Ch. 26: Pentose Phosphate Pathway

NADPH

key product of PPP, source of reducing power for biosynthesis and antioxidants in the cells

Oxidative Phase of PPP

Generation of NADPH

Non-oxidative phase of PPP

interconversion of sugars

glucose 6-phosphate dehydrogenase (G6PDH)

converts glucose 6-phosphate into 6-phosphoglucono-S-lactone and reduces NADP+ to NADPH, rate limiting enzyme in PPP

Phase I PPP input

Glucose 6-Phosphate, 2 NADP+, and H2O

Phase I PPP output

Ribulose 5-phosphate, 2 NADPH, 2 H+, and CO2

phosphopentose isomerase

isomerizes ribulose 5-phosphate into ribose 5-phosphate

phosphopentose epimerase

converts ribulose 5-phosphate into xylulose 5-phosphate

reaction 1 non-oxidative phase PPP input

ribose 5-phosphate and xylulose 5-phosphate

reaction 1 non-oxidative phase PPP output

glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate

reaction 1 of non-oxidative phase PPP overview

C5 + C5 = C3 + C7

reaction 2 of non-oxidative phase PPP input

glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate

reaction 2 of non-oxidative phase PPP output

Fructose 6-phosphate and erythrose 4-phosphate

reaction 2 of non-oxidative phase PPP overview

C3 + C7 = C6 + C4

reaction 3 of non-oxidative phase PPP input

Xylulose 5-phosphate and erythrose 4-phosphate

reaction 3 of non-oxidative phase PPP output

Fructose 6-phosphate and glyceraldehyde 3-phosphate

Sum of Reactions of PPP

3 Ribose 5-phosphate = 2 Fructose 6-phosphate+ Glyceraldehyde 3-phosphate

NADP+

most important regulatory factor of PPP

Mode 1 of PPP: ribose 5-phosphate needs exceed needs of NADPH

(Low NADP+ and Low Ribose 5-phosphate), Takes fructose 6-phosphate from glycolysis and shuttles it into non-oxidative phase of PPP to generate ribose 5-phosphate

Mode 2 of PPP: Ribose 5-phosphate and NADPH needs are balanced

(High NADP+, Low Ribose 5-phosphate) oxidative phase of PPP generates both ribose 5-phosphate and NADPH

Mode 3 of PPP: NADPH needs exceed needs of ribose 5-phosphate

(High NADP+, High Ribose 5-phosphate, High ATP), oxidative phase forms 2 NADPH and 1 ribose 5-phosphate, non-oxidative phase converts that ribose 5-phosphate to F6-P and GAP, gluconeogenesis converts these into additional glucose 6-phsophate

Mode 4: NADPH and ATP needed

(High NADP+, High ribose 5-phosphate, low ATP), oxidative phase forms 2 NADPH and 1 ribose 5-phosphate, non-oxidative phase converts that ribose 5-phosphate to F6-P and GAP, glycolysis converts F6-P and GAP to pyruvate which can be oxidized to generate ATP

purpose of generating NADPH through PPP

for biosynthesis/antioxidants

purpose of generating ribose 5-phosphate through PPP

for nucleic acid synthesis

Glutathione

made up of Glutamine, Cystiene, and Glycine: helps prevent damage to reactive oxygen species generated in metabolism, reduces and eliminates toxins