Pentose Phosphate Pathway and Nicotinamide Adenine Dinucleotide Phosphate

Pentose Phosphate Pathway and Nicotinamide Adenine Dinucleotide Phosphate

OVERVIEW

The pentose phosphate pathway (also called the hexose monophosphate shunt) occurs in the cytosol of the cell. It includes two irreversible oxidative reactions, followed by a series of reversible sugar–phosphate interconversions (Figure 13.1). No adenosine triphosphate (ATP) is directly consumed or produced in the cycle. Carbon 1 of glucose 6-phosphate is released as CO₂, and two reduced nicotinamide adenine dinucleotide phosphates (NADPHs) are produced for each glucose 6-phosphate molecule entering the oxidative part of the pathway. The rate and direction of the reversible reactions of the pentose phosphate pathway are determined by the supply of and demand for intermediates of the cycle. The pathway provides a major portion of the body’s NADPH, which functions as a biochemical reductant. It also produces ribose 5-phosphate, required for the biosynthesis of nucleotides, and provides a mechanism for the metabolic use of five-carbon sugars obtained from the diet or the degradation of structural carbohydrates.

Figure 13.1 Pentose phosphate pathway shown as a component of the metabolic map (see Figure 8.2, for a more detailed view of the metabolic pathways). P = phosphate; DHAP = dihydroxyacetone phosphate.