Allosteric Enzymes and Kinetics: Models, Regulation, and Inhibition

Ligand

A molecule that binds to a specific site on a larger molecule.

Allosteric transition

The ligand-induced conformational change in a protein.

Allosteric enzyme

An enzyme whose activity is affected by the binding of effector molecules.

Sigmoidal binding curve

An S-shaped graph that shows the binding of a molecule to a protein.

Allosteric regulation

A process where a molecule, called an allosteric effector, binds to a protein at a site other than the active site, causing a change in the protein's shape (conformation) that either increases or decreases its activity.

Concerted model

The conformations of all the protein's subunits are believed to change simultaneously when the first effector binds.

Sequential model

The binding of a ligand to one flexible subunit in a multisubunit protein triggers a conformational change that is sequentially transmitted to adjacent subunits.

Allosteric site

A binding site on a protein, distinct from its active site, where molecules can bind to alter the protein's function.

Enzymes

A biomolecule that catalyzes a biochemical reaction.

Emergence

Emergent properties of a complex system have different characteristics from those of its component parts.

Effective concentrations

The concentration of a substance that produces a biological effect.

Substrate

The reactant in an enzyme-catalyzed reaction that is converted to a product.

Transition state

In catalysis, the unstable intermediate formed by the enzyme that has altered the substrate so that it now shares properties of both the substrate and the product.

Catalyst

A substance that enhances the rate of a chemical reaction but is not permanently altered by the reaction.

Lock-and-key model

Each enzyme binds to a single type of substrate because the active site and the substrate have complementary structures.

Induced-fit model

The substrate does not fit precisely into a rigid active site. Instead, noncovalent interactions between the enzyme and substrate change the three-dimensional structure of the active site.

Oxidoreductase

An enzyme that catalyzes an oxidation/reduction reaction.

Lyase

An enzyme that catalyzes the cleavage of C—O, C—C, or C—N bonds, thereby producing a product that contains a double bond.

Ligase

An enzyme that catalyzes the joining of two molecules.

Transferase

An enzyme that catalyzes the transfer of a functional group from one molecule to another.

Isomerase

An enzyme that catalyzes the conversion of one isomer to another.

Vitamins

An organic molecule required by organisms in minute quantities; some vitamins are coenzymes required for the function of certain enzymes.

Velocity

The rate of a biochemical reaction; the change in the concentration of a reactant or product per unit time.

Kinetics

Measure the affinity of enzymes for substrates and inhibitors and provide insight into reaction mechanisms.

Lineweaver-Burk plot

The reciprocals of the initial velocities are plotted as functions of the reciprocals of substrate concentrations, generating a straight line.

Competitive inhibitor

A reversible type of enzyme inhibition in which the inhibitor molecule competes with the substrate for occupation of the active site.

Uncompetitive inhibitor

An inhibitor binds only to the enzyme-substrate complex; a rare type of noncompetitive inhibition.

Noncompetitive inhibitor

Inhibition of an enzyme in which the inhibitor binds to both the free enzyme and the enzyme-substrate complex.

Reversible inhibitor

A form of enzyme inhibition in which the inhibitory effect of a compound can be counteracted by increasing the substrate or removing the inhibitor while the enzyme remains intact.

Irreversible inhibitor

A form of enzyme inhibition in which an inhibitor molecule permanently impairs an enzyme, usually through binding via a covalent bond.

Genetic control

The synthesis of enzymes in response to changing metabolic needs, a process referred to as enzyme induction, allows cells to respond efficiently to changes in their environment.

Zymogen

The inactive form of an enzyme that is converted into an active enzyme by a proteolytic cleavage.

Ping-pong mechanism

The first product is released before the second substrate binds. In such a reaction, the enzyme is altered by the first phase of the reaction.