Question 1

Which of the following is the primary function of the oxidative phase of the pentose phosphate pathway?

Accepted Answer

To produce NADPH and ribulose-5-phosphate

Question 2

Which enzyme catalyzes the first and rate-limiting step of the pentose phosphate pathway, and what is the product of this reaction?

Accepted Answer

Glucose-6-phosphate dehydrogenase; 6-phosphogluconolactone

Question 3

In the oxidative phase, which molecule is reduced and which is oxidized in the reaction catalyzed by glucose-6-phosphate dehydrogenase?

Accepted Answer

NADP$$^{+} is $$reduced; glucose-6-phosphate is oxidized

Question 4

Which of the following best describes the role of NADPH produced by the pentose phosphate pathway in red blood cells?

Accepted Answer

It maintains reduced glutathione to protect against oxidative damage

Question 5

A patient with a deficiency in transketolase would most likely have impaired synthesis of which molecule, and why?

Accepted Answer

Fructose-6-phosphate, because transketolase transfers two-carbon units to form it in the nonoxidative phase

Question 6

Which cofactor is essential for the activity of transketolase in the pentose phosphate pathway?

Accepted Answer

Thiamine pyrophosphate (TPP)

Question 7

If a cell requires more ribose-5-phosphate than NADPH, which phase of the pentose phosphate pathway is primarily utilized?

Accepted Answer

The nonoxidative phase, converting glycolytic intermediates to ribose-5-phosphate

Question 8

Which of the following equations correctly represents the overall reaction of the oxidative phase of the pentose phosphate pathway?

Accepted Answer

$$Glucose-6-phosphate + 2 $$NADP^{+}$$ \rightarrow Ribulose-5-phosphate + CO_{2} + 2 NADPH + 2 H$$^{+}$$

Question 9

Which regulatory mechanism controls the activity of glucose-6-phosphate dehydrogenase in the pentose phosphate pathway?

Accepted Answer

Allosteric inhibition by NADPH

Question 10

A cell is rapidly dividing and requires large amounts of ribose-5-phosphate for nucleotide synthesis but little NADPH. Which metabolic adaptation will the cell employ?

Accepted Answer

Utilize the nonoxidative phase to convert fructose-6-phosphate and glyceraldehyde-3-phosphate to ribose-5-phosphate

Question 11

Which of the following is NOT a product of the pentose phosphate pathway?

Accepted Answer

ATP

Question 12

Which reaction in the pentose phosphate pathway involves the decarboxylation of 6-phosphogluconate?

Accepted Answer

6-phosphogluconate $$\rightarrow$$ ribulose-5-phosphate + CO$$_2$$

Question 13

Which enzyme catalyzes the transfer of a two-carbon unit from a ketose donor to an aldose acceptor in the nonoxidative phase?

Accepted Answer

Transketolase

Question 14

A deficiency in thiamine (vitamin B1) would most directly impair which step of the pentose phosphate pathway?

Accepted Answer

The transketolase-catalyzed transfer of two-carbon units

Question 15

Which of the following best explains why the pentose phosphate pathway is important for fatty acid synthesis?

Accepted Answer

It produces NADPH, which is required for reductive biosynthesis

Question 16

Which of the following is a correct description of the nonoxidative phase of the pentose phosphate pathway?

Accepted Answer

It interconverts sugars of varying carbon lengths, such as ribose-5-phosphate, xylulose-5-phosphate, and fructose-6-phosphate

Question 17

Which of the following scenarios would most likely increase the flux through the pentose phosphate pathway?

Accepted Answer

High demand for NADPH due to increased fatty acid synthesis

Question 18

Which of the following is the correct net reaction for the nonoxidative phase when 3 ribulose-5-phosphate molecules are rearranged?

Accepted Answer

$$2 xylulose-5-phosphate + ribose-5-phosphate \rightarrow 2 fructose-6-phosphate + glyceraldehyde-3-phosphate$$

Question 19

Which of the following best describes the fate of ribose-5-phosphate when both NADPH and ATP are needed but not ribose-5-phosphate?

Accepted Answer

It is converted to glycolytic intermediates such as fructose-6-phosphate and glyceraldehyde-3-phosphate

Pentose Phosphate Pathway (Hexose Monophosphate Shunt): Structure, Function, and Regulation

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