Six Groups of Enzymes

The six groups of enzymes, based on the chemical reaction catalyzed, are described in more detail in this section.

1. Oxidoreductases

Oxidoreductases are enzymes that oxidize or reduce substrates by transfer of hydrogen or electrons or by addition of oxygen. The systematic name is formed as ''donor:acceptor oxidoreductase." One example for alcohol dehydrogenase (EC 1.1.1.1) is shown in Eq. (1) above, in which the donor substrate is ethanol which contributes two hydrogens to NAD+, the acceptor substrate (and cofactor) with the formation of acetal-

dehyde and NADH2; NADH2 then loses a proton (H+) based on the pH of the reaction. Another example is the catalase reaction in which one H2O2 molecule serves as the donor substrate and the second H2O2 molecule serves as the acceptor substrate to form O2 and H2O [Eq. (2)]. The systematic name is "hydrogen peroxide:-hydrogen peroxide oxidoreductase'' (EC 1.11.1.6).

Whitaker (2) has suggested dividing the oxidoreduc-tases into eight subgroups based on mechanistic considerations. But the International Commission on Enzymes considers only the overall reaction in naming enzymes.

2. Transferases

Transferases are enzymes that remove groups (not including H, which belongs in group 1, the oxidoreduc-tases) from substrate(s) and transfers the group to acceptor substrates (not including water). The systematic name is formed as "donor:acceptor group-trans-ferred-transferase.'' The types of groups transferred are given by the second digit in EC 2. of Table 1. A specific example of a transferase is shown in Eq. (3).

OH OH

D-glucose D-glucose-6-phosphate

OH OH

D-glucose D-glucose-6-phosphate

The systematic name of the enzyme that catalyzes the reaction in Eq. (3) is "ATP:D-glucose 6-phosphotrans-ferase'' (EC 2.7.1.2). The trivial name is glucokinase. The position to which the group (phosphate) is transferred to the glucose is given in the systematic name when more than one possibility exists, such as D-glu-cose-1-phosphate.

3. Hydrolases

Hydrolases are enzymes in which water (H2O) is the second substrate. Water participates in the breakage of covalent bonds; for example, peptide bonds in proteins, glycosidic bonds in carbohydrates, ester bonds in lipids, and phosphodiester bonds (and others) in nucleic acids. The systematic name is formed as "substrate hydrolase.'' The elements (H and OH) appear in the products of the reaction as shown in Eq. (4)

CH3C(0)NH CH3COOH + CH3C(NH2,H)C(0)0CH2CH3

CH3CHC(0)0CH2CH3 + H2O^

CH3CH2OH + CH3C(0)NH

CH3CHC(0)0H

When the enzyme specificity is limited to a single group (of two or more susceptible groups), the name of the group is given as a prefix of hydrolase. For example, enzymatic hydrolysis of a compound such as N-acetyl-L-alanine ethyl ester [Eq. (4)] can occur at either the acetyl-nitrogen bond or the ethoxy-oxygen bond, depending on the specificity of the enzyme.

In reaction 1 [Eq. (4)], the enzyme is an acyl amino-hydrolase (EC 3.5.) while in reaction 2 [Eq. (4)], the enzyme is an acyl oxygenhydrolase (esterase, EC 3.1.). There is no crossreactivity between the two enzymes, as they are specific for hydrolysis of only one of the scis-sile bonds, unlike HCl.

4. Lyases

Lyases are enzymes that remove groups from their substrates (not by hydrolysis) to give a double bond in the product, or which conversely add groups to double bonds. The systematic name is formed as "substrate prefix-lyase.'' Prefixes such as "hydro-" and "ammonia-" are used to indicate the type of reaction—for example, ''L-malate hydro-lyase'' (EC 4.2.1.2). Decarboxylases are named as ''carboxylases.'' A hyphen is always written before ''lyase'' to avoid confusion with hydrolases, carboxylases, etc. An example of a lyase is the removal of HOH from malic acid to give fumaric acid [Eq. (5)]:

hoch2cooh hoocch

ch2cooh chcooh

Malic acid Fumaric acid

by the enzyme (s)-malate hydro-lyase (fumarate hydra-tase; EC 4.2.1.2; formerly known as fumarase).

5. Isomerases

Isomerases are enzymes that cause rearrangement of one or more groups on substrates without changing the atomic composition of the product. The general systematic name is formed as ''substrate prefix-isomer-ase.'' The prefix indicates the type of isomerization involved, for example, ''maleate cis-trans-isomerase'' (EC 5.2.1.1) [Eq. (6)].

Table l Key to Numbering and Classification of Enzymesa

1. Oxidoreductases

1.1 Acting on the CH-OH group of donors

1.2 Acting on the aldehyde or oxo group of donors

1.3 Acting on the CH-CH group of donors

1.4 Acting on the CH-NH2 group of donors

1.5 Acting on the CH-NH group of donors

1.6 Acting on NADH or NADPH

1.7 Acting on other nitrogenous compounds as donors

1.8 Acting on a sulfur group of donors

1.9 Acting on a heme group of donors

1.10 Acting on diphenols and related substances as donors

1.11 Acting on hydrogen peroxide as acceptor

1.12 Acting on hydrogen as donor

1.13 Acting on single donors with incorporation of molecular oxygen (oxygenases)

1.14 Acting on paired donors with incorporation of molecular oxygen

1.15 Acting on superoxide radicals as acceptor

1.16 Oxidizing metal ions

1.17 Acting on CH2 groups

1.18 Acting on reduced ferredoxin as donor

1.19 Acting on reduced flavodoxin as donor 1.97 Other oxidoreductases

2. Transferases

2.1 Transferring one-carbon groups

2.2 Transferring aldehyde or ketone residues

2.3 Acyltransferases

2.4 Glycosyltransferases

2.5 Transferring alkyl or aryl groups, other than methyl groups

2.6 Transferring nitrogenous groups

2.7 Transferring phosphorous-containing groups

2.8 Transferring sulfur-containing groups

3. Hydrolases

3.1 Acting on ester bonds

3.2 Glycosidases

3.3 Acting on ether bonds

3.4 Acting on peptide bonds (peptidases)

3.5 Acting on carbon-nitrogen bonds, other than peptide bonds

3.6 Acting on acid anhydrides

3.7 Acting on carbon-carbon bonds

3.8 Acting on halide bonds

3.9 Acting on phosphorus-nitrogen bonds

3.10 Acting on sulfur-nitrogen bonds

3.11 Acting on carbon-phosphorus bonds

4. Lyases

4.1 Carbon-carbon lyases

4.2 Carbon-oxygen lyases

4.3 Carbon-nitrogen lyases

4.4 Carbon-sulfur lyases

4.5 Carbon-halide lyases

4.6 Phosphorus-oxygen lyases 4.99 Other lyases

5. Isomerases

5.1 Racemases and epimerases

5.2 cis-trans-isomerases

5.3 Intramolecular oxidoreductases

5.4 Intramolecular transferases (mutases)

5.5 Intramolecular lyases 5.99 Other isomerases

6. Ligases

6.1 Forming carbon-oxygen bonds

6.2 Forming carbon-sulfur bonds

6.3 Forming carbon-nitrogen bonds

6.4 Forming carbon-carbon bonds

6.5 Forming phosphoric ester bonds

The third and fourth levels of classification are given in Ref. 1:v-xi.

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