Pentose Phosphate Pathway (aka Hexose monophosphate shunt)

Two main functions of the Pentose Phosphate Pathway Produce NADPH (for fatty acid synthesis, cholesterol synthesis, detoxification via CYP450, glutathione activity) Produce ribose-5-phosphate for nucleic acid synthesis

Two phases of the PPP The net reaction of PPP is: 3 G6P + 6 NADP+ + 3 H2O  6 NADPH + 6H+ + 3CO2 + 2 F6P + GAP Oxidative phase (3G6P  3Ru-5-P) Steps 1-3: generation of NADPH equivalents Irreversible process Non-oxidative phase (3Ru-5-P  2 F6P + GAP) Steps 4-8: generation of R5P and/or glycolytic intermediates Reversible process

Step 1 3 G6P enters to PPP at one time, this (and succeeding reactions happen 3 times) The enzyme is highly specific for NADP+; the Km for NAD+ is 1000 greater than for NADP+. Allosterically controlled by NADPH/NADP+ ratio Inducible enzyme by insulin Committed step in PPP

Step 2

Step 3 A second NADPH is formed Oxidative steps are done, a total of 6 NADPH is formed

Steps 4-8

Steps 4 and 5 3 Ru-5-P  2 Xu-5-P + R-5-P (or 3 R-5-P) Ru-5-P  Xu-5-P needs an epimerase Ru-5-P  R-5-P needs an isomerase Relative amounts of Xu-5-P and R-5-P depends on the need of the cell.

Steps 6-8 Recall: Xu-5-P and R5P is generated in the ratio 2:1

Transketolase catalyzes the transfer of C2 from Xu-5-P to R-5-P forming S-7-P and GAP. Requires TPP as cofactor Goes through a TPP-Xu-5-P adduct as intermediate

Transaldolase catalyzes the transfer of C3 from S-7-P to GAP forming E-4-P and F-6-P. NO TPP required

A second transketolase catalyzes the transfer of C2 from Xu-5-P to E-4-P forming a second F-6-P and GAP. Requires TPP as cofactor Goes through a TPP-Xu-5-P adduct as intermediate

Regulation of PPP G6PD is the most regulated enzyme: inhibition by NADPH, expression is dependent on insulin thus it is only expressed at high glucose concentration Since the non-oxidative pathway is reversible, the direction is dependent on the need of the cell for ATP / acetyl CoA (energy / fatty acid synthesis) vs Ribose-5-P (nucleic acid synthesis)

CASE 1: Rapidly dividing cells require more ribose 5- phosphate than NADPH.

Case 2: The need for NADPH and ribose 5-phosphate is balanced.

Case 3: More NADPH is needed than ribose 5-phosphate; Fatty acid synthesis in adipose cells.

Case 4: The cell needs both NADPH and ATP

NADPH production in RBC Spontaneous oxidation of Fe2+ in Hb occurs around 1% per hour, forming superoxides: Hb-Fe2+-O2 -> Hb-Fe3+ + O2-. O2- and H2O2 may cause hemolysis if not turned reduced to H2O via glutathione. Glucose-6-Phosphate dehydrogenase deficiency is a disease characterized by hemolytic anemia