Enzyme Catalysis, Thermodynamics, and Kinetics Vocabulary

Comprehensive vocabulary flashcards covering enzyme kinetics, catalytic strategies, thermodynamic principles, and enzyme inhibition from the lecture transcripts.

Method of Initial Rates

An experimental technique that measures reaction velocity ($v_0$) at the very beginning of a reaction when product concentration is zero to prevent the reverse reaction from occurring.

Saturation Kinetics

The kinetic behavior observed when high substrate concentration occupies all available active sites on an enzyme, causing the reaction velocity to approach a theoretical maximum ($V_{max}$).

Michaelis Constant ($K_m$)

The specific substrate concentration at which an enzyme operates at half of its maximum velocity ($\frac{1}{2}V_{max}$); expressed in terms of rate constants as $K_m = \frac{k_{-1} + k_2}{k_1}$.

Michaelis-Menten Equation

The fundamental rate equation for single-substrate enzyme reactions relating initial velocity ($v_0$) to substrate concentration ($[S]$): $v_0 = \frac{V_{max}[S]}{K_m + [S]}$.

Lineweaver-Burk Plot

A double reciprocal linear transformation of the Michaelis-Menten equation ($\frac{1}{v_0}$ versus $\frac{1}{[S]}$) where the $y$-intercept equals $\frac{1}{V_{max}}$ and the $x$-intercept equals $-\frac{1}{K_m}$.

Turnover Number ($k_{cat}$)

A first-order rate constant (equivalent to $k_2$ in the simple Michaelis-Menten model) that defines the maximum number of substrate molecules converted to product per unit time per active site when the enzyme is fully saturated.

Specificity Constant

The ratio $\frac{k_{cat}}{K_m}$, which measures an enzyme's catalytic efficiency and substrate preference under physiological (non-saturating) conditions.

Kinetically Perfect Enzyme

An enzyme whose reaction rate is restricted only by the rate at which substrate diffuses into the active site and product diffuses out, attaining specificity constants around $10^8\text{ M}^{-1}\text{s}^{-1}$.

Promiscuous Enzymes

Enzymes that possess the ability to catalyze reactions with a range of structurally related substrates rather than reacting strictly with one single molecule.

Allosteric Regulation

Regulation of enzyme activity caused by the binding of an effector molecule at a regulatory site distinct from the active site, inducing a conformational change between inactive ($T$ state) and active ($R$ state) forms.

Homotopic Allosteric Regulation

A form of allosteric regulation in which the enzyme's substrate itself functions as the allosteric regulatory molecule.

Heterotropic Allosteric Regulation

A form of allosteric regulation in which the regulatory effector is a molecule distinct from the enzyme's substrate.

Concerted Model

An allosteric binding model proposing that all subunits or active sites in a multimeric enzyme transition simultaneously between the low-activity ($T$) state and high-activity ($R$) state in an all-or-none manner.

Sequential Model

An allosteric binding model proposing that substrate binding at one active site induces a conformational change in that specific domain, converting individual subunits from low to high activity sequentially.

General Acid-Base Catalysis

A catalytic strategy in which functional groups or amino acid side chains within the enzyme act directly as proton donors (acids) or proton acceptors (bases) to stabilize reaction intermediates.

Covalent Catalysis

A catalytic strategy involving the temporary formation of a transient covalent bond between the enzyme and substrate to provide a lower energy alternative pathway for the reaction.

Schiff Base

An imine functional group containing a carbon-nitrogen double bond ($C=N$) formed as a temporary covalent intermediate between an enzyme amine group and a substrate carbonyl.

Metalloenzymes

Enzymes containing tightly bound transition metal ions with variable oxidation states that participate directly in catalysis by mediating electron transfer in redox reactions.

Metal-Activated Enzymes

Enzymes that loosely bind group 1 or group 2 cations (such as $Mg^{2+}$) to perform structural roles, assist in substrate orientation, or shield negative electrical charges on substrates like $ATP$.

Cofactors

A broad category encompassing non-protein chemical helpers—including inorganic metal ions and organic coenzymes—that enzymes recruit to assist in catalysis.

Coenzymes

Organic non-protein cofactors derived from dietary vitamins that assist enzymes in carrying out chemical transformations.

Electrostatic Catalysis

A catalytic mechanism where the nonpolar, low dielectric environment of the enzyme active site excludes water and enhances electrical interactions between polar or charged groups on the enzyme and substrate.

Proximity and Orientation Effects

Catalytic enhancement achieved by holding substrates in close proximity and aligning their reactive functional groups in the precise spatial geometry needed for a productive collision.

Transition State Stabilization

The primary mechanism of enzyme catalysis in which the active site binds most tightly to the transition state structure, lowering the activation energy barrier ($\text{Delta } G^{\text{double dagger}}$).

Activation Energy ($ ext{Delta } G^{ ext{double dagger}}$)

The energy barrier representing the difference in free energy between the initial reactants/substrates and the high-energy transition state species.

Competitive Inhibitor

A reversible inhibitor that competes directly with the substrate for binding at the active site; its effect can be overcome at high substrate concentrations, maintaining an unchanged $V_{max}$.

Uncompetitive Inhibitor

A reversible inhibitor that binds exclusively to the enzyme-substrate ($ES$) complex after substrate has bound, preventing the complex from proceeding to form product.

Mixed Inhibitor

A reversible inhibitor capable of binding both to the free enzyme ($E$) and to the enzyme-substrate ($ES$) complex at a site distinct from the active site.

Biochemical Standard State ($ ext{Delta } G^{ ext{prime zero}}$)

A standardized reference state for thermodynamic calculations defined at $298\text{ K}$, $1\text{ atm}$ pressure, $1\text{ M}$ solute concentrations, and a physiological $pH\text{ of }7.0$.