BI2.1-5 | Enzyme — Glossary

A biological catalyst — usually a protein — that accelerates a specific chemical reaction without being permanently altered, by lowering the activation energy

The specific pocket or cleft on an enzyme where the substrate binds and catalysis occurs, formed by a precise arrangement of amino acid residues

The minimum energy barrier that must be overcome for a chemical reaction to proceed; enzymes lower this barrier to accelerate reactions

The protein portion of an enzyme that is catalytically inactive without its cofactor

The complete, catalytically active form of an enzyme consisting of the apoenzyme plus its cofactor or coenzyme

A non-protein component required for enzyme activity; can be an inorganic metal ion or an organic coenzyme

An organic cofactor, often derived from a vitamin, that participates in the enzymatic reaction as a co-substrate (e.g., NAD+, FAD, CoA)

A cofactor that is tightly or covalently bound to the enzyme and not released during the reaction (e.g., haem, biotin, PLP)

Different molecular forms of the same enzyme that catalyse the same reaction but differ in amino acid sequence, tissue distribution, and kinetic properties

The substrate concentration at which the reaction velocity is half of Vmax; a measure of enzyme-substrate affinity — low Km means high affinity

The maximum reaction velocity achieved when all enzyme molecules are saturated with substrate

The currently accepted model (Koshland, 1958) where the enzyme active site changes shape upon substrate binding to achieve optimal complementarity

Inhibition where the inhibitor competes with the substrate for the active site; increases apparent Km but Vmax is unchanged and can be overcome by excess substrate

Inhibition where the inhibitor binds at a site other than the active site, reducing Vmax without changing Km

Permanent enzyme inactivation by covalent modification of the active site, as seen with aspirin (COX), organophosphates (AChE), and penicillin (transpeptidase)

Regulation of enzyme activity by binding of an effector molecule at a site other than the active site, causing conformational change between active (R) and inactive (T) states

A cardiac-specific protein (I and T isoforms) that is the gold standard serum marker for myocardial infarction, remaining elevated for 7-14 days

The cardiac isoform of creatine kinase, useful for diagnosing myocardial infarction and detecting re-infarction due to its short elevation window (normalises at 72 hours)

A double reciprocal plot of 1/v vs 1/[S] that linearises the Michaelis-Menten equation, allowing graphical determination of Km and Vmax

A bacterial enzyme used as a thrombolytic agent that activates plasminogen to plasmin, dissolving fibrin clots in acute myocardial infarction and stroke

A therapeutic enzyme that depletes blood asparagine, selectively starving ALL cells that cannot synthesise their own asparagine

Treatment of lysosomal storage diseases by infusing recombinant forms of the deficient enzyme (e.g., imiglucerase for Gaucher disease)

A liver and pancreatic beta-cell hexokinase isoform with high Km (~10 mM) that acts as a glucose sensor, becoming active only when blood glucose rises above normal