Biochem 11.3 The Pentose Phosphate Pathway PDF

## The Pentose Phosphate Pathway ### Introduction The pentose phosphate pathway, sometimes called the hexose monophosphate shunt, acts on intermediates of the glycolysis pathway to ultimately convert glucose molecules into pentose phosphate molecules (i.e., phosphorylated five-carbon sugars). The p...

## The Pentose Phosphate Pathway ### Introduction The pentose phosphate pathway, sometimes called the hexose monophosphate shunt, acts on intermediates of the glycolysis pathway to ultimately convert glucose molecules into pentose phosphate molecules (i.e., phosphorylated five-carbon sugars). The pentose phosphates can then be used to form nucleosides and nucleotides. In addition, part of the pentose phosphate pathway can be used to produce NADPH, which can then be used by the cell as a reducing agent. The pentose phosphate pathway consists of two related but separate sub-pathways. These sub-pathways are sometimes called the different "phases" of the pentose phosphate pathway. Each phase can operate independently, and both can create pentose phosphates. Alternatively, they can work together in sequence, with one phase creating pentose phosphates and the second phase consuming them. ### Glycolysis - Glucose - Glucose 6-phosphate - Fructose 6-phosphate - Glyceraldehyde 6-phosphate - Pyruvate ### Pentose Phosphate Pathway - NADPH +H+ - Pentose phosphates ### 11.3.01 The Oxidative Phase The oxidative phase of the pentose phosphate pathway (PPP) is a sequence of three irreversible enzyme-catalyzed reactions that ultimately result in the production of a pentose phosphate, two molecules of NADPH, and a CO2 molecule. The substrate molecule that enters the oxidative phase is glucose 6-phosphate. Because all three steps of the oxidative phase are irreversible, the first step in the sequence (acting on glucose 6-phosphate) is the earliest committed (and therefore rate-determining) step. The enzyme that catalyzes this reaction is glucose-6-phosphate dehydrogenase (G6PD). As its name implies, G6PD is an oxidoreductase that oxidizes (or dehydrogenates) the anomeric carbon of glucose 6-phosphate into a cyclic ester called 6-phosphoglucono-d-lactone. Unlike the enzymes of glycolysis, G6PD does not use NAD+ as a coenzyme; instead, it uses a related but different coenzyme called NADP+, which becomes NADPH. ### 11.3.02 The Nonoxidative Phase The nonoxidative phase of the pentose phosphate pathway (PPP) consists of a sequence of reversible reactions that convert intermediates of glycolysis into pentose phosphates. Unlike the oxidative phase, in which some mass is lost as CO2 and some reducing power is transferred to NADPH, the nonoxidative phase uses only reversible transfer reactions, so no mass or reducing power is lost to other molecules. The reactions of the nonoxidative phase are unlikely to be tested on the exam; however, a common misconception is that the nonoxidative phase serves only as a continuation of the oxidative phase to re-enter glycolysis, rather than as a separate sequence of reactions that can operate independently of the oxidative phase. To emphasize this important property, the nonoxidative phase is presented here proceeding from the intermediates of glycolysis toward production of pentose phosphates. Unlike the oxidative phase, which produces one pentose phosphate from one hexose phosphate, the nonoxidative phase requires multiple substrate molecules and produces multiple pentose phosphate molecules. The stoichiometry of the overall reaction can be easily remembered as 5 hexose phosphates 6 pentose phosphates. ### Concept Check 11.4 A resting cell receives a signal to upregulate one of the following processes. Based on the metabolic needs of the process, will the cell experience increased flux (in either direction, if applicable) through the oxidative phase, the nonoxidative phase, or both phases of the pentose phosphate pathway? - Increased DNA replication (requires both pentose phosphates and NADPH) - Increased gene transcription (requires pentose phosphates only) - Increased fatty acid synthesis (requires NADPH only)