CHEM 350 Lecture 15: Enzyme Kinetics, Inhibition & Control

Vocabulary practice flashcards covering the six enzyme classes, catalytic principles, Michaelis-Menten kinetics, Lineweaver-Burk plots, multi-substrate reactions, and enzyme inhibition mechanisms from CHEM 350.

Oxidoreductases

A class of enzymes that catalyze oxidation-reduction (redox) reactions; most are dehydrogenases, while others are classified as oxidases, peroxidases, oxygenases, or reductases.

Lactate dehydrogenase (LDH)

An oxidoreductase that catalyzes the reversible conversion of lactate to pyruvate coupled to the reduction of the coenzyme $NAD^+$ to $NADH + H^+$.

Transferases

A class of enzymes that catalyze group transfer reactions, often requiring coenzymes, in which the substrate frequently forms a covalent bond to the enzyme or coenzyme.

Kinases

An important class of transferases that specifically catalyze the transfer of a phosphoryl group.

Alanine transaminase (ALT)

A transferase enzyme that catalyzes the transfer of an amino group from alanine to $\alpha$-ketoglutarate, forming pyruvate and L-glutamate.

Hydrolases

A class of enzymes that catalyze hydrolysis reactions, which operate as the reverse of condensation reactions.

Pyrophosphatase

A hydrolase enzyme that catalyzes the cleavage of pyrophosphate (diphosphate) with water into two inorganic phosphates.

Lyases

A class of enzymes that catalyze the lysis of a substrate by cleaving a bond to generate a new double bond via a nonhydrolytic, non-redox elimination reaction (or catalyzing the reverse addition across a double bond).

Pyruvate decarboxylase

A lyase enzyme that catalyzes the scission of a C-C bond in pyruvate, yielding acetaldehyde and carbon dioxide while generating a new double bond.

Isomerases

A class of enzymes that catalyze structural rearrangements within a single molecule; their catalyzed reactions are the simplest enzymatic reactions because they contain only one substrate and one product.

Alanine racemase

An isomerase enzyme that catalyzes the optical interconversion between L-alanine and D-alanine.

Ligases

A class of enzymes (also called synthetases) that catalyze the joining or ligation of two substrates, typically coupling the reaction to NTP (e.g., ATP) hydrolysis to provide the necessary free energy.

Glutamine synthetase

A ligase that utilizes free energy derived from ATP hydrolysis to join an ammonia group to L-glutamate, forming L-glutamine.

Lock-and-key model

An enzymatic model where the enzyme active site and the substrate have preformed, rigid complementary shapes prior to binding.

Induced fit model

An enzymatic model wherein substrate binding induces a conformational distortion in the enzyme active site to align with the transition state conformation.

Initial rate ($v_0$)

The velocity of an enzymatic reaction measured at the very beginning of the reaction as a function of substrate concentration $[S]$, reflecting the rate of product formation.

Michaelis-Menten equation

The mathematical relationship describing enzyme-catalyzed reaction velocity as a function of substrate concentration: $v_0 = \frac{V_{\max}[S]}{K_m + [S]}$.

Michaelis constant ($K_m$)

The substrate concentration at which reaction velocity is half of $V_{\max}$; it serves primarily as a kinetic parameter measuring enzyme-substrate binding affinity, expressed mechanistically as $K_m = \frac{k_{-1} + k_2}{k_1}$.

Maximum velocity ($V_{\max}$)

The upper limiting reaction rate reached when substrate concentration is saturating ($[S] \gg K_m$) and all enzyme active sites are occupied, proportional to total enzyme concentration: $V_{\max} = k_{\text{cat}}[E]_{\text{total}}$.

Steady-state assumption

The kinetic assumption in the Michaelis-Menten derivation stating that the intermediate enzyme-substrate complex concentration ($[ES]$) remains constant over time because its rate of formation equals its rate of decomposition: $k_1([E]_{\text{total}} - [ES])[S] = (k_{-1} + k_2)[ES]$.

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

A catalytic rate constant defined as $k_{\text{cat}} = \frac{V_{\max}}{[E]_{\text{total}}}$, indicating the number of substrate molecules converted to product per active site per unit time when the enzyme is saturated.

Catalytic efficiency

The second-order rate constant given by the ratio $\frac{k_{\text{cat}}}{K_m}$, which measures how effectively an enzyme turns over substrate at very dilute substrate concentrations ($[S] \approx 0$).

Diffusion-limited reaction

An enzymatic condition where catalytic efficiency approaches the theoretical upper limit of $\sim 10^8\,(\text{mol/L})^{-1} s^{-1}$, meaning catalytic rate is limited purely by the speed at which enzyme and substrate diffuse together.

Lineweaver-Burk equation

The linearized double-reciprocal form of the Michaelis-Menten equation: $\frac{1}{v_0} = \left(\frac{K_m}{V_{\max}}\right)\frac{1}{[S]} + \frac{1}{V_{\max}}$, where the slope equals $\frac{K_m}{V_{\max}}$, the y-intercept is $\frac{1}{V_{\max}}$, and the x-intercept is $-\frac{1}{K_m}$.

Ordered sequential reaction

A bisubstrate mechanism where all substrates bind to the enzyme in a strictly mandatory sequence to yield a ternary complex before any products are released in a specific sequence.

Random sequential reaction

A bisubstrate mechanism where substrates can bind in any order to form the central ternary complex before products depart.

Ping-pong reaction

A bisubstrate kinetic mechanism where the first substrate ($A$) binds and yields the first product ($P$), leaving behind a modified enzyme form ($F$) that subsequently binds substrate $B$ to generate product $Q$ and regenerate free enzyme $E$.

Irreversible inhibitors

Inhibitors (also termed inactivators) that permanently abolish enzymatic activity, often by forming covalent bonds or destroying essential functional groups in the active site.

Inhibitor dissociation constant ($K_i$)

The equilibrium dissociation constant reflecting the binding affinity between free enzyme and a reversible inhibitor: $K_d = K_i = \frac{[E][I]}{[EI]}$.

Classical competitive inhibition

A form of reversible inhibition where the substrate ($S$) and inhibitor ($I$) directly compete for binding to the same active site on the enzyme.

Nonclassical competitive inhibition

A form of inhibition where the inhibitor binds to a separate site on the enzyme, inducing a conformational change that prevents substrate binding at the active site, and vice versa.

Uncompetitive inhibition

A reversible inhibition mode where inhibitor ($I$) binds exclusively to the preformed enzyme-substrate ($ES$) complex, halting catalytic production of product.

Noncompetitive inhibition

A reversible inhibition mode where inhibitor ($I$) binds equally to free enzyme ($E$) or the enzyme-substrate complex ($ES$); substrate can still bind the $EI$ complex, but turnover to product is blocked.

Competitive inhibition kinetics

Kinetic profile where $V_{\max}$ remains unchanged because excess substrate can outcompete the inhibitor, while the apparent Michaelis constant increases ($K_m^{\text{app}} > K_m$), pivoting Lineweaver-Burk plots at the y-intercept.

Product inhibition

A regulatory mechanism belonging to competitive inhibition where the enzymatic reaction product ($P$) binds to the enzyme active site and turns off further turnover.

Transition state analog

A competitive inhibitor engineered to structurally mimic the transition state geometry of a reaction, binding to the active site with extremely high affinity to hijack the enzyme.