Enzyme Structure and Catalytic Mechanisms (1A) - MCAT Biological and Biochemical Foundations of Living Systems

Noncompetitive (pure) inhibition. Binding at a separate site affects enzyme function without altering substrate affinity.

$K_m$ increases; $V_{max}$ unchanged. Inhibitor decreases apparent affinity but maximum velocity is achievable with excess substrate.

Competitive inhibition. Inhibitor competes with substrate for the active site, reducing available enzyme for catalysis.

$\frac{k_{cat}}{K_m}$. This ratio integrates turnover and affinity, enabling comparison of enzymes under non-saturating conditions.

Catalytic cycles per enzyme per second at saturation: $k_{cat}=\frac{V_{max}}{[E]T}$. $k{cat}$ measures enzyme efficiency by quantifying substrate conversions per unit time at saturation.

Maximum rate when enzyme active sites are saturated with substrate. $V_{max}$ quantifies the enzyme's full catalytic capacity when all sites are occupied.

High apparent affinity (less substrate needed to reach $\frac{V_{max}}{2}$). Lower $K_m$ means stronger binding, requiring less substrate for significant activity.

$v_0 = \frac{V_{max}[S]}{K_m + [S]}$. This hyperbolic equation models enzyme kinetics under steady-state assumptions for single-substrate reactions.

Substrate concentration where $v_0 = \frac{V_{max}}{2}$. $K_m$ reflects enzyme-substrate affinity, indicating half-maximal velocity substrate level.

$\Delta G$ is unchanged; $E_a$ is decreased. Enzymes affect reaction kinetics but not the overall free energy change or equilibrium position.

They lower the activation energy $E_a$ by stabilizing the transition state. By reducing $E_a$, enzymes increase the fraction of molecules with sufficient energy to react.

The highest-energy, unstable intermediate along the reaction pathway. It represents the peak energy barrier that must be overcome for reactants to form products.

Active site is preformed and complementary to the substrate shape. Rigid enzyme structure ensures precise substrate recognition without conformational changes during binding.

Substrate binding triggers a conformational change that improves fit. Enzyme flexibility allows adjustment to substrate shape, enhancing binding specificity and catalytic efficiency.

$V_{max}$ decreases; $K_m$ unchanged. Inhibitor reduces functional enzyme concentration without impacting substrate binding affinity.

Enzymes change $E_a$ (decrease it); they do not change $K_{eq}$. Enzymes accelerate both forward and reverse reactions equally, preserving equilibrium but reducing energy barrier.

General acid-base catalysis. Active-site amino acids act as acids or bases to stabilize intermediates via proton transfer.

Covalent catalysis. Temporary bond formation facilitates reaction by altering substrate reactivity.

Apoenzyme: protein alone; holoenzyme: apoenzyme plus required cofactor(s). Apoenzyme lacks cofactors and is inactive; holoenzyme is the complete, functional form.

Coenzyme binds transiently; prosthetic group is tightly or covalently bound. Binding duration distinguishes transient helpers from permanently integrated ones in enzyme structure.

A nonprotein helper required for activity (metal ion or organic molecule). Cofactors provide essential chemical functionalities that the protein component lacks for catalysis.

Both $K_m$ and $V_{max}$ decrease. Apparent affinity increases, but overall catalytic capacity is reduced due to trapped complexes.

Uncompetitive inhibition. Inhibitor stabilizes the enzyme-substrate complex, preventing product release.

A biological catalyst that increases reaction rate without being consumed. Enzymes accelerate biochemical reactions by lowering activation energy while remaining unaltered post-reaction.