CHM 3270: Enzyme Kinetics, Inhibition, and Catalytic Mechanisms

Comprehensive vocabulary flashcards covering enzyme kinetics, Michaelis-Menten and Lineweaver-Burk parameters, modes of inhibition, enzyme regulation, Enzyme Commission classes, and catalytic mechanisms from Chapter 7.

Michaelis-Menten Equation

The fundamental kinetic equation $v = \frac{V_{\max}[S]}{K_M + [S]}$, used to calculate reaction velocity at a given substrate concentration.

Turnover Number ($k_{\text{cat}}$)

A kinetic parameter defined as $k_{\text{cat}} = \frac{V_{\max}}{[E]_T}$, measuring the number of substrate molecules converted to product per enzyme active site per second.

Catalytic Efficiency ($\frac{k_{\text{cat}}}{K_M}$)

The specificity constant measuring both substrate binding and catalytic velocity; for catalytically perfect enzymes, it reaches values between $10^8\,M^{-1}s^{-1}$ and $10^{10}\,M^{-1}s^{-1}$.

$$V_{\max}$$

The maximum reaction velocity achieved when all enzyme active sites are fully saturated with substrate.

$K_M$ (Michaelis Constant)

The substrate concentration at which reaction velocity is half of maximum ($v = \frac{V_{\max}}{2}$); lower values indicate higher substrate affinity and tighter binding.

Lineweaver-Burk Equation

The double-reciprocal form of the Michaelis-Menten equation given by $\frac{1}{v} = \frac{K_M}{V_{\max}}\frac{1}{[S]} + \frac{1}{V_{\max}}$, where slope is $\frac{K_M}{V_{\max}}$, y-intercept is $\frac{1}{V_{\max}}$, and x-intercept is $-\frac{1}{K_M}$.

Competitive Inhibition

Reversible inhibition where the inhibitor binds directly to the active site ($E + I \rightleftharpoons EI$), increasing apparent $K_M$ while $V_{\max}$ remains unchanged, yielding the same y-intercept on a Lineweaver-Burk plot.

Uncompetitive Inhibition

Reversible inhibition where the inhibitor binds exclusively to the enzyme-substrate complex ($ES + I \rightleftharpoons ESI$), decreasing both apparent $K_M$ and $V_{\max}$ and producing parallel lines on a Lineweaver-Burk plot.

Noncompetitive Inhibition

Inhibition in which the inhibitor binds free enzyme and the enzyme-substrate complex with equal affinity, leaving $K_M$ unchanged while decreasing $V_{\max}$, yielding the same x-intercept on a Lineweaver-Burk plot.

Mixed Inhibition

Inhibition where the inhibitor binds both free enzyme and the enzyme-substrate complex with different affinities, causing $V_{\max}$ to decrease and apparent $K_M$ to increase or decrease, with lines intersecting left of the y-axis on a Lineweaver-Burk plot.

Allosteric Regulation

Regulation of enzyme activity occurring when an effector binds to a site other than the active site, resulting in a sigmoidal (S-shaped) kinetic curve rather than a hyperbolic Michaelis-Menten curve.

Feedback Inhibition

A regulatory mechanism where the end product of a metabolic pathway inhibits an upstream enzyme to prevent excessive product formation and maintain homeostasis.

Covalent Modification

The reversible alteration of enzyme catalytic activity by chemical modification, such as phosphorylation catalyzed by kinases and dephosphorylation catalyzed by phosphatases.

Lock-and-Key Model

An early, rigid model of enzyme action proposing that the active site is preformed and perfectly complementary to the substrate prior to binding.

Induced Fit Model

The widely accepted, flexible model of enzyme action proposing that the enzyme changes its conformation upon substrate binding to establish an optimal catalytic fit.

Oxidoreductases

Enzyme Commission (EC) Class 1 enzymes that catalyze oxidation-reduction reactions, such as lactate dehydrogenase.

Transferases

Enzyme Commission (EC) Class 2 enzymes that catalyze the transfer of functional groups from one molecule to another, such as kinases.

Hydrolases

Enzyme Commission (EC) Class 3 enzymes that catalyze bond cleavage through the addition of water, such as proteases.

Lyases

Enzyme Commission (EC) Class 4 enzymes that catalyze the addition or elimination of functional groups without hydrolysis or oxidation, such as enolase.

Isomerases

Enzyme Commission (EC) Class 5 enzymes that catalyze structural rearrangements within a single molecule, such as triose phosphate isomerase.

Ligases

Enzyme Commission (EC) Class 6 enzymes that catalyze the joining of two molecules coupled to the consumption of ATP, such as DNA ligase.

Activation Energy ($\Delta G^\ddagger$)

The free energy barrier determining reaction rate; enzymes increase reaction rates by lowering $\Delta G^\ddagger$ without altering overall reaction favorability ($\Delta G$) or the equilibrium constant.

Rate Enhancement

The factor by which an enzyme accelerates a reaction, determined by $\text{Rate Enhancement} = e^{(\Delta\Delta G^\ddagger/RT)}$, where a $5.7\,kJ\,mol^{-1}$ decrease in $\Delta G^\ddagger$ at $25^\circ\text{C}$ yields approximately a $10$-fold rate increase.

Preferential Transition-State Binding

The catalytic strategy in which the enzyme binds the transition state more tightly than the ground-state substrate, representing the primary source of enzymatic rate acceleration.

Serine Protease Catalytic Triad

A conserved active-site network of three amino acid residues—Asp102, His57, and Ser195—that cooperatively facilitates peptide bond cleavage via covalent and acid-base catalysis.

Triose Phosphate Isomerase (TIM)

A catalytically perfect enzyme that interconverts dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP) using Glu165 and His95 via an enediol intermediate.

Ribonuclease A (RNase A)

An enzyme that cleaves RNA using general acid-base catalysis and electrostatic stabilization mediated by active-site residues His12, His119, and Lys41.

Enolase

A glycolytic enzyme that converts 2-phosphoglycerate to phosphoenolpyruvate, utilizing $Mg^{2+}$ ions to stabilize a carbanion intermediate during water elimination.