Pentose Phosphate Pathway

What are the two major functions of the pentose phosphate pathway?

1. Biosynthesis of pentose sugars, which are required for the synthesis of nucleotides and nucleic acids 2. Formation of NADPH, which is required for the synthesis of fatty acids and steroids, for regeneration of the reduced form of the enzyme glutathione, and under some circumstances, generation of reactive oxygen.

What is the major regulatory step in the pentose phosphate pathway?

The conversion of glucose-6-phosphate and NADP+ to 6-phosphogluconolactone and NADPH by the enzyme glucose-6-phosphate dehydrogenase.

Which pentose phosphate pathway enzyme irreversibly hydrolyzes 6-phosphogluconolactone to 6-phosphogluconate?

Lactonase.

Which pentose phosphate pathway enzyme catalyzes the oxidative decarboxylation of 6-phosphogluconate to form ribulose-5-phosphate?

6-phosphogluconate dehydrogenase.

What is the main allosteric inhibitor of 6-phosphogluconate dehydrogenase?

NADPH.

Which enzymes generate NADPH in the pentose phosphate pathway?

Glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase.

Ribulose-5-phosphate can be directly converted to which two pentose phosphate pathway metabolites?

Xylulose-5-phosphate (catalyed by phosphopentose epimerase) or ribose-5-phosphate (catalyzed by phosphopentose isomerase).

True or False. In the pentose phosphate pathway, the conversion of ribose-5-phosphate to glycolytic intermediates generates NADPH.

False. Only the first stage of the pentose phosphate pathway (from glucose-6-phosphate to ribulose-5-phosphate) generates NADPH.

What coenzyme is required for transketolase activity?

Thiamine pyrophosphate (TPP).

Measuring the activity of which pentose phosphate pathway enzyme can be used to assay for thiamine deficiency?

Erythrocyte transketolase.

In general, the enzyme transketolase transfers fragments from a ketose to an aldose. How many carbons in length are these fragments?

2 carbons (one way to remember this is to think 'transkeTWOlase').

In general, the enzyme transaldolase transfers fragments from a ketose to an aldose. How many carbons in length are these fragments?

3 carbons.

Which (and how many) glycolytic intermediates are produced by the rearrangement of three ribulose-5-phosphate molecules by transketolase and transaldolase?

Two fructose-6-phosphate molecules and one glyceraldehyde-3-phosphate molecule are produced.

What is the net balanced chemical equation of the pentose phosphate pathway?

3 glucose-6-phosphate + 6 NADP -> 2 fructose-6-phosphate + glyceraldehyde-3-phosphate + 3 CO2 + 6 NADPH + 6 H+.

What is the structural difference between NADH and NADPH?

NADPH has an additional phosphate group at the 2'-position of the ribose linked to adenine. This phosphate group allows enzymes to discriminate between NADH and NADPH.

If the NAD+/NADH ratio in a cell is very high, the NADP+/NADPH ratio in the same cell is ___________ (very high/very low/the same).

Very low. A cell with a high concentration of NAD+ will therefore also have a high concentration of NADPH.

List the tissues in which the pentose phosphate pathway is most active.

Liver, adipose tissue, adrenal cortex, phagocytes and erythrocytes (red blood cells).

What is the primary function of the pentose phosphate pathway in erythrocytes (red blood cells)?

It generates NADPH, which plays a role in protecting erythrocytes from oxidative damage.

The generation of reactive oxygen species (ROS) by phagocytes is an important mechanism used to kill engulfed bacteria. Which enzyme generates a superoxide anion from oxygen?

NADPH oxidase.

What is the reaction catalyzed by the enzyme superoxide dismutase (SOD)?

2 superoxide anions + 2 protons -> hydrogen peroxide (H2O2) + oxygen (O2).

Which enzyme generates hypochlorite (bleach) from hydrogen peroxide and chloride anions during the oxidative burst of a neutrophil?

Myeloperoxidase.

Which enzyme catalyzes the decomposition of hydrogen peroxide into water and oxygen?

Catalase.

Which family of enzymes reduces peroxides by catalyzing the transfer of electrons from an electron donor to the peroxide?

Peroxidases.

Glutathione is an important coenzyme for which enzyme?

Glutathione peroxidase (GSH peroxidase).

What reaction does glutathione peroxidase (GSH peroxidase) catalyze?

GSH peroxidase reduces hydrogen peroxide to two water molecules.

The NADPH produced in the pentose phosphate pathway plays what role in the reduction of hydrogen peroxide to water by GSH peroxidase?

As GSH peroxidase reduces peroxide, it oxidizes the coenzyme glutathione. The NADPH produced by glucose-6-phosphate dehydrogenase is used to regenerate the reduced form of glutathione.

A patient is developing severe acute hemolytic anemia after reportedly taking the antimalarial drug primaquine. You suspect a deficiency of which enzyme?

Glucose-6-phosphate dehydrogenase.

List some foods and classes of drugs that can trigger episodes of hemolytic anemia in people with glucose-6-phosphate dehydrogenase deficiency.

Fava beans, antimalarial drugs, and some antibiotics and antipyretics.

Which enzyme not found in erythrocytes produces NADPH in a pathway other than the pentose phosphate shunt?

NADP-linked malic enzyme (also known as NADP-linked malate dehydrogenase).

What are the substrates and products of the reaction catalyzed by NADP-linked malic enzyme (also known as NADP-linked malate dehydrogenase)?

Malate + NADP+ -> pyruvate + CO2 + NADPH + H+.