Enzyme Inhibition and Regulation (5E) - MCAT Chemical and Physical Foundations of Biological Systems

$\frac{k_{cat}}{K_m}$. Catalytic efficiency measures how well an enzyme converts substrate to product at low [S], combining turnover rate kcat with substrate affinity via Km.

$V_{max}=k_{cat}[E]_T$. Vmax is the maximum reaction rate, achieved when all enzyme is saturated, and equals the turnover number kcat times total enzyme concentration [E]T.

$K_m$ is the $[S]$ at which $v_0=\frac{1}{2}V_{max}$. Km represents the substrate concentration at which the reaction rate is half-maximal, reflecting enzyme-substrate affinity in Michaelis-Menten kinetics.

$\frac{1}{v_0}=\frac{K_m}{V_{max}}\frac{1}{[S]}+\frac{1}{V_{max}}$. This linear transformation of the Michaelis-Menten equation allows plotting 1/v0 vs 1/[S] to determine Km and Vmax from slope and intercepts.

$v_0=\frac{V_{max}[S]}{K_m+[S]}$. The equation describes hyperbolic enzyme kinetics, where initial velocity v0 approaches Vmax as [S] increases, with Km as the [S] yielding half Vmax.

Uncompetitive inhibition. Uncompetitive inhibitors bind only to the ES complex, stabilizing it and reducing both Vmax and apparent Km by the same factor.

Pure noncompetitive inhibition. Pure noncompetitive inhibitors bind a site other than the active site on both E and ES equally, reducing effective enzyme concentration and thus Vmax, without affecting substrate affinity or Km.

Competitive inhibition. Competitive inhibitors bind the active site, competing with substrate and requiring higher [S] to reach half Vmax, thus increasing Km while Vmax is unchanged at saturating [S].

Higher apparent substrate affinity. A lower Km means the enzyme achieves half Vmax at lower [S], indicating stronger binding affinity for the substrate.

Active site (competes with substrate). Competitive inhibitors mimic substrate structure and bind directly to the enzyme's active site, preventing substrate access.

Binds only the $ES$ complex. Uncompetitive inhibitors preferentially bind the enzyme-substrate complex, often stabilizing it and reducing productive catalysis.

Competitive inhibition. In Lineweaver-Burk plots, competitive inhibition increases the slope and x-intercept shift, but Vmax is unchanged, so y-intercept remains the same.

Pure noncompetitive inhibition. Pure noncompetitive inhibition decreases Vmax (increases y-intercept) but does not affect Km, leaving the x-intercept unchanged in Lineweaver-Burk plots.

Uncompetitive inhibition. Uncompetitive inhibition causes parallel lines in Lineweaver-Burk plots because it proportionally affects both Km and Vmax, preserving the slope Km/Vmax.

Covalent or permanent inactivation reduces active $[E]_T$. Irreversible inhibitors form stable bonds with the enzyme, permanently decreasing the pool of functional enzyme and thus reducing activity persistently.

Allosteric enzymes show sigmoidal (not hyperbolic) kinetics. Allosteric enzymes exhibit cooperative binding, resulting in a sigmoidal v vs [S] curve, unlike the hyperbolic curve of non-cooperative Michaelis-Menten enzymes.

Decreases apparent $K_{0.5}$ (left-shifts the curve). Allosteric activators enhance enzyme affinity for substrate, shifting the sigmoidal curve leftward and lowering the [S] needed for half-maximal velocity.

Increases apparent $K_{0.5}$ (right-shifts the curve). Allosteric inhibitors reduce enzyme affinity for substrate, shifting the sigmoidal curve rightward and raising the [S] required for half-maximal velocity.

End product allosterically inhibits an earlier enzyme in the pathway. Feedback inhibition regulates pathways by having the final product bind allosterically to inhibit an upstream enzyme, preventing overproduction.

Inhibition is overcome; $V_{max}$ can still be reached. At high [S], substrate can outcompete the inhibitor for the active site, allowing the enzyme to reach its uninhibited maximum velocity.

No; $V_{max}$ remains decreased. Pure noncompetitive inhibitors reduce the effective enzyme concentration, permanently lowering Vmax regardless of [S] increase.

Competitive inhibition. The tripling of Km with unchanged Vmax indicates competition for the active site, characteristic of competitive inhibition.

Pure noncompetitive inhibition. Halving of Vmax with unchanged Km suggests binding to a non-active site that reduces catalytic capacity without affecting substrate affinity.

Uncompetitive inhibition. Proportional decrease in both parameters occurs when the inhibitor binds only ES, stabilizing it and altering kinetics uniformly.