Chapter 26 - Pentose Phophate Pathway

Question 1

Which of the following enzymes is the regulatory enzyme in the pentose phosphate pathway?

a - 6-Phosphogluconate dehydrogenase

b - Glyceraldehyde 3-phosphate dehydrogenase

c - Glucose 6-phosphate dehydrogenase

d - Glucose 6-phosphatase

e - Lactonase

Answer 1

Glucose 6-phosphate dehydrogenase

Question 2

The pentose phosphate pathway is composed of two distinct phases.

What are the two phases, and what are their roles?

Answer 2

The oxidative phase generates NADPH and is irreversible

The non-oxidative phase allows for the interconversion of phosphorylated sugars.

Question 3

What is the key regulatory enzyme in the pentose phosphate pathway, and what is its most prominent regulatory signal?

Answer 3

Glucose 6-phosphate dehydrogenase

NADP+

Question 4

Glucose is normally completely oxidized to CO2 in the mitochondria. Under what circumstance can glucose be completely oxidized to CO2 in the cytoplasm?

Answer 4

When much NADPH is required.

The oxidative phase of the pentose phosphate pathway is followed by the nonoxidative phase. The resulting fructose 6-phosphate and glyceraldehyde 3-phosphate are used to generate glucose 6-phosphate through gluconeogenesis and the cycle is repeated until the equivalent of one glucose molecule is oxidized to CO2

Question 5

Liver and other organ meats contain large quantities of nucleic acids. In the course of digestion, RNA is hydrolyzed to ribose, among other chemicals. Explain how ribose can be used as a fuel.

Answer 5

The nonoxidative phase of the pentose phosphate pathway can be used to convert three molecules of ribose 5-phosphate into two molecules of glyceraldehyde 3-phosphate.

These molecules are components of the glycolytic pathway

Question 6

Why do deficiencies in glucose 6-phosphate dehydrogenase frequently present as anemia?

Answer 6

Because red blood cells do not have mitochondria and the only means to obtain NADPH is through the pentose phosphate pathway. There are biochemical means to convert mitochondrial NADH into cytoplasmic NADPH

Question 7

Explain why the Calvin cycle and the pentose phosphate pathway are almost mirror images of each other.

Answer 7

The Calvin Cycle begins with the fixation of CO2 and proceeds to use NADPH in the synthesis of glucose.

The pentose phosphate pathway begins with the oxidization of a glucose-derived carbon to CO2 and concomitantly generates NADPH.

The regeneration phase of the Calvin cycle converts C6 and C3 molecules back into the starting material - the C5 molecule ribulose 1,5-bisphosphate.

The pentose phosphate pathway converts a C5 molecule ribose 5-phosphate, into C6 and C3 intermediates of the glycolytic pathway.