Enzymes: Principles, Kinetics, and Regulation

Vocabulary practice flashcards covering enzyme classification, structural properties, catalytic mechanisms, kinetics, inhibition, and clinical diagnostic applications.

Enzymes

Protein catalysts that increase the velocity of a chemical reaction by lowering the energy of activation of the transition state, accelerating rates by $10^5$ to $10^{17}$ fold without being consumed in the process.

Ribozymes

Catalytic RNA molecules that represent the rare exception to the rule that enzymes are proteins.

Oxidoreductases

The class of enzymes that catalyze oxidation-reduction reactions, such as lactate dehydrogenase.

Transferases

The class of enzymes that catalyze the transfer of carbon-, nitrogen-, or phosphorus-containing groups from one molecule to another, such as serine hydroxymethyl transferase.

Hydrolases

The class of enzymes that catalyze the cleavage of chemical bonds through the addition of water, such as urease.

Lyases

The class of enzymes that catalyze the cleavage of $C-C$, $C-S$, and certain $C-N$ bonds, such as pyruvate decarboxylase.

Isomerases

The class of enzymes that catalyze the racemization or geometric rearrangement of optical or geometric isomers, such as methylmalonyl CoA mutase.

Ligases

The class of enzymes that catalyze bond formation between carbon and $O$, $S$, or $N$, coupled with the hydrolysis of high-energy phosphates, such as pyruvate carboxylase.

Zymogens

Larger, inactive enzyme precursors that require cleavage to become catalytically active once they reach their target tissue.

Holoenzyme

An active enzyme complex consisting of the protein component (apoenzyme) combined with its non-protein component (cofactor or coenzyme).

Apoenzyme

The inactive protein portion of an enzyme that requires the binding of a specific cofactor or coenzyme to become catalytically active.

Cofactor

A non-protein, inorganic component—typically a metal ion such as $Fe^{2+}$ or $Zn^{2+}$—required for an enzyme's catalytic activity.

Coenzyme

A non-protein, small organic molecule, frequently vitamin-derived (e.g., $NAD^+$ from niacin, CoA from pantothenic acid, and FAD from riboflavin), required for enzyme activity.

Active Site

A special pocket or cleft on an enzyme molecule containing amino acid side chains that directly participate in substrate binding and chemical catalysis.

Induced Fit

The conformational change that occurs in an enzyme upon substrate binding, which properly aligns catalytic groups to facilitate the reaction.

Free Energy of Activation

The energy barrier separating reactants from the high-energy transition state; enzymes accelerate reactions by lowering this barrier without altering overall reaction free energy ($\Delta G$) or equilibrium.

Michaelis-Menten Kinetics

Kinetic behavior of enzymes characterized by a hyperbolic curve when plotting reaction velocity ($v_o$) against substrate concentration ($[S]$), approaching a maximal rate ($V_{max}$) at saturating substrate levels.

Allosteric Enzymes

Enzymes that show a sigmoidal velocity curve and are regulated by noncovalent binding of effectors or modifiers at a regulatory site other than the active site.

Competitive Inhibition

A reversible inhibition mechanism in which the inhibitor directly competes with the substrate by binding to the enzyme's active site (e.g., statin drugs like pravastatin competing with HMG-CoA).

Noncompetitive Inhibition

A reversible inhibition mechanism where the inhibitor binds to a site other than the active site, altering the enzyme's conformation and rendering the active site unsuitable for catalysis.

Feedback Inhibition

A regulatory mechanism in which the end product of a biochemical pathway inhibits an enzyme catalyzing an earlier step in the pathway (e.g., ATP inhibiting phosphofructokinase-1).

Covalent Modification

A method of enzyme regulation involving the reversible addition or removal of chemical groups, most commonly the phosphorylation or dephosphorylation of specific serine, threonine, or tyrosine residues.

Cardiac Troponin and CK-MB

Intracellular proteins and enzymes released into the blood plasma following myocardial damage, serving as clinical diagnostic markers for acute myocardial infarction.