Enzymes: Nature, Kinetics, Classification, and Regulation

Enzyme

A biological catalyst, usually a three-dimensional globular protein, that speeds up the rate of biochemical reactions without being consumed in the process.

Activation Energy

The minimum input of energy required before reactant molecules can undergo a chemical reaction.

Anabolism

The set of metabolic pathways that build complex biological molecules from simpler ones, requiring an input of energy.

Catabolism

The set of metabolic pathways that break down complex organic molecules into simpler ones, releasing energy.

Necrosis

The localized death of living tissue, which damages cell membranes and causes intracellular enzymes to leak out into the bloodstream.

Active Site

A specialized pocket or cleft in an enzyme lined with amino acid side chains where substrate binding and catalysis occur through noncovalent forces.

Substrate

The specific reactant molecule that binds to an enzyme's active site and is converted into product.

Ribozyme

A non-protein catalytic RNA molecule that speeds up cellular reactions involving nucleic acids, such as the processing of RNA.

Apoenzyme

The inactive, protein portion of a conjugated or complex enzyme that requires a non-protein cofactor or coenzyme to function.

Cofactor

A non-protein, inorganic substance, such as metal ions like $\text{Fe}^{2+}$, $\text{Fe}^{3+}$, $\text{Cu}^{2+}$, or $\text{Zn}^{2+}$, bound within an enzyme to assist catalytic activity.

Holoenzyme

The complete, catalytically active enzyme system consisting of an apoenzyme combined with its essential cofactor or coenzyme.

Coenzyme

A small organic molecule, frequently derived from vitamins, that acts as a carrier molecule for chemical groups during enzymatic reactions.

Cosubstrate

A type of coenzyme that binds loosely or transiently to an enzyme near the active site and dissociates from it in an altered state (e.g., $\text{NAD}^+$).

Prosthetic Group

A non-protein cofactor or coenzyme that is tightly or permanently bound to an enzyme structure (e.g., FAD).

EC Numbering System

A systematic nomenclature established by the Enzyme Commission where each enzyme is described by four numbers preceded by EC, specifying its main class, subclass, sub-subclass, and serial number.

Oxidoreductases

The main class of enzymes (EC 1) that catalyze oxidation-reduction reactions, such as the transfer of electrons, hydride ions, or hydrogen atoms.

Phenolase

A copper-containing oxidoreductase enzyme (polyphenoloxidase) responsible for the enzymatic browning of cut fruits and vegetables exposed to air.

Transferases

The main class of enzymes (EC 2) that catalyze the transfer of functional groups (such as methyl or phosphate groups) between donor and acceptor molecules.

Hydrolases

The main class of enzymes (EC 3) that catalyze the cleavage of chemical bonds by adding water across them (hydrolysis).