Enzyme Nomenclature

EC 1 Introduction

EC 1. Oxidoreductases

To this class belong all enzymes catalysing oxido-reductions. The substrate oxidized is regarded as hydrogen or electron donor. The classification is based on 'donor:acceptor oxidoreductase'. The Accepted name is 'dehydrogenase', wherever this is possible; as an alternative, 'acceptor reductase' can be used. 'Oxidase' is used only where O2 is an acceptor. Classification is difficult in some cases, because of the lack of specificity towards the acceptor.

EC 1.1 Acting on the CH-OH Group of Donors

This subclass contains all dehydrogenases acting on primary alcohols, secondary alcohols and hemi-acetals. They are further classified according to the acceptor which can be NAD+ or NADP+ (subclass EC 1.1.1), cytochrome (EC 1.1.2), oxygen (EC 1.1.3), a disulfide (EC 1.1.4), quinone (EC 1.1.5) or another acceptor (EC 1.1.99).

EC 1.2 Acting on the Aldehyde or Oxo Group of Donors

This subclass contains all enzymes oxidizing aldehydes to the corresponding acids; when this acid is concomitantly phosphorylated or acetylates CoA, this is indicated in parentheses. Oxo groups may be oxidized either with addition of water and cleavage of a carbon-carbon bond or, in the case of ring compounds, by addition of the elements of water and dehydrogenation. This subclass is subdivided according to the acceptor, which may be NAD(P)+ (EC 1.2.1), a cytochrome (EC 1.2.2), oxygen (EC 1.2.3), a disulfide (EC 1.2.4), an iron-sulfur protein (EC 1.2.7) or some other acceptor (EC 1.2.99).

EC 1.3 Acting on the CH-CH Group of Donors

In this subclass are listed all enzymes introducing a double-bond into the substrate by direct dehydrogenation at a carbon-carbon single bond. Sub-subclasses are based on the acceptor; EC 1.3.1 with NAD(P)+, EC 1.3.2 with cytochrome, EC 1.3.3 with oxygen, EC 1.3.5 with a quinone or related compound, EC 1.3.7 with an iron-sulfur protein and EC 1.3.99 with other acceptors.

EC 1.4 Acting on the CH-NH2 Group of Donors

These are the amino-acid dehydrogenases and the amine oxidases. In most cases, the imine formed is hydrolysed to give an oxo-group and NH3. This is indicated as (deaminating). Sub-subclasses are formed according to acceptor: EC 1.4.1 with NAD(P)+, EC 1.4.2 with a cytochrome, EC 1.4.3 with oxygen, EC 1.4.4 with a disulfide, EC 1.4.5 with a quinone or similar compound as acceptor, EC 1.4.7 with an iron-sulfur protein as acceptor, and EC 1.4.99 with other acceptors.

EC 1.5 Acting on CH-NH Group of Donors

Enzymes dehydrogenating secondary amines, introducing a C=N double bond as the primary reaction. In some cases, this is later hydrolysed. Sub-subclasses are EC 1.5.1 with NAD(P)+, EC 1.5.3 with oxygen as acceptor, EC 1.5.4 with a disulfide as acceptor, EC 1.5.5 with a quinone as acceptor, EC 1.5.57 with an iron-sulfur protein as acceptor, EC 1.5.8 with a flavin as acceptor and EC 1.5.99 with other acceptors.

EC 1.6 Acting on NADH or NADPH

In general, enzymes using NADH or NADPH to reduce a substrate are classified according to the reverse reaction, in which NAD+ or NADP+ is formally regarded as acceptor. In subclass EC 1.6, only those enzymes are listed in which some other redox carrier is the acceptor. This can be either NAD(P)+, in sub-subclass EC 1.6.1; a heme protein in EC 1.6.2; oxygen in EC 1.6.3; a disulfide in EC 1.6.4 ; a quinone in EC 1.6.5; a nitrogenous group in EC 1.6.6; a flavin in EC 1.6.8 ; or some other acceptor in EC 1.6.99.

EC 1.7 Acting on other Nitrogenous Compounds as Donors

A small group of enzymes oxidizing diverse nitrogenous substrates with a cytochrome (EC 1.7.2 ), oxygen (EC 1.7.3), an iron-sulfur protein (EC 1.7.7 ), or with other acceptors (EC 1.7.99 ).

EC 1.8 Acting on a Sulfur Group of Donors

A small subclass acting either on inorganic substrates or organic thiols. Sub-subclasses again depend on the acceptor; EC 1.8.1 with NAD+ or NADP+; EC 1.8.2 with a cytochrome; EC 1.8.3 with oxygen; EC 1.8.4 with a disulfide; EC 1.8.5 with a quinone; EC 1.8.7 with an iron-sulfur protein; EC 1.8.98 with other, known, acceptors; and EC 1.8.99 with other acceptors.

EC 1.9 Acting on a Heme Group of Donors

These are the cytochrome oxidases and nitrate reductases. Sub-subclasses: EC 1.9.3 with oxygen, EC 1.9.6 with a nitrogenous compound as acceptor and EC 1.9.99 with other acceptors.

EC 1.10 Acting on Diphenols and Related Substances as Donors

These enzymes catalyse the oxidation of diphenols or ascorbate. There are four sub-subclasses: EC 1.10.1 with NAD+ or NADP+; EC 1.10.2 with cytochromes; EC 1.10.3 with oxygen as acceptor; and EC 1.10.99 with other acceptors. Some enzymes catalysing the oxidation of phenols are oxygenases (sub-subclass EC 1.14.18).

EC 1.11 Acting on Peroxide as Acceptor

This single sub-subclass (EC 1.11.1) contains the peroxidases.

EC 1.12 Acting on Hydrogen as Donors

Hydrogenases using iron-sulfur compounds as donor for the reduction of H+ to H2 are listed in subclass EC 1.18. Other hydrogenases have been left in sub-subclasses EC 1.12.1 (with NAD+ or NADP+ as acceptor), EC 1.12.2 (with cytochromes), EC 1.6.5 (with quinone), EC 1.12.7 (with iron-sulfur protein), EC 1.12.99 (with other known acceptors) or EC 1.12.99 (with other acceptors).

EC 1.13 Acting on Single Donors with Incorporation of Molecular Oxygen (Oxygenases)

The enzymes in this subclass differ from all those in earlier subclasses in that oxygen is actually incorporated from O2 into the substance oxidized; they differ from those in EC 1.14 in that a second hydrogen donor is not required. Sub-subclasses are EC 1.13.11, when two atoms of oxygen are incorporated; EC 1.13.12, when only one oxygen atom is used, and EC 1.13.99, for other cases. This classification replaces an earlier version. Accepted names in this subclass are of the form 'monooxygenase' and 'dioxygenase'.

EC 1.14 Acting on Paired Donors with Incorporation of Molecular Oxygen

The enzymes in this subclass all act on two hydrogen-donors, and oxygen from O2 is incorporated into one or both of them. In sub-subclass EC 1.14.11, 2-oxoglutarate is one donor and one atom of oxygen goes into each donor; in EC 1.14.12, NADH or NADPH is one donor, and two atoms of oxygen go into the other donor; in EC 1.14.13, NADH or NADPH is again one donor, but only one atom of oxygen goes into the other donor; in sub-subclasses EC 1.14.14-1.14.18, one atom of oxygen is incorporated into one donor, the other donor being respectively a reduced flavin or flavoprotein, a reduced iron-sulfur protein, a reduced pteridine, ascorbate, or some other compound. Sub-subclass EC 1.14.19 differs from others in subclass EC 1.14 in that hydrogen atoms removed from the two donors are combined with molecular oxygen to form two molecules of water. Sub-subclass EC 1.14.20 has 2-oxoglutarate as one donor, and the other is dehydrogenated. Sub-subclass EC 1.14.21 has NADH or NADPH as one donor, and the other dehydrogenated. Sub-subclass EC 1.14.99 is for cases where information about the second donor is incomplete.

EC 1.15 Acting on Superoxide Radicals as Acceptor

Sub-subclass EC 1.15.1 contains a single enzyme that brings about the dismutation of superoxide radicals.

EC 1.16 Oxidizing Metal Ions

Metal ions act as 'donor', being oxidized to a higher valency state. Two sub-subclasses are known: EC 1.16.1 with NAD+ or NADP+ as acceptor, EC 1.16.3, with oxygen as acceptor, and EC 1.16.8, with flavin as acceptor.

EC 1.17 Acting on CH or CH2 Groups

The -CH2- group of donors is oxidized by these enzymes to -CHOH- (or CH to COH); in the reverse direction, they are involved in the formation of deoxysugars. Five sub-subclasses are known: EC 1.17.1, with NAD+ or NADP+ as acceptor; EC 1.17.3, with oxygen as acceptor; EC 1.17.4, with a disulfide as acceptor; EC 1.17.5, with a quinone or siilar compound as acceptor; and EC 1.17.99, with other acceptors.

EC 1.18 Acting on Iron-Sulfur Proteins as Donors

Three sub-subclasses are known: EC 1.18.1, with NAD+ or NADP+ as acceptor; EC 1.18.6 , with dinitrogen as acceptor; and EC 1.18.99, with H+ acceptors.

EC 1.19 Acting on Reduced Flavodoxin as Donor

The only sub-subclass is EC 1.19.6, with dinitrogen as acceptor.

EC 1.20 Acting on Phosphorus or Arsenic in Donors

Four sub-subclasses are known: EC 1.20.1, with NAD+ or NADP+ as acceptor; EC 1.20.4 , with disulfide as acceptor; EC 1.20.98, with other, known acceptors; and EC 1.20.99, with other acceptors.

EC 1.21 Acting on X-H and Y-H to form an X-Y bond

Three sub-subclasses are known: EC 1.21.3, with oxygen as acceptor; EC 1.21.4, with a disulphide as acceptor; and EC 1.20.99, with other acceptors.

EC 1.22 Acting on Halogen in Donors

The only sub-subclass is EC 1.22.1, with NAD+ or NADP+ as acceptor.

EC 1.97 Other Oxidoreductases

This subclass (EC 1.97) is reserved for oxidoreductases not included in the previous categories.

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