Each round of two-carbon addition in fatty acid synthesis consumes reducing equivalents. What is the principal source of cytosolic NADPH for de novo palmitate synthesis in adipose tissue?

• A Mitochondrial electron transport through complex I
• B Beta-oxidation of preformed fatty acids
• C The hexose monophosphate shunt and malic enzyme ✓
• D Direct reduction of NADP+ by citrate lyase

Explanation

Synthesis of one palmitate molecule requires 14 NADPH. Most comes from the pentose phosphate pathway (glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase), with the remainder supplied by malic enzyme, which converts cytosolic malate to pyruvate while generating NADPH as part of the citrate shuttle. Electron transport produces NADH and ATP, not NADPH, and citrate lyase cleaves citrate at the expense of ATP without redox chemistry.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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