BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 15. THE PENTOSE PHOSPHATE PATHWAY AND GLUCONEOGENESIS

15.5. The Rate of the Pentose Phosphate Pathway Is Regulated by the Concentration of NADP⁺

The first reaction of the oxidative branch of the Pentose Phosphate Pathway, the dehydrogenation of glucose-6-phosphate, is essentially irreversible. Under physiological conditions, this reaction is rate-limiting and serves as a "control point". The most crucial regulatory factor is the concentration of NADP⁺, the electron acceptor in The oxidation of glucose-6-phosphate to 6-phosphoglucono-lactone. In addition, NADPH competes with NADP⁺ for binding to the enzyme, and ATP competes with glucose-6-phosphate. The ratio of the concentration of NADP⁺ to that of NADPH in the Cytosol of Liver Cells from well-fed rats is approximately 0.014, which is several orders of magnitude lower than the [NAD⁺]/[NADH] ratio, which under the same conditions is 700. The pronounced effect of the NADP⁺ concentration on the rate of reactions in the oxidative branch of The pentose phosphate pathway confirms that NADPH generation is tightly coupled to its utilization in reductive biosyntheses. The question regarding the Regulation of the nonoxidative branch of the pentose phosphate pathway remains open to this day.

15.6. The Fate of Glucose-6-Phosphate Depends on the Demand for NADPH, Ribose-5-Phosphate, and ATP

Let us trace The Fate of glucose-6-phosphate in four different situations.

1. The demand for ribose-5-phosphate significantly exceeds the demand for NADPH. The greater part of glucose-6-phosphate is converted into fructose-6-phosphate and glyceraldehyde-3-phosphate via The Glycolytic Pathway. Subsequently, two molecules of fructose-6-phosphate and one molecule of glyceraldehyde-3-phosphate are converted into three molecules of ribose-5-phosphate through the action of transaldolase and transketolase by the Reversal of the reaction described earlier. The stoichiometry of this transformation (Fig. 15.2, A) is as follows:

5Glucose-6-phosphate + 5ATP → 6Ribose-5-phosphate + 5ADP + Н⁺.

Class="center">Fig. 15.2. Four mechanisms of the pentose phosphate pathway. The names of the main products are shaded

2. The demand for NADPH and ribose-5-phosphate is balanced. Under these conditions, the predominant reaction is The formation of two molecules of NADPH and one molecule of ribose-5-phosphate from one molecule of glucose-6-phosphate via the oxidative branch of the pentose phosphate pathway. The stoichiometry of this transformation (Fig. 15.2, B) is described by the equation

Glucose-6-phosphate + 2NADP⁺ + Н₂O → Ribose-5-phosphate + 2NАDРН + 2Н⁺ + СO₂. '

3. The demand for NADPH significantly exceeds the demand for ribose-5-phosphate; glucose-6-phosphate is completely oxidized to СO₂. In this situation, three groups of reactions proceed actively. First, two NADPH and one ribose-5-phosphate are formed via the oxidative branch of the pentose phosphate pathway. Next, ribose-5-phosphate is converted into fructose-6-phosphate and glyceraldehyde-3-phosphate under the action of transketolase and transaldolase. Finally, glucose-6-phosphate is resynthesized from fructose-6-phosphate and glyceraldehyde-3-phosphate via The Gluconeogenesis pathway (discussed later in this chapter). The stoichiometry of these reactions (Fig. 15.2, C) is described by the following equations:

6Glucose-6-phosphate + 12NADP⁺ + 6Н₂О → 6Ribose-5-phosphate + 12NADPH + 12Н⁺ + 6СО₂,

6Ribose-5-phosphate → 4Fructose-6-phosphate + 2Glyceraldehyde-3-phosphate,

4Fructose-6-phosphate + 2Glyceraldehyde- 3-phosphate + Н₂O → 5Glucose-6-phosphate + Рi.

Summing these reactions, we obtain

Glucose-6-phosphate + 12NADP⁺ + 7Н₂O → 6СO₂ + 12NADPH + 12Н⁺ + Рi.

Thus, an equivalent of glucose-6-phosphate can be completely oxidized to СO₂ with the simultaneous generation of NADPH. The Significance of these reactions is that ribose-5-phosphate, formed via the pentose phosphate pathway, is converted back into glucose-6-phosphate through the action of transketolase, transaldolase, and certain Enzymes of gluconeogenesis.

4. The demand for NADPH significantly exceeds the demand for ribose-5-phosphate: glucose-6-phosphate is converted into Pyruvate. Another pathway is also possible: ribose-5-phosphate formed via the oxidative branch of the pentose phosphate pathway is converted into pyruvate (Fig. 15.2, D). Fructose-6-phosphate and glyceraldehyde-3-phosphate derived from ribose-5-phosphate enter the glycolytic pathway of METABOLISM rather than undergoing conversion back into glucose-6-phosphate. According to the outlined mechanism, there is simultaneous generation of ATP and NADPH, and five out of the six carbon atoms of glucose-6-phosphate appear in pyruvate:

3Glucose-6-phosphate + 6NADP⁺ + 5NAD⁺ + 5Рi + 8ADP → 5Pyruvate + 3СO₂ + 6NADPH + 5NADH + 8ATP + 2Н₂O + 8Н⁺.

The pyruvate formed in these reactions can be oxidized to yield additional ATP or can be used as a building block in various biosynthetic processes.

15.7. The Activity of the Pentose Phosphate Pathway Is Considerably Higher in Adipose Tissue Than in Muscle

Radioactive tracer experiments make it possible to measure the proportion of glucose-6-phosphate metabolized via the pentose phosphate pathway versus the glycolytic pathway coupled with The Tricarboxylic Acid Cycle. To achieve this, one tissue homogenate sample is incubated with glucose labeled with ¹⁴С at the C-1 position, and another sample with glucose labeled with ¹⁴С at the C-6 position, after which the radioactivity of the CO₂ produced in both samples is compared. The rationale behind the experiment is that decarboxylation via the pentose phosphate pathway occurs exclusively at C-1. In contrast, during glucose degradation mediated by the pyruvate dehydrogenase complex in the glycolytic pathway followed by the tricarboxylic acid cycle, decarboxylation at C-1 and C-6 proceeds with equal intensity. The equivalent rate of decarboxylation at C-1 and C-6 in this latter group of reactions is due to the rapid interconversion of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate catalyzed by Triosephosphate isomerase.

This experimental approach revealed that pentose phosphate pathway activity is extremely low in Skeletal Muscle, yet exceptionally high in adipose tissue. These findings support the hypothesis that the primary role of the pentose phosphate pathway is to generate NADPH for reductive biosyntheses. Adipose tissue consumes large amounts of NADPH for the reductive synthesis of Fatty acids from acetyl-CoA (Chap. 17).