Biochemistry
Core metabolic and molecular concepts
Biochemistry questions reward a map-like understanding of pathways. Know glycolysis, pyruvate dehydrogenase, the tricarboxylic acid cycle, oxidative phosphorylation, gluconeogenesis, glycogen metabolism, the pentose phosphate pathway, fatty acid oxidation, ketogenesis, the urea cycle, and amino acid catabolism. For molecular biology, understand DNA replication, transcription, RNA processing, translation, and common mutations. Enzyme kinetics appears as Michaelis-Menten behavior, competitive versus noncompetitive inhibition, and the meaning of Km and Vmax.
Cofactors and classic traps
Many questions test vitamin-derived cofactors. Thiamine pyrophosphate is used by pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase. Biotin is used by carboxylases. Vitamin B12 and folate intersect in one-carbon metabolism; B12 deficiency can cause megaloblastic anemia with neurologic signs, while folate deficiency causes megaloblastic anemia without the same neurologic pattern. Do not confuse the rate-limiting enzyme with the regulated or irreversible step. For glycolysis, phosphofructokinase-1 is the key regulated enzyme. For gluconeogenesis, the four bypass reactions avoid the irreversible glycolytic steps.
Worked example
A patient with alcohol use disorder develops lactic acidosis. Pyruvate dehydrogenase requires thiamine, so thiamine deficiency shunts pyruvate to lactate. The rule of thumb: before giving glucose in a malnourished or alcoholic patient, give thiamine, because glucose load can worsen thiamine-dependent neurologic injury. This combines pathway logic with clinical sequencing.