Pentose Phosphate Pathway

Transcription

1 Pentose Phosphate Pathway MDSC1101 Digestion & Metabolism Dr. J. Foster Biochemistry Unit, Dept. Pre-clinical Sciences Facult yof Medical Sciences U.W.I., St. Augustine

2 Lecture objectives (33) State the major products of the pentose phosphate pathway and discuss the importance of this pathway, its intermediates, and by-products. (34) Outline the steps involved in the pentose phosphate pathway.

3 Introduction to the Pathway What is the Pentose Phosphate Pathway (PPP)? alternative catabolic pathway for glucose 6-phosphate alternative names: phosphogluconate pathway, hexose monophosphate shunt called a shunt as it diverts G-6-P from glycolytic pathway into pentoses which can later be returned to glycolysis

4 Introduction to the Pathway What is the Pentose Phosphate Pathway (PPP)? significance - produces ribose 5-phosphate & NADPH unlike glycolysis, it does NOT produce ATP

5 Introduction to the Pathway Significance of PPP differs from cell to cell, e.g. R-5-P needed for nucleic acid & coenzyme synthesis (NAD, FADH etc.) in rapidly dividing cells (e.g. mucosa, skin) NADPH used for reductive synthesis of fatty acids in tissue e.g. liver, adipose, mammary glands Liver, adrenal glands, gonads also use NADPH for cholesterol & steroid anabolism

6 Introduction to the Pathway Significance of PPP differs from cell to cell, e.g. NADPH also used to create protective reducing atmosphere in oxygen-exposed tissue (erythrocytes, eye lens, cornea) NADPH + H + G-S-S-G 2H 2 O PPP FAD gluthathione Se reductase gluthathione peroxidase 2H + NADP + 2G-SH H 2 O 2

7 Introduction to the Pathway The PPP can be partitioned into 2 phases Oxidative (irreversible) phase Nonoxidative (reversible) phase Both phases occur in the cytosol (with glycolysis & gluconeogenesis)

8 Oxidative Phase Results in production of R-5-P, NADPH & CO 2 Involves several enzymes & intermediate products, and is summarized as: G-6-P + 2NADP + + H 2 O R-5-P + CO 2 + 2NADPH + 2H +

9 Oxidative Phase: Reaction 1 G 6-P oxidized to 6-phosphogluconolactone by glucose 6- phosphate dehydrogenase (G6PD) produces the first molecule of NADPH rxn is irreversible and regulatory step of pathway G6PD highly specific for NADP + G6PD strongly inhibited by fatty acid esters of CoA & NADPH

10 Oxidative Phase: Reaction 1 G 6-P oxidized to 6-phosphogluconolactone by glucose 6- phosphate dehydrogenase (G6PD) Adapted from Biochemistry 2 nd ed., Garret & Grisham

11 Oxidative Phase: Reaction 2 6-phosphogluconolactone hydrolyzed by gluconolactonase, opening the ring ring hydrolysis is actually spontaneous due to lactone s unstable structure lactonase significantly speeds up process product formed is 6-phospho-D-gluconate (a sugar acid)

12 Oxidative Phase: Reaction 2 6-phosphogluconolactone hydrolyzed by gluconolactonase, opening the ring Adapted from Biochemistry 2 nd ed., Garret & Grisham

13 Oxidative Phase: Reaction 3 6-phospho-D-gluconate is oxidatively decarboxylated by 6- phosphogluconolactonate dehydrogenase in 2 steps sugar acid is dehydrogenated to a ketoacid using NADP + second molecule of NADPH produced ketoacid then decarboxylated to the pentose sugar ribulose 5- phosphate

14 Oxidative Phase: Reaction 3 6-phospho-D-gluconate is oxidatively decarboxylated by 6- phosphogluconolactonate dehydrogenase in 2 steps Adapted from Biochemistry 2 nd ed., Garret & Grisham

15 Oxidative Phase Reaction 4 Finally ribose-5-p ketoisomerase converts ribulose-5-p to ribose-5-p via an enediol intermediate Adapted from Biochemistry 2 nd ed., Garret & Grisham

16 Nonoxidative Phase Some tissues require more NADPH than pentose phosphate In these tissues the nonoxidative phase is used to recycle glucose 6-P from ribose 5-P Reversible phase of the PPP

17 Nonoxidative Phase: Reaction 1 1 st, ribulose 5-phosphate is epimerised to xylulose 5- phosphate by ribulose 5-phosphate 3-epimerase : epimerase

18 Nonoxidative Phase: Rearrangement Next, a set of cyclic rearrangement reactions involving the xylulose 5-phosphate and several carbon sugars occur Reactions catalysed by either one of 2 enzymes transketolase & transaldolase At the end a net of six 5-carbon sugars are converted to five 6-carbon sugars

19 Oxidative phase PPP Glucose 6-P phosphohexose isomerase Xylulose 5-P Glyceraldehyde 3-P Fructose 6-P Ribulose 5-P TRANSKETOLASE (TPP-dependent) TRANSALDOLASE Ribose 5-P Sedoheptulose 7-P Erythrose 4-P TPP = thiamine pyrophosphate (derived from vitamin B) 2x 5c == 1x 6C

20 2 nd round of non-oxidative phase in PPP Glyceraldehyde 3-P Glyceraldehyde 3-P Xylulose 5-P TRANSKETOLASE (TPP-dependent) Fructose 6-P Fructose 6-P Erythrose 4-P phosphohexose isomerase Glucose 6-P Oxidative phase PPP 3x 5c == 2x 6C

21 PPP Mathematics! Oxidative Phase is: Glucose 6 P + H 2 O + 2 NADP + Ribulose 5 P + CO NADPH + 2H + Nonoxidative Phase is: 3 R 5 P 2 Fructose 6 P + Glyceraldehyde 3 P

22 PPP Mathematics! As 3 mol of Ribulose 5-P go into the nonoxidative phase, the oxidative equation becomes: 3 G-6-P + 3 H 2 O + 6 NADP + 3 R-5-P + 3CO NADPH + 6 H + Thus the sum total of oxidative & nonoxidative phases is: 3 G-6-P + 3 H 2 O + 6 NADP + 2 F-6-P + Glyceraldehyde-3-P + 6 NADPH H + + 3CO 2

23 Further Reading Harper s Biochemistry, 26 th Ed pp