5: Enzyme Information

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Enzyme Classification

For ease of communication, enzymes have been classified into 6 classes. Every enzyme has a systematic name and code number, and a common name (or working name- typically less cumbersome). The systematic name tells you something about the reaction, whereas the common name may or may not. Below is a list of the systematic classes of enzyme names and the basis for their classification.

1. Oxidoreductase- Transfer of electrons, oxidation/reduction reactions

Nomenclature: donor:acceptor oxidoreductase example: lactate:\(\mathrm{NAD^{+}}\) oxidoreductase

(For reactions that reduce/oxidize \(\mathrm{NAD^{+}}\), \(\mathrm{NAD^{+}}\) is always written as acceptor independent of physiological direction of the reaction).

Common names in this class:

dehydrogenase Transfer of hydrogen from a donor to an acceptor other than oxygen.

oxidase Transfer of hydrogen (or elect.) from a donor to oxygen as the acceptor

oxygenase Oxygen incorporated into substrate

2. Transferase Transfer of a group from one compound (called the donor) to another (the acceptor)

Nomenclature: donor:acceptor grouptransferase example: alanine: \(\alpha\)-KG aminotransferase

(ATP always written as donor independent of physiological direction of reaction).

(\(\alpha\)-KG (= \(\alpha\)ketoglutarate) always written as acceptor independent of physiological direction).

Common names in this class:

Kinase (=phosphotransferase) Transfers a phosphoryl group from one compound to another

transaminase(=aminotransferase) Transfers an amino group to a keto group

3. Hydrolase Hydrolytic cleavage-(can be thought of as transfer to \(\mathrm{H_{2}O}\)). \(\mathrm{H_{2}O}\) used to split a single bond with the inclusion of water into two products.

Nomenclature: substrate grouphydrolase

Example: pyrophosphate phosphohydrolase

Common subtypes in this class:

phosphatase Removal of phosphate esters example: pyrophosphatase

4. Lyase Cleavage of C-C, C-O, C-N, and other bonds by elimination, leaving a double bond, or the reverse reaction- the addition of groups to double bonds.

Nomenclature: substrate group-lyase (hyphen is essential in name)

This is always written in the direction of cleavage. Thus substrate is the molecule that gets cleaved. What is written as the group in the name? Typically, the small group is written. When the two cleavage products are of equivalent size, then the group that underwent electronic rearrangement is designated.

Common leaving groups:

hydro-lyase (NOT hydrolase) carboxy-lyase

thiol-lyase ammonia-lyase

5. Isomerase Intramolecular rearrangement

Nomenclature: substrate type of rearrangementisomerase

e.g., cis-transisomerase/ ketolisomerase

Common subtypes in this class:

mutase Intramolecular transfer of a group

epimerase Interconversion of chirality at one C

6. Ligase Joining of two molecules with hydrolysis of nucleoside triphosphate

Nomenclature: X:Y ligase (____ forming)

another name for this type of reaction synthetase (all ligases could also be called synthetases)

CAREFUL: The term synthase is not a systematic class name, but a common name. Synthase does not tell you the reaction class, but synthetase indicates specifically the ligase class. Synthetase is synonymous with ligase.