The Pentose Phosphate Pathway | Plants

In this article we will discuss about the Pentose Phosphate Pathway (Respiration). After reading this article you will learn about: 1. Sequence of Reactions in the Pentose Phosphate Pathway 2. Regulation of the Pentose Phosphate Pathway.

Sequence of Reactions in the Pentose Phosphate Pathway:

The enzymes of the pentose phosphate cycle occur in the extra-mitochondrial soluble portion of the cell, the cytosol.

It is not known whether they occupy a compartment separated from the enzymes of glycolysis which are also located in the cytoplasm. As in glycolysis, it is dehydrogenation by which oxidation is performed, but the pentose phosphate pathway differs from glycolysis in utilizing NADP⁺ and not NAD⁺ as the hydrogen acceptor.

The direct degradation of glucose is achieved in two phases:

The first one consists of the oxidative conversion of glucose-6-P to pentose phosphates and the second non-oxidative phase involves the regeneration of glucose-6-phosphate from pentose phosphate, while glucose-6-phosphate enters the cycle again (Fig. 9.25).

(i) Oxidative Phase:

The starting substance is glucose-6-P which is oxidized at carbon atom 1 by NADP⁺-dependent enzyme glucose-6-P dehydrogenase. NADP⁺ is reduced to NADPH and 6-phosphogluconolactone formed is either spontaneously or enzymatically converted to 6-phosphogluconic acid by gluconolactone hydrolase or lactonase.

6-Phosphogluconate is the substrate for the second oxidative step, this time at carbon atom 3 and the enzyme is 6-phosphogluconate dehydrogenase, which also requires NADP⁺ as hydrogen acceptor. An unstable compound, 3-keto-6-phosphogluconate is formed first which is readily converted to ribulose-5-phosphate by decarboxylation at carbon atom 1.

This terminates the first phase, i.e., the oxidation process. Two reducing equivalents (NADPH) are formed, and carbon atom I is eliminated as CO₂ (Fig. 9.26).

(ii) Non-oxidative Phase:

The ribulose-5-phosphate is further metabolized by a series of reactions involving inter-conversions of 3-, 4-, 5-, 6- and 7-carbon mono-saccharides which are catalyzed by several enzymes.

In the first instance, ribulose-5-P serves as substrate for two different enzymes. Ribulose- 5-P isoflierase converts ribulose-5-P to the corresponding aldopentose, ribose-5-P. The other enzyme ribulose-5-P epimerase alters the configuration about carbon 3 of ribulose-5-P and forms the epimer xylulose-5-P, another ketopentose.

The two products of the epimerase and isomerase reactions now serve as substrates for the next enzyme, transketolase which catalyzes the following reaction:

The enzyme requires the coenzyme thiamine pyrophosphate (TPP) in addition to Mg²⁺ ions and the reaction mechanism is similar to that of pyruvate decarboxylase. A 2-carbon active glycol-aldehyde moiety bound to TPP is transferred from xylulose-5-P to ribose-5-P producing the 7-carbon ketose sedoheptulose-7-P and the aldose glyceraldehyde-3-P.

Transketolase is the first enzyme characteristic of the non-oxidative phase. We are familiar with transketolase action from photosynthetic CO₂ reduction cycle in which it, however, favours the reverse reaction, i.e., the formation of pentose phosphates.

These two products of transketolase reaction enter another reaction catalyzed by transaldolase which effects the transfer of a 3-carbon active dihydroxyacetoe moiety from the donor ketose sedoheptulose-7-P to the acceptor aldose glyceraldehyde-3-P to form the ketose fructose-6-P and the 4-carbon aldose erythrose-4-P. Transaldolase is the second enzyme characteristic of non-oxidative phase.

Another reaction involving transketolase takes place in which xylulose-5-P again serves as a donor of active glycol aldehyde whereas erythrose-4-P, however, acts as the acceptor. The products of the reaction are fructose-6-P and glyceraldehyde-3-P.

This is once again the reversal of a reaction involved in photosynthetic CO₂ reduction cycle. The molecule of glyceraldehyde-3-P left over in the second transketolase reaction may either enter glycolysis or may be used to regenerate a molecule of glucose-6-P. In the latter case, four of the enzymes of glycolysis are also required and thus a cyclic nature of the pentose phosphate cycle can be visualized.

These enzymes are:

(i) Triose phosphate isomerase to catalyse inter-conversion of glyceraldehyde-3-P and dihydroxyacetone phosphate,

(ii) Aldolase for the formation of fructose-1, 6-bisphosphate from these two triose phosphates,

(iii) Fructose-1,6-bisphosphatase to hydrolyze fructose-1,6-bisphosphate to fructose-6-P,

(iv) Hexose phosphate isomerase to convert fructose -6- P to glucose-6-P.

These enzymes, however, function in the reverse order of the glycolytic sequence in a phenomenon known as gluconeogenesis. It can be visualized that three molecules of glucose-6-P give rise to three molecules of CO₂ and three pentose phosphates and the latter can be used to regenerate two molecules of glucose-6-P and one molecule of triose glyceraldehyde-3-P is left over.

In order to regenerate the third molecule of glucose-6-P, a glyceraldehyde-3-P is to combine with another triose which must be provided by another three pentose phosphates derived from another set of three glucose-6-P molecules.

Thus for the operation of the cycle and to oxidize glucose completely to CO₂ through pentose phosphate cycle, six molecules of glucose-6-P should enter the cycle, five of the six molecules are recovered, while one is completely degraded to CO₂ and NADH-bound hydrogen.

Regulation of the Pentose Phosphate Pathway:

Pentose phosphate pathway represents an alternate route for the breakdown of carbohydrates.

Glucose-6-P dehydrogenase, the first committed step of pentose phosphate pathway, represents a strategic point of control. It catalyses a non-equilibrium reaction and is strongly inhibited by NADPH.

Variations in the concentration of NADPH, together with opposite changes in NADP+, may be the main controlling factor of glucose-6-P dehydrogenase and may regulate the sequence of reactions through pentose phosphate pathway.

Increased demand for NADPH, as shown by decrease in NADPH/NADP+ ratio, would serve to erase the inhibition of gIucose-6-P dehydrogenase and thus increase the rate of reactions through pentose phosphate pathway.