CHAPTER 26 The Pentose Phosphate Pathway

Pentose Phosphate pathway is active when there is excess glucose 6-phosphate

The oxidative phase results in production of NADPH, used for reductive biosythesis and Ribose 5-phosphate, used to synthesize DNA and RNA

Glucose 6-phosphate dehydrogenase initiates the oxidative phase of the pentose phosphate pathway with the conversion of glucose 6-phosphate into 6- phosphoglucono-δ-lactone. In the process, glucose 6-phosphate dehydrogenase reduces NADP + to NADPH. A second NADPH is generated in the oxidative phase when 6-phosphogluconate is converted into ribulose 5-phosphate and CO 2.

Ribulose 5-phosphate, generated by the oxidative phase, is isomerized into ribose 5- phosphate.

The nonoxidative phase results in the conversion of three 5-carbon molecules to two 6-carbon molecules and a 3-carbon molecule

The nonoxidative phase consists of three reactions:

Xylulose 5-phosphate, one of the substrates for the transketolase reaction, is formed from its epimer ribulose 5-phosphate.

Two five-carbon sugars are converted into a three-carbon and seven-carbon sugar.

Transaldolase forms a six-carbon and a four-carbon sugar from a three-carbon and a seven-carbon sugar.

Transketolase converts a four-carbon sugar and a five-carbon sugar into a six-carbon sugar and a three-carbon sugar.

The sum of the reactions converts 3 five-carbon sugars into components of the glycolytic and gluconeogenic pathways.

The pentose phosphate pathway can operate in four distinct modes that result from various combinations of the oxidative phase, the nonoxidative phase, glycolysis, and gluconeogenesis. 1.Ribose 5-phosphate needs exceed the needs for NADPH. 2.The NADH and ribose 5-phosphate needs are balanced. 3.More NADPH is needed than ribose 5-phosphate. 4.NADPH and ATP are both required.

If the need for ribose 5-phosphate is greater than the need for NADPH… Two molecules of fructose 6-phosphate and one molecule of glyceraldehyde 3-phosphate are converted to three molecules of ribose 5-phosphate by reversal of the pentose phosphate pathway reactions.

If the need for NADPH and ribose 5-phosphate are similar… One molecule of glucose 6-phosphate is converted to two molecules of NADPH and one molecule of ribose 5-phosphate in the oxidative phase of the pentose phosphate pathway.

If the need for NADPH is greater than the need for ribose 5-phosphate… The oxidative phase of the pentose phosphate pathway forms two molecules of NADPH and one molecule of ribose 5-phosphate (x3). The nonoxidative phase forms two molecules of fructose 6-phosphate and one molecule of glyceraldehyde 3-phosphate from three ribose 5-phosphates. Glucose 6-phosphate is formed from fructose 6-phosphate and glyceraldehyde 3-phosphate.

If both NADPH and ATP are needed… The oxidative phase of the pentose phosphate pathway forms two molecules of NADPH and one molecule of ribose 5-phosphate (x3). Ribose 5-phosphate is converted into fructose 6-phosphate and glyceraldehyde 3-phosphate. Glycolysis produces ATP from fructose 6-phosphate and glyceraldehyde 3-phosphate.