Biochemistry and Metabolism: Energetics, Enzyme Kinetics, and Pathway Logic
Core concepts
Know the major catabolic and anabolic routes and their regulation: glycolysis, pyruvate dehydrogenase, the citric acid cycle, oxidative phosphorylation, gluconeogenesis, glycogen metabolism, the pentose phosphate pathway, fatty acid oxidation and synthesis, and amino acid transamination. Track the key regulated enzymes (phosphofructokinase-1, pyruvate dehydrogenase, isocitrate dehydrogenase, glycogen phosphorylase, acetyl-CoA carboxylase) and their allosteric and covalent controls. Understand thermodynamic coupling, redox carriers (NAD+/NADH, FAD/FADH2, NADP+/NADPH), and how substrate-level phosphorylation differs from oxidative phosphorylation. Enzyme kinetics-Michaelis-Menten, Lineweaver-Burk interpretation, allosteric sigmoidal behavior-is fair game.
Traps
Items frequently test compartmentation: fatty acid oxidation occurs in mitochondria while synthesis occurs in the cytosol, and NADPH (not NADH) drives reductive biosynthesis. Another trap is confusing the fate of pyruvate under anaerobic conditions (lactate in animals, ethanol in yeast) and miscounting ATP equivalents. Watch for questions that assume a pathway is running forward when the cell's energy charge would favor reversal, and for inhibition types mislabeled on reciprocal plots.
Worked rule of thumb
Apply the energy-charge heuristic: high ATP/AMP ratio favors anabolism and inhibits catabolic flux at committed steps; low ratio does the reverse. When a question asks "what happens to flux," identify the committed, regulated step and ask whether the stated effector activates or inhibits that enzyme-then propagate downstream. This avoids memorizing every intermediate.
Study strategy
Build a one-page map linking pathways at shared metabolites (glucose-6-phosphate, pyruvate, acetyl-CoA, oxaloacetate) so you can answer crossover questions confidently.