classifications of enzymes

Enzyme (Definition)

Biocatalyst that increases reaction rates in living cells without being consumed or permanently altered.

Substrate (Definition)

The reactant molecule upon which an enzyme acts to catalyze a reaction.

Apoenzyme

The inactive protein component of an enzyme, lacking its required cofactor.

Holoenzyme

The active, complete enzyme form consisting of an apoenzyme bound to its cofactor.

Cofactor vs Coenzyme vs Prosthetic Group

Cofactor = general non-protein helper (metal or organic); Coenzyme = loosely bound organic helper; Prosthetic group = tightly bound organic or inorganic helper.

Coenzyme (Binding & Origin)

Binds loosely to the enzyme and is usually organic (often derived from vitamins).

Prosthetic Group (Binding & Composition)

Binds tightly to the enzyme; non-polypeptide unit that can be organic (vitamin, sugar, lipid) or inorganic (metal ion).

Historical Milestone: 1833

Diastase (now known as amylase) was partially isolated.

Historical Milestone: 1878

Kühne coined the term "enzyme" (from Greek "enzumé" meaning "in yeast") to replace "ferment".

Historical Milestone: 1897

Büchner demonstrated sugar fermentation using cell-free yeast extract, proving living cells are not required.

Historical Milestone: 1926

Sumner crystallized the first enzyme (urease from Jack-bean extracts).

Traditional Enzyme Naming Rules

Usually end in "-ase" based on the substrate (e.g., lactase), except proteolytic enzymes which usually end in "-in" (e.g., trypsin).

Fumarase Reaction Type

Hydrates fumarate to form malate (does NOT catalyze a hydrolytic reaction).

EC Main Classes (Mnemonic: OTHLIL-T)

1: Oxidoreductases, 2: Transferases, 3: Hydrolases, 4: Lyases, 5: Isomerases, 6: Ligases, 7: Translocases.

EC 7 Class Addition Date

Translocases were officially added as the 7th main EC class in August 2018.

EC 1: Oxidoreductases (Function)

Catalyze oxidation/reduction reactions (transfer of electrons or hydrogen).

EC 2: Transferases (Function)

Catalyze the transfer of a specific atom or functional group between two molecules.

EC 3: Hydrolases (Function)

Catalyze cleavage of bonds by adding water (hydrolysis).

EC 4: Lyases (Function)

Catalyze group removal or bond cleavage without hydrolysis or oxidation.

EC 5: Isomerases (Function)

Catalyze geometric, structural, or positional rearrangement within a single molecule.

EC 6: Ligases (Function)

Catalyze the joining of two molecules coupled with the breakdown of a pyrophosphate bond in ATP/NTP.

EC 1 (Oxidoreductases) Subgroups (2nd Digit)

1: >CHOH, 2: >C=O, 3: -CH=CH-, 4: Primary amine (-CHNH2), 5: Secondary amine (>CHNH-), 6: NADH/NADPH.

EC 1 (Oxidoreductases) Acceptors (3rd Digit)

1: NAD+/NADP+, 2: Fe3+ (cytochromes), 3: O2, 99: Otherwise unclassified acceptor.

Lactate Dehydrogenase (EC Code & Reaction)

E.C. 1.1.1.27; Catalyzes interconversion of L-lactate and pyruvate using NAD+/NADH.

Amino Acid Oxidase (EC Code & Reaction)

E.C. 1.4.3.3; Catalyzes conversion of D-amino acid, H2O, and O2 into oxo acid, NH4+, and H2O2.

EC 2 (Transferases) Groups Transferred (2nd Digit)

1: 1-carbon group, 2: Aldehyde/Ketone, 3: Acyl, 4: Glycosyl, 7: Phosphate group.

EC 2.7 Phosphotransferases (Kinases) Trivial Name

Usually end in "-kinase" (e.g., Hexokinase = E.C. 2.7.1.1).

EC 2.7 Phosphotransferases Acceptors (3rd Digit)

2.7.1: Alcohol acceptor, 2.7.2: Carboxyl acceptor, 2.7.3: Nitrogenous group acceptor.

Hexokinase (EC Code & Reaction)

E.C. 2.7.1.1; Transfers a phosphate group from ATP to D-hexose (forming D-hexose-6-phosphate + ADP).

EC 3 (Hydrolases) Bonds Cleaved (2nd Digit)

1: Ester, 2: Glycosidic, 4: Peptide, 5: C-N bonds other than peptides.

Alkaline Phosphatase (EC Code & Class)

E.C. 3.1.3.1; Orthophosphoric monoester phosphohydrolase (cleaves organic phosphate to alcohol + inorganic phosphate).

EC 4 (Ly