Biochemistry CH 16

What is the name of the pathway that converts noncarbohydrate molecules into glucose?

A) pentose phosphate pathway

B) glycogenolysis

C) citric acid cycle

D) gluconeogenesis

E) None of the answers is correct.

D) gluconeogenesis

The Embden Meyerhof pathway is also known as ________.

A) pentose phosphate pathway

B) glycolysis

C) citric acid cycle

D) gluconeogenesis

E) None of the answers is correct.

B) glycolysis

What type of organism cannot survive in the presence of oxygen?

A) obligate anaerobes

B) facultative anaerobes

C) obligate aerobes

D) facultative aerobes

E) None of the answers is correct.

A) obligate anaerobes

Which of the following must be regenerated for glycolysis to proceed?

A) pyruvate

B) ATP

C) NAD+

D) UDP-glucose

E) All of the answers are correct.

C) NAD+

Which of the following intermediates is needed for the conversion of galactose into glucose by reacting with galactose 1-phosphate?

A) glucose 1-phosphate

B) fructose 6-phosphate

C) glucose 6-phosphate

D) UDP-galactose

E) UDP-glucose

E) UDP-glucose

Which of the following is an allosteric inhibitor of phosphofructokinase?

A) NADH

B) ATP

C) AMP

D) fructose 1,6-bisphosphate

E) None of the answers is correct.

B) ATP

What transporter is responsible for fructose uptake in the intestine?

A) GLUT1

B) GLUT2

C) GLUT3

D) GLUT4

E) GLUT5

E) GLUT5

Which of the following conditions results from genetic deficiency of a single transferase enzyme?

A) lactose intolerance

B) galactosemia

C) lactic acidosis

D) hypoglycemia

E) None of the answers is correct.

B) galactosemia

What nutrient is required for the carboxylation of pyruvate in humans?

A) biotin

B) thiamine

C) riboflavin

D) niacin

E) folic acid

A) biotin

Which of the following is an allosteric inhibitor of gluconeogenesis?

A) NADH

B) ATP

C) AMP

D) fructose 6-phosphate

E) phosphoenolpyruvate

C) AMP

Which of the following are reasons that glucose is a common metabolic fuel used by living organisms?

A) In its stable ring structure, glucose is unlikely to nonezymatically glycosylate proteins.

B) It is one of the fuels formed under prebiotic conditions, thus available for primitive

biochemical systems.

C) It is the only sugar used by the brain and kidney as a fuel source.

D) It is a ketose and thus a nonreducing sugar.

E) Both A and B are correct.

E) Both A and B are correct.

What is the purpose of phosphorylating glucose in the cytosol?

A) to trap glucose in the cell

B) to destabilize glucose and facilitate the next series of metabolic steps

C) to convert it to a more soluble form

D) to slow the transport of glucose into the cell

E) Both A and B are correct.

E) Both A and B are correct.

What two 3-carbon molecules are generated by the cleavage of fructose-1,6-bisphosphate?

A) glyceraldehyde-3-phosphate and 3-phosphoglycerate

B) glyceraldehyde-3-phosphate and dihydroxyacetone phosphate

C) pyruvate and phosphoenolpyruvate

D) enolase and 2-phosphoglycerate

E) glyceraldehyde-3-phosphate and pyruvate

B) glyceraldehyde-3-phosphate and dihydroxyacetone phosphate

What is a common mechanistic feature of kinases?

A) Phosphoryl groups are transferred from a substrate with high phosphoryl-transfer potential to AMP.

B) Binding of substrate causes a large conformation change that results in cleft closing.

C) All phosphorylation reactions are specific for monosaccharides.

D) Phosphorylation always uses ATP as a substrate.

E) None of the answers is correct.

B) Binding of substrate causes a large conformation change that results in cleft closing.

What reaction is catalyzed by aldolase?

A) isomerization of DHAP to GAP

B) hydrolysis of either GAP or DHAP

C) reversible cleavage of F-1,6-BP to DHAP and GAP

D) cleavage of DHAP to GAP

E) irreversible aldol condensation of DHAP and GAP

C) reversible cleavage of F-1,6-BP to DHAP and GAP

What is the function of glyceraldehyde 3-phosphate dehydrogenase?

A) oxidation of an aldehyde by NAD+ and formation of acyl-phosphate

B) oxidation of an alcohol to an aldehyde by NAD+

C) dehydration and dephosphorylation of GAP

D) hydrolysis of GAP

E) removal of the 3-phosphoryl-group from GAP

A) oxidation of an aldehyde by NAD+ and formation of acyl-phosphate

What is the function of a thioester intermediate such as the one formed from GAP?

A) It speeds up the actual reaction so that more product can be made.

B) The thioester shifts the equilibrium of the first stage of the reaction.

C) The thioester allows the two-step reaction to be coupled so the second reaction, the energetically unfavorable phosphorylation, can proceed.

D) The thioester intermediate induces a conformational change that alters the enzymespecificity.

E) The thioester prevents the formation of metabolically unfavorable side products.

C) The thioester allows the two-step reaction to be coupled so the second reaction, the energetically unfavorable phosphorylation, can proceed.

What is substrate level phosphorylation?

A) phosphorylation of AMP by ATP

B) ATP synthesis when the phosphate donor is a substrate with high phosphoryl transfer potential

C) phosphorylation of glycolytic intermediates

D) phosphorylation of ATP coupled to an ion gradient

E) ATP and AMP synthesis from two molecules of ADP

B) ATP synthesis when the phosphate donor is a substrate with high phosphoryl transfer potential

What is the additional metabolite that is required for the conversion of 3-phosphoglycerate to 2-phosphoglycerate?

A) 1-phosphoglycerate

B) diacylglycerol

C) NADH

D) 2,3-bisphosphoglycerate

E) 1,3-bisphosphoglycerate

D) 2,3-bisphosphoglycerate

What are the primary metabolic fates of pyruvate?

A) ethanol

B) lactate

C) acetyl CoA

D) All of the answers are correct.

E) ethanol and lactate

D) All of the answers are correct.

Fructose can enter glycolysis at two distinct points, depending on the tissue. How is fructose metabolized in adipose tissue?

A) Fructose is cleaved to two molecules of GAP.

B) Fructose is converted to fructose-1-phosphate.

C) Fructose is converted to fructose-6-phosphate.

D) Fructose is cleaved to GAP and DHAP.

E) Fructose is converted to glucose, which enters the pathway.

C) Fructose is converted to fructose-6-phosphate.

Lactose intolerance is caused by a deficiency of

A) lactase.

B) elastase.

C) lactose.

D) sucrase.

E) None of the answers is correct.

A) lactase.

How are the glycolytic enzymes regulated?

A) transcriptional control

B) reversible phosphorylation

C) allosteric control

D) All of the answers are correct.

E) reversible phosphorylation and allosteric control

D) All of the answers are correct.

Two primary raw materials for gluconeogenesis in the human liver are

A) galactose and sucrose.

B) acetate and oxaloacetate.

C) lactate and alanine.

D) fructose and alanine.

E) lactose and lactate.

C) lactate and alanine.

How many high-energy phosphate bonds are expended in gluconeogenesis?

A) three

B) six

C) two

D) four

E) one

B) six

What astounding discovery was made by the Buchners?

When the Buchners demonstrated that fermentation can occur in yeast extracts, they were

the first to dispute Pasteur's assertion that fermentation required intact cells.

Why do muscles need to generate ATP under aerobic and anaerobic conditions?

Muscles initially function aerobically. However, when bursts of energy are required, the oxygen supply does not meet the demand. In order to generate sufficient ATP for energy

demands during periods of extreme activity, the muscle depends on anaerobic metabolism.

Both hexokinase and glucokinase phosphorylate glucose. The function of glucokinase is to

phosphorylate glucose in liver cells as a means to regulate blood-sugar levels. Would you expect

its KM to be higher or lower than hexokinase?

Glucokinase must be responsive to elevated blood-glucose concentrations, thus it should

have a higher KM. This lower binding affinity allows this enzyme to become more active at high

glucose concentrations, which saturate hexokinase. Hexokinase serves to phosphorylate glucose

(and other hexoses) in the cytosol, and it has higher affinity for glucose, or a lower KM value.

What two isomerization reactions occur in glycolysis? Why are these steps necessary?

Glucose-6-phosphate is isomerized to fructose-6-phosphate, converting an aldose to a ketose, which then allows phosphorylation at the number 1 carbon. Later in the pathway, dihydroxyacetone-phosphate is converted to glyceraldehyde-3-phosphate, utilizing both of the molecules formed from fructose-1,6-bisphosphate cleavage.

At equilibrium, there is far more DHAP than GAP. Yet the conversion of DHAP by triose

phosphate isomerase proceeds readily. Why?

The GAP formed is immediately removed by subsequent reactions, resulting in conversion of DHAP into GAP by the enzyme. The removal of the product drives the reaction in

one direction.

How is the conversion of phosphoenolpyruvate to pyruvate accompanied by ATP formation?

The enol phosphate possesses very high potential for phosphoryl transfer, which is due to

the driving force of the tautomerization of the enol to the more stable ketone.

Describe the biochemical explanation for galactosemia.

Galactose is metabolized by conversion to Gal-1-P by galactokinase. The enzyme,

galactose-1-phosphate uridyl transferase transfers a uridyl group from UDP-Glc to Gal-1-P to

produce UDP-Gal. Then UDP-Gal is epimerized to UDP-Glc. UDP-galactose is a necessary intermediate in the metabolism of galactose. Individuals deficient in galactose-1-phosphate uridyl transferase activity cannot metabolize galactose, which leads to elevated levels of galactose in the blood and urine. This genetic disease is referred to as galactosemia.

Give the reactions by which glycerol (from fats) can be metabolized into pyruvate or

synthesized into glucose.

After the glycerol is converted into DHAP, it is isomerized into GAP, which can then either

proceed through glycolysis to pyruvate or through gluconeogenesis to glucose.

How is glycolysis maintained under anaerobic conditions?

Pyruvate can be reduced to either lactate or ethanol, and this reaction is accompanied by

the oxidation of NADH to regenerate NAD+.

How does citrate influence glycolysis?

Phosphofructokinase is inhibited by citrate, which is an intermediate in the citric acid cycle. Thus, if citrate levels are high, the enzyme is inhibited, and fewer glucose molecules are metabolized. In this sense, citrate serves as a cell indicator. High levels of citrate in the

cytoplasm means that biosynthetic precursors are abundant, and so there is no need to degrade

additional glucose for this purpose.

Why is it more sensible for phosphofructokinase to be an important control step, rather than

hexokinase?

Phosphofructokinase catalyzes the first committed step in the glycolytic pathway. At this point, the molecule is committed to entering the glycolytic path. In contrast, production of G6P is the first step in many different paths. Thus, glycolytic control would not be maintained by tight regulation of hexokinase.

What two functions are attributed to substrate cycles?

The substrate cycles regulate glycolytic path flux by amplifying metabolic signals, and they generate body heat produced by the hydrolysis of ATP.

Which metabolic steps differ between glycolysis and gluconeogenesis?

There are three irreversible steps in glycolysis, which require four different steps in gluconeogenesis: pyruvate conversion to phosphoenolpyruvate via an oxaloacetate intermediate,

fructose 1,6-bisphosphate hydrolysis, and the hydrolysis of glucose 6-phosphate.

How are gluconeogenesis and glycolysis regulated reciprocally?

The enzymes involved in two substrate cycles are control points. Figure 16.29 in the text shows the glycolytic path activation by F-2,6-BP, AMP, and F-1,6-BP; whereas ATP, alanine, citrate, and protons inhibit glycolysis. Gluconeogenesis is activated by citrate and acetyl CoA

and inhibited by F-2,6-BP, AMP, and ADP.

Describe the two isoforms of lactate dehydrogenase.

Two forms exist, called M and H, which predominate in skeletal and heart muscle, respectively. The two forms are products of different genes, but they are similar in structure and can form tetramers in various H:M ratios. The two forms differ in their sensitivity to pyruvate.

H4 functions primarily to oxidize lactate to pyruvate, which serves as a fuel for aerobic metabolism. In contrast, M4 produces lactate so that glycolysis can continue under anaerobic conditions.

During exercise, how is glycolysis regulated by feedforward stimulation?

Exercise increases the abundance of fructose-6-phosphate which leads to higher concentrations of fructose-2,6-bisphosphate. Fructose-2,6-bisphosphate increases the affinity of

phosphofructokinase for fructose 6-phosphate, leading to an acceleration of glycolysis if glucose

is abundant.