Biochemistry: Enzymes and Enzyme Kinetics

Enzymes are the biological catalysts that make life's reactions fast enough to matter, and the MCAT loves to test how they behave. The high-yield skills are reading a Michaelis-Menten curve, knowing what Vmax and Km mean, and predicting how each class of inhibitor shifts them. Master those and the Bio/Biochem enzyme questions become almost mechanical.

Core Idea

• Enzymes lower the activation energy of a reaction by stabilizing the transition state; they speed up both the forward and reverse reactions equally.
• Enzymes are not consumed and are regenerated unchanged, and they do not alter the equilibrium (Keq) or the overall ΔG of the reaction — they only change how fast equilibrium is reached.
• Enzymes are specific, saturable, and highly regulated, which lets the cell control metabolic flux precisely.

How Enzymes Work: Active Site and Induced Fit

An enzyme binds its substrate at the active site, a small pocket of specific amino acid residues. The older "lock-and-key" idea has been replaced on the MCAT by the induced-fit model: substrate binding causes the active site to change shape and mold around the substrate, optimizing catalysis. Enzymes are named by the reaction they catalyze — the six conceptual classes are oxidoreductases (redox), transferases (move functional groups), hydrolases (hydrolysis), lyases (add/remove groups without hydrolysis or redox), isomerases (rearrange within a molecule), and ligases (join molecules, usually using ATP).

Factors Affecting Activity

• Temperature: activity rises with temperature until the optimum (~37°C for humans); beyond it the protein denatures and activity crashes.
• pH: each enzyme has an optimum pH (pepsin ~2, most cytosolic enzymes ~7.4); extremes disrupt charges and denature the enzyme.
• Substrate concentration: rate rises with [S] until active sites saturate, producing the plateau at Vmax.
• Enzyme concentration: with excess substrate, rate is directly proportional to enzyme amount.

Michaelis-Menten Kinetics

Plotting reaction velocity (v) against substrate concentration [S] gives a hyperbolic curve. Two constants define it:

• Vmax is the maximum rate, reached when all enzyme is saturated with substrate.
• Km is the substrate concentration at which velocity is half of Vmax. Km is an inverse measure of affinity: a low Km means high affinity (little substrate needed to run at half-speed), and a high Km means low affinity.

The Lineweaver-Burk plot is the double-reciprocal linearization (1/v vs 1/[S]). Its y-intercept = 1/Vmax and its x-intercept = -1/Km, which makes it easy to read how an inhibitor changes each value.

Cofactors, Coenzymes, and Inhibition

Many enzymes need helpers. Cofactors are inorganic ions (Zn²⁺, Mg²⁺, Fe²⁺); coenzymes are small organic molecules, often derived from vitamins (NAD⁺ from niacin, FAD from riboflavin, coenzyme A). A tightly, often covalently bound cofactor is a prosthetic group. The enzyme without its cofactor is an apoenzyme; the complete active form is a holoenzyme.

Inhibitors are a favorite MCAT topic. Track their effect on Vmax and Km:

• Competitive: binds the active site, competing with substrate. Overcome by adding more substrate, so Vmax is unchanged but Km increases (apparent affinity drops).
• Noncompetitive: binds an allosteric site on free enzyme or the ES complex equally; Vmax decreases, Km unchanged.
• Uncompetitive: binds only the ES complex; both Vmax and Km decrease.
• Mixed: binds free enzyme and ES with different affinities; Vmax decreases and Km increases or decreases depending on preference.

Regulation: Allostery and Feedback

Allosteric enzymes have regulatory sites separate from the active site and give a sigmoidal (S-shaped) curve because of cooperative subunit binding. Feedback (end-product) inhibition is the classic control loop: the final product of a pathway inhibits an early, committed enzyme, preventing overproduction. Enzymes are also toggled by covalent modification (phosphorylation) and by zymogen cleavage (activating inactive precursors like pepsinogen).

High-Yield Exam Patterns

• "Which increases Km without changing Vmax?" is always the competitive inhibitor — and its effect is reversible by adding substrate.
• Remember low Km = high affinity; the exam repeatedly flips this to trap you.
• On a Lineweaver-Burk plot, competitive inhibitors share the same y-intercept (Vmax constant); noncompetitive inhibitors share the same x-intercept (Km constant).
• Enzymes do not change ΔG or Keq — if an answer says an enzyme makes a reaction "more favorable" or shifts equilibrium, it is wrong.
• A sigmoidal v vs [S] curve signals cooperativity / allosteric regulation (think hemoglobin-style behavior).
• Vitamin-derived coenzymes (NAD⁺, FAD, CoA) are common answer choices for what an enzyme "requires."

Common Traps to Avoid

• Thinking enzymes are consumed or permanently altered — they are regenerated each cycle.
• Confusing high Km with high affinity; it is the opposite.
• Assuming any inhibitor lowers Vmax — competitive inhibitors leave Vmax unchanged.
• Believing an enzyme changes the equilibrium position; it only changes the rate to equilibrium.
• Mixing up cofactor (inorganic) with coenzyme (organic), or apoenzyme (incomplete) with holoenzyme (complete).