.

2. In addition, alanine ___ in muscle under conditions of high metabolic demand, where glycolysis must still function to provide ___.

most

precursor

activated

ATP

Only gluconeogenic tissues contain appreciable levels of glucose-6-phosphatase.

The glucose-6-phosphatase [plays] a role only in primarily liver because its function is to manufacture glucose for export.

FILL IN THE BLANK 12.23

Glycogen breakdown is under nonhormonal as well as hormonal control. Recall that phosphorylase b is relatively inactive, existing primarily in its T state. This form of the enzyme is activated allosterically by 5′−___(but not by cAMP). This activation does not usually occur in the cell because ___, which is far more abundant and does not activate phosphorylase b, competes with ___ for binding to the enzyme. However, under energy-deprived conditions, AMP may accumulate at the expense of ATP breakdown. ___ binding shifts the conformational equilibrium of phosphorylase b to the more active R state. ATP and___, signs of adequate energy status, shift the equilibrium of phosphorylase b back to the less active T state.

Once glycogen phosphorylase b is phosphorylated to its more active a state, it exists primarily in the R form and is unresponsive to most metabolite effectors. However, glucose and ___ act synergistically on phosphorylase a, shifting its equilibrium slightly back toward the T state. In the T state, the phosphoserine side chains are more accessible to phosphoprotein phosphatase 1, so that the T state is more readily dephosphorylated than the R state.

The ___ mechanism, which does not involve a metabolic cascade, stimulates glycogenolysis in response to a low-energy charge, whereas the ___ mechanism predominates when the need is to rapidly augment energy generation.

AMP

ATP

AMP

AMP

glucose-6-phosphate

glucose-6-phosphate

nonhormonal

hormonal