Chapter 6: Enzymes - The Catalysts of Life

Vocabulary practice flashcards covering fundamental enzyme principles, active site properties, classification, regulation, ribozymes, and enzyme kinetics.

Activation Energy ($E_A$)

The minimum amount of energy required before collisions between reactant molecules will give rise to products.

Transition State

An intermediate chemical stage during a reaction that has a higher free energy than that of the initial reactants.

Metastable State

A state in which thermodynamically unstable reactants lack sufficient activation energy ($E_A$) to react without a catalyst.

Active Site

A characteristic cluster of amino acids within an enzyme formed by 3D protein folding where substrates bind and catalysis takes place.

Cofactor

A nonprotein component located at the active site of an enzyme, such as a metal ion or prosthetic group, that is indispensable for catalytic activity.

Coenzyme

A small organic molecule derivative of vitamins that functions as a cofactor during enzymatic reactions.

Group Specificity

The ability of an enzyme to accept an entire group of closely related substrates sharing a common structural feature.

Oxidoreductases

A class of enzymes that catalyze oxidation-reduction reactions involving electron transfer.

Transferases

A class of enzymes that catalyze the transfer of functional groups from one molecule to another.

Hydrolases

A class of enzymes that catalyze hydrolytic cleavage of one molecule into two molecules.

Lyases

A class of enzymes that catalyze the removal of a functional group from, or addition of a group to, a molecule without hydrolytic cleavage.

Isomerases

A class of enzymes that catalyze the movement or rearrangement of a functional group within a single molecule.

Ligases

A class of enzymes that catalyze the joining of two molecules to form a single molecule.

Lock-and-Key Model

An older model of enzyme action proposing that the active site is rigid and fits the substrate precisely like a key in a lock.

Induced-Fit Model

A model proposing that substrate binding at the active site induces a conformational change in the enzyme to optimize the fit for catalysis.

Substrate Activation

The process of increasing substrate reactivity through active site mechanisms such as bond distortion, proton transfer, or electron transfer.

Ribozymes

Catalytic RNA molecules capable of accelerating specific cellular chemical reactions.

Substrate-Level Regulation

Direct control of enzymatic reaction rates where increases in substrate concentration produce increased reaction rates.

Feedback Inhibition

A regulatory mechanism where the final product of an enzyme pathway negatively regulates an earlier step in the pathway.

Allosteric Regulation

Control of enzyme activity through non-covalent binding of an effector molecule at an allosteric site separate from the active site.

Allosteric Site

A regulatory site on an enzyme, distinct from the active site, where effector molecules bind to alter enzyme conformation.

Protein Kinase

An enzyme that catalyzes phosphorylation by transferring a phosphate group from ATP to a protein substrate.

Protein Phosphatase

An enzyme that catalyzes dephosphorylation by removing phosphate groups from a protein.

Zymogen

An inactive precursor form of an enzyme that is activated by one-time, irreversible proteolytic cleavage.

Competitive Inhibitor

A reversible inhibitor that directly competes with the substrate by binding to the active site of an enzyme.

Noncompetitive Inhibitor

A reversible inhibitor that binds to an enzyme outside the active site, causing a conformational change that reduces catalytic activity.

Irreversible Inhibitor

An inhibitor that covalently binds to an enzyme, causing permanent loss of catalytic activity.

Initial Reaction Velocity ($v$)

The reaction rate measured over a brief initial time interval before substrate concentration decreases enough to affect the rate.

Michaelis Constant ($K_m$)

The specific substrate concentration at which the reaction velocity reaches half of $V_{\max}$, measuring an enzyme's affinity for its substrate.

Maximum Velocity ($V_{\max}$)

The upper limiting rate of an enzymatic reaction achieved when the enzyme is completely saturated with substrate.

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

The rate at which substrate molecules are converted into product by a single enzyme molecule per unit time at maximum velocity ($V_{\max}$).

Lineweaver–Burk Plot

A double-reciprocal plot of $\frac{1}{v}$ versus $\frac{1}{[S]}$ used to visualize kinetic data and determine $K_m$ and $V_{\max}$.

Thomas Cech

The researcher who in 1981 discovered autocatalysis with self-splicing RNA molecules in Tetrahymena.

Sidney Altman

The researcher who showed in the early 1980s that the RNA component of Ribonuclease P (RNase P) alone is capable of cleaving tRNA precursors.

Harry Noller

The researcher who in 1992 demonstrated that rRNA on the large ribosomal subunit contains the active site for peptidyl transferase activity.

Edmond H. Fischer and Edwin G. Krebs

Scientists who won the 1992 Nobel Prize for discovering reversible protein phosphorylation using glycogen phosphorylase.