Enzyme Structure and Catalysis (5E) - MCAT Chemical and Physical Foundations of Biological Systems

Substrate binding causes enzyme conformational change to fit better. This model explains how enzymes adapt their shape upon substrate binding to enhance catalytic efficiency.

Active site is preformed and complementary to the substrate. This rigid model describes perfect geometric complementarity between enzyme and substrate without conformational changes.

Maximum rate when enzyme is saturated with substrate. $V_{max}$ represents the enzyme's turnover rate when all active sites are occupied by substrate.

It lowers the activation energy $E_a$. By stabilizing the transition state, enzymes reduce the energy barrier required for the reaction to proceed.

Enzymes do not change $\Delta G$ of the reaction. Catalysts like enzymes accelerate reactions without altering the overall free energy change between reactants and products.

They stabilize the transition state and provide a lower-$E_a$ pathway. Enzymes accelerate reactions by lowering the activation energy through transition state stabilization.

$v_0=\frac{V_{max}[S]}{K_m+[S]}$. This equation models the hyperbolic relationship between initial velocity and substrate concentration in enzyme kinetics.

Apoenzyme lacks cofactor; holoenzyme is the active complete enzyme. The apoenzyme requires the cofactor to form the functional holoenzyme for catalytic activity.

A tightly bound cofactor that remains associated with the enzyme. Prosthetic groups enhance enzyme function by remaining covalently or tightly bound during catalysis.

Serine, histidine, and aspartate. These residues work together to facilitate nucleophilic attack and proton transfer in peptide bond hydrolysis.

The highest-energy, unstable configuration between reactants and products. The transition state represents the peak of the energy barrier that must be overcome for the reaction to occur.

$[S]$ at which $v_0=\frac{V_{max}}{2}$. $K_m$ reflects the enzyme's affinity for the substrate, where half-maximal velocity is achieved.

A nonprotein helper, often a metal ion, required for activity. Cofactors assist in catalysis by participating in redox reactions or stabilizing intermediates.

An organic cofactor, often vitamin-derived, required for activity. Coenzymes act as carriers of chemical groups or electrons during enzymatic reactions.

$v_0=\frac{V_{max}}{2}$. At $[S]=K_m$, the enzyme operates at half its maximum capacity according to Michaelis-Menten kinetics.

$v_0\approx V_{max}$. High substrate concentrations saturate the enzyme, making the reaction rate independent of $[S]$.

$v_0\approx \frac{V_{max}}{K_m}[S]$. Low substrate levels make the reaction rate directly proportional to $[S]$, following first-order kinetics.

$\frac{1}{v_0}=\frac{K_m}{V_{max}}\frac{1}{[S]}+\frac{1}{V_{max}}$. This linear transformation allows determination of $K_m$ and $V_{max}$ from intercepts on a double-reciprocal plot.

$\frac{1}{V_{max}}$. The y-intercept corresponds to the reciprocal of the maximum velocity in the Lineweaver-Burk equation.

$-\frac{1}{K_m}$. The x-intercept is the negative reciprocal of $K_m$, indicating substrate affinity in the plot.

Inhibitor binds active site; increases $K_m$; $V_{max}$ unchanged. Competitive inhibitors compete for the active site, requiring higher $[S]$ to achieve the same velocity.

Inhibitor binds allosteric site; decreases $V_{max}$; $K_m$ unchanged. Noncompetitive inhibitors reduce enzyme activity by binding elsewhere, without affecting substrate binding affinity.

Inhibitor binds $ES$; decreases $K_m$ and decreases $V_{max}$. Uncompetitive inhibitors bind only to the enzyme-substrate complex, altering both apparent affinity and maximum rate.

The specific region where substrate binds and catalysis occurs. The active site facilitates catalysis by providing a microenvironment that stabilizes the transition state through specific substrate interactions.