Essential Glossary — CB-02 Terms
Activation Energy: The minimum energy required for a chemical reaction to occur (to reach the transition state). Enzymes lower this barrier.
Alpha-Helix: A right-handed coiled secondary protein structure stabilized by backbone hydrogen bonds between every 4th amino acid (i to i+4). Example: keratin.
Amphipathic: Having both hydrophilic and hydrophobic regions. Property of phospholipids that drives bilayer formation.
Apoenzyme: The protein portion of an enzyme that requires a cofactor for catalytic activity. Inactive alone.
Beta-Pleated Sheet: A secondary protein structure in which extended polypeptide chains run parallel or antiparallel, stabilized by inter-chain hydrogen bonds. Example: silk fibroin.
Cofactor: A non-protein component required by some enzymes for catalytic activity. Types: coenzymes, prosthetic groups, metal ions.
Coenzyme: A loosely bound organic cofactor (often derived from vitamins). Examples: N (from niacin), FAD (from riboflavin), Coenzyme A.
Competitive Inhibition: Inhibition in which the inhibitor structurally resembles the substrate and competes for the active site. Km increases; Vmax unchanged; overcome by excess substrate.
Denaturation: Irreversible disruption of protein secondary/tertiary structure (not primary) by heat, extreme pH, or chemicals, resulting in loss of function.
Disaccharide: A carbohydrate consisting of two monosaccharide units joined by a glycosidic bond. Examples: sucrose, lactose, maltose.
Disulphide Bond (–S–S–): A covalent bond between two cysteine residues. Stabilizes tertiary/quaternary protein structure.
Enzyme Specificity: The ability of an enzyme to act on a particular substrate (or class of substrates) due to complementarity between the active site and the substrate.
Glycosidic Bond: A covalent bond formed between two monosaccharides by dehydration synthesis, linking one sugar's –OH to another's –H.
Holoenzyme: The complete, catalytically active form of an enzyme: apoenzyme + cofactor.
Induced Fit Model: The currently accepted model of enzyme action in which the active site is flexible and reshapes upon substrate binding (Koshland, 1958).
Km (Michaelis Constant): The substrate concentration at which the reaction velocity equals half of Vmax (½ Vmax). A measure of enzyme-substrate affinity (low Km = high affinity).
Lock-and-Key Model: An early model of enzyme action (Fischer, 1894) proposing a rigid active site complementary to the substrate. Now largely replaced by the induced fit model.
Monosaccharide: The simplest (non-hydrolysable) carbohydrate unit. Examples: glucose, fructose (hexoses); ribose, deoxyribose (pentoses).
Non-Competitive Inhibition: Inhibition in which the inhibitor binds an allosteric site (not the active site), causing a conformational change that reduces Vmax. Km is unchanged; cannot be overcome by excess substrate.
Peptide Bond: The covalent bond formed between the carboxyl group of one amino acid and the amino group of another, with the loss of water. Defines primary protein structure.
Polysaccharide: A carbohydrate polymer of many monosaccharides linked by glycosidic bonds. Examples: starch, glycogen (storage); cellulose, chitin (structural).
Primary Structure: The linear sequence of amino acids in a polypeptide, defined by peptide bonds.
Prosthetic Group: A tightly (often covalently) bound non-protein cofactor. Example: haem in haemoglobin and cytochromes.
Quaternary Structure: The association of two or more polypeptide subunits to form a functional multi-subunit protein. Example: haemoglobin (4 subunits).
Ribozyme: An RNA molecule with catalytic activity. Proof that not all enzymes are proteins. Discovered by Cech and Altman (Nobel Prize 1989).
Tertiary Structure: The 3D conformation of a single polypeptide chain, maintained by R-group interactions (disulphide bonds, hydrophobic interactions, ionic bonds, hydrogen bonds).
Transition State: The unstable, highest-energy intermediate between reactants and products in a chemical reaction. Enzymes stabilize the transition state, lowering the activation energy.
Vmax: The maximum velocity of an enzyme-catalysed reaction, achieved when all enzyme active sites are saturated with substrate.
Zymogen (Proenzyme): An inactive precursor of an enzyme that requires specific cleavage (proteolytic activation) to become active. Example: pepsinogen → pepsin.