Biochemistry: Metabolism
Free MCAT study guide — Biological and Biochemical Foundations of Living Systems
Glycolysis
Location: Cytoplasm. Net yield per glucose: 2 ATP, 2 NADH, 2 pyruvate.
Key Point: Three irreversible steps: Hexokinase (step 1), PFK-1 (step 3, rate-limiting), Pyruvate kinase (step 10).
TCA Cycle
Location: Mitochondrial matrix. Per turn: 3 NADH, 1 FADH₂, 1 GTP, 2 CO₂.
Key Point: Isocitrate dehydrogenase is rate-limiting. Activated by ADP, inhibited by ATP and NADH.
Oxidative Phosphorylation
NADH → Complex I → CoQ → Complex III → Cyt c → Complex IV → O₂ + H₂O
FADH₂ → Complex II → CoQ (bypasses Complex I)
~2.5 ATP per NADH | ~1.5 ATP per FADH₂ | ~30-32 ATP per glucose total
Key Point: Cyanide/CO inhibit Complex IV. Oligomycin inhibits ATP synthase. DNP/2,4-DNP uncouple (dissipate H⁺ gradient).
Other Pathways
| Pathway | Location | Key Product | When Active |
|---|---|---|---|
| Gluconeogenesis | Liver cytosol/mito | Glucose | Fasting (glucagon) |
| Glycogenolysis | Liver, muscle | Glucose/G6P | Fasting, exercise |
| β-Oxidation | Mito matrix | Acetyl-CoA, NADH, FADH₂ | Fasting |
| Ketogenesis | Liver mito | Ketone bodies | Prolonged fasting/starvation |
| PPP | Cytoplasm | NADPH, Ribose-5-P | When NADPH or nucleotides needed |
| Fatty acid synthesis | Cytoplasm | Palmitate | Fed state (insulin) |
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