MCAT Biochemistry: The Complete High-Yield Review

Biochemistry is one of the highest-yield subjects on the MCAT and one of the most intimidating. Once you understand the underlying logic — energy, regulation, and metabolism — the content becomes manageable.

How Biochemistry Is Tested on the MCAT

MCAT biochemistry appears primarily in the Bio/Biochem section and partially in Chem/Phys. It's tested through passages describing experiments (enzyme kinetics studies, metabolic research) and standalone questions on specific concepts. You need to understand mechanisms, not just memorize reactions.

Amino Acids: What You Actually Need to Know

You do not need to memorize all 20 amino acid structures, but you need to know the charged ones (his, arg, lys are positive at physiological pH; asp and glu are negative), the nonpolar ones that form the hydrophobic core of proteins, and the special cases: cysteine (disulfide bonds), proline (disrupts alpha helices), glycine (smallest, most flexible).

Enzyme Kinetics: Km, Vmax, and Inhibition

This appears on almost every MCAT. Km is the substrate concentration at half-maximal velocity — lower Km means higher affinity. Vmax is the maximum reaction rate at substrate saturation.

Metabolic Pathways: The Big Picture

Glycolysis (cytoplasm)

Glucose → 2 pyruvate, net 2 ATP, 2 NADH. Rate-limiting enzyme: phosphofructokinase-1 (PFK-1). Inhibited by ATP and citrate. Activated by AMP and fructose-2,6-bisphosphate.

Krebs Cycle / TCA (mitochondrial matrix)

Acetyl-CoA + oxaloacetate → 2 CO2, 3 NADH, 1 FADH2, 1 GTP per turn. Key enzymes: isocitrate dehydrogenase (rate-limiting), alpha-ketoglutarate dehydrogenase. Know the carbon counting.

Oxidative Phosphorylation (inner mitochondrial membrane)

NADH yields ~2.5 ATP; FADH2 yields ~1.5 ATP. Complex I–IV form the electron transport chain. Proton gradient drives ATP synthase (Complex V). Uncouplers (DNP) dissipate the gradient as heat.

Gluconeogenesis

Occurs in liver and kidney cortex. Uses pyruvate, lactate, glycerol, and glucogenic amino acids. Key enzymes bypass the irreversible glycolysis steps: PEPCK, fructose-1,6-bisphosphatase, glucose-6-phosphatase.

Regulation You Must Know

• High energy state (high ATP/ADP ratio) → inhibits glycolysis, stimulates gluconeogenesis
• Insulin → activates glycolysis, glycogen synthesis, fatty acid synthesis
• Glucagon/epinephrine → activates glycogenolysis, gluconeogenesis, fatty acid oxidation
• Fasting state → fatty acids become primary fuel, ketone bodies produced