Top 50 High-Yield MCAT Biochemistry Topics for 2025

Section 1: Enzyme Kinetics (Questions 1–8)

• 1. Km definition: concentration of substrate at half-maximal velocity (Vmax/2). High Km = low affinity.
• 2. Vmax: the maximum reaction rate when enzyme is saturated with substrate.
• 3. Competitive inhibition: inhibitor binds active site, increases apparent Km, Vmax unchanged.
• 4. Noncompetitive inhibition: inhibitor binds allosteric site, decreases Vmax, Km unchanged.
• 5. Uncompetitive inhibition: inhibitor binds enzyme-substrate complex, decreases both Km and Vmax.
• 6. Allosteric regulation: binding at a non-active site changes enzyme conformation and activity.
• 7. Cooperativity (e.g., hemoglobin): binding of one ligand increases affinity for subsequent ligands — sigmoidal curve.
• 8. Feedback inhibition: end product of a pathway inhibits an early enzyme — common exam question type.

Section 2: Metabolism (Questions 9–20)

• 9. Glycolysis: 10 steps, net yield: 2 ATP, 2 NADH, 2 pyruvate. Occurs in cytoplasm.
• 10. Pyruvate dehydrogenase complex: converts pyruvate to acetyl-CoA + CO₂ + NADH. Mitochondrial matrix.
• 11. TCA cycle (Krebs): per turn: 3 NADH, 1 FADH₂, 1 GTP, 2 CO₂. Runs twice per glucose.
• 12. Electron transport chain: NADH → Complex I, FADH₂ → Complex II, final electron acceptor is O₂.
• 13. ATP synthase (Complex V): uses proton gradient (chemiosmosis) to produce ATP. ~32–34 ATP per glucose.
• 14. Gluconeogenesis: makes glucose from pyruvate, lactate, glycerol, amino acids. Liver and kidney only.
• 15. Glycogen synthesis/breakdown: glycogenesis (fed state), glycogenolysis (fasted/exercise).
• 16. Beta-oxidation: fatty acid breakdown in mitochondria. Each cycle: 1 acetyl-CoA, 1 NADH, 1 FADH₂.
• 17. Ketone body synthesis: occurs in liver during prolonged fasting. Brain uses ketones when glucose is low.
• 18. Pentose phosphate pathway: produces NADPH (reducing equivalents for biosynthesis) and ribose-5-phosphate.
• 19. Hormone signaling in metabolism: insulin (fed state, anabolic), glucagon (fasted, catabolic), epinephrine (exercise).
• 20. Reactive oxygen species (ROS): byproducts of ETC, neutralized by catalase, superoxide dismutase, glutathione.

Section 3: Molecular Biology & Genetics (Questions 21–32)

• 21. DNA replication: semiconservative, 5'→3' synthesis only, leading vs lagging strand, Okazaki fragments.
• 22. DNA polymerase III: main replication enzyme in prokaryotes. Requires a primer. Proofreads 3'→5'.
• 23. Transcription: DNA → pre-mRNA. RNA polymerase II (eukaryotes). Promoter, elongation, termination.
• 24. mRNA processing: 5' cap, poly-A tail, splicing of introns by spliceosomes.
• 25. Translation: mRNA → protein. Start codon AUG (Met). Stop codons UAA, UAG, UGA.
• 26. Gene regulation in eukaryotes: enhancers, silencers, transcription factors, chromatin remodeling.
• 27. Epigenetics: methylation (silencing), acetylation (activation). Heritable but not DNA sequence changes.
• 28. Mutation types: silent, missense, nonsense, frameshift. Transitions vs transversions.
• 29. DNA repair: mismatch repair, nucleotide excision repair (UV damage), base excision repair.
• 30. Mendelian genetics: dominant, recessive, incomplete dominance, codominance, sex-linked traits.
• 31. Hardy-Weinberg: p + q = 1, p² + 2pq + q² = 1. Assumes no selection, mutation, migration, drift.
• 32. Recombinant DNA techniques: PCR, restriction enzymes, gel electrophoresis, ELISA, Southern/Northern/Western blot.

Section 4: Cell Biology (Questions 33–42)

• 33. Cell membrane: fluid mosaic model. Cholesterol increases fluidity stability. Saturated fatty acids decrease fluidity.
• 34. Membrane transport: simple diffusion, facilitated diffusion, active transport (requires ATP), osmosis.
• 35. Signal transduction: receptor tyrosine kinases (RTKs), G-protein coupled receptors (GPCRs), second messengers (cAMP, IP₃, DAG, Ca²⁺).
• 36. Cell cycle and cancer: cyclin-CDK complexes, tumor suppressors (p53, Rb), proto-oncogenes vs oncogenes.
• 37. Apoptosis: programmed cell death. Caspase cascade. Bcl-2 family regulates. Important in development and cancer.
• 38. Endoplasmic reticulum: rough ER (protein synthesis & folding), smooth ER (lipid synthesis, detox).
• 39. Golgi apparatus: protein modification, sorting, and secretion. Cis (receives from ER) → trans (secretes).
• 40. Lysosome: acidic compartment containing hydrolytic enzymes. Degrades old organelles (autophagy).
• 41. Cytoskeleton: microfilaments (actin), intermediate filaments (structural), microtubules (cell shape, mitotic spindle, cilia).
• 42. Endocytosis types: phagocytosis (large particles), pinocytosis (fluid), receptor-mediated endocytosis (clathrin-coated pits).

Section 5: Protein Structure & Amino Acids (Questions 43–50)

• 43. Amino acid classification: polar, nonpolar, acidic (Asp, Glu), basic (Lys, Arg, His), aromatic (Phe, Tyr, Trp).
• 44. Protein structure levels: primary (sequence), secondary (α-helix, β-sheet), tertiary (3D folding), quaternary (subunits).
• 45. Protein folding: driven by hydrophobic effect. Chaperones (HSP70) prevent misfolding. Denaturation disrupts non-covalent bonds.
• 46. Disulfide bonds: covalent bonds between cysteine residues. Stabilize tertiary and quaternary structure.
• 47. Hemoglobin vs myoglobin: Hb is tetrameric with cooperative O₂ binding (sigmoidal curve). Mb is monomeric (hyperbolic).
• 48. Bohr effect: CO₂ and H⁺ decrease Hb affinity for O₂ (right shift of O₂ dissociation curve). Promotes O₂ delivery to tissues.
• 49. Enzyme active site: lock-and-key vs induced-fit model. Transition state stabilization is key to catalysis.
• 50. Post-translational modifications: phosphorylation (activation/inactivation), glycosylation, ubiquitination (degradation tag).