Biomolecules · Lesson 11 of 12
Enzyme Classes, Cofactors and Functional Enzyme Complexes
“Six classical reaction classes, modern translocases, apoenzymes, prosthetic groups, coenzymes and metal cofactors.”
• Classify enzymes using the chemical transformations they catalyse. • Distinguish the six classical classes and the modern translocase category. • Explain the distinction between an apoenzyme and a holoenzyme. • Compare prosthetic groups, coenzymes and metal-ion cofactors. • Identify the roles of haem, NAD/NADP and zinc in enzyme systems.
Why enzymes are classified by reactions
Enzymes vary greatly in shape and tissue location, but the reactions they catalyse provide a systematic classification. A reaction involving electron transfer differs from one that hydrolyses a bond or joins two compounds. The Enzyme Commission numbering system gives an enzyme-catalysed reaction a hierarchical designation: a major class followed by progressively finer categories. This lets scientists classify chemically similar reactions even when their catalysts have different amino-acid sequences.
The chapter source presents six classes, the classical arrangement. The modern International Union of Biochemistry and Molecular Biology classification also includes a seventh class, translocases. Both statements can be true: the source illustrates an older framework, and a current reference describes the expanded system.
An enzyme catalysing an oxidation–reduction reaction, where reducing equivalents such as electrons are transferred between molecules.
Oxidoreductases and transferases
An oxidoreductase catalyses a reaction in which one species is oxidised while another is reduced. Dehydrogenases are important examples that transfer hydrogen-associated reducing equivalents in metabolic chemistry. The source's general representation is: reduced S + oxidised S′ → oxidised S + reduced S′. This expresses electron-transfer logic rather than one specific balanced chemical equation.
An enzyme transferring a chemical functional group from a donor to an acceptor.
A transferase can move a phosphate, methyl, amino or other group. In simplified notation S–G + S′ → S + S′–G, the group G moves from one molecule to another. This allows enzyme networks to attach functional groups to proteins, sugars or other molecules. It is distinct from joining whole molecules through a newly created bond using an energy-coupled ligase reaction.
Hydrolases, lyases and isomerases
An enzyme catalysing cleavage of a chemical bond by hydrolysis with water.
A hydrolase uses water to split chemical linkages. Protein-digesting proteases, such as trypsin, are examples because they hydrolyse peptide bonds. The classification is based on the reaction, not simply on whether the enzyme happens to work in an aqueous solution.
An enzyme catalysing removal or addition of groups by mechanisms distinct from ordinary hydrolysis or redox, often forming or using double bonds.
Lyases may remove groups without adding water across the cleaved bond, yielding double bonds; the reverse can involve adding groups across a double bond. Although some lyases split molecules, they are not hydrolases unless their chemistry fits water-mediated cleavage.
An enzyme converting a molecule to another isomer by changing its internal arrangement without altering the overall elemental formula.
Isomerases convert one arrangement of atoms into another, such as changing the position of a functional group or a stereochemical configuration. These reactions are important in carbohydrate metabolism. Isomerisation does not require two separate molecules to exchange a group.
Ligases and modern translocases
An enzyme catalysing the formation of a bond between molecules, usually coupled to expenditure of chemical energy.
Ligases join molecules through energy-coupled reactions, commonly using ATP or related compounds. A DNA ligase forms covalent links in a DNA backbone during repair or replication-associated processes. The characteristic is bond formation; the fact that ATP is used does not itself determine whether every ATP-using enzyme is a ligase.
A modern EC class covering certain enzyme-catalysed movements of ions or molecules across membranes, coupled to energy-transforming reactions.
Translocases were added as EC class 7 after the historical six-class framework used by the textbook. Examples include catalysts that transport ions across membranes using coupled energy. Keeping the seventh class separate from the six source classes makes the evolution of scientific nomenclature clear.
| Class | What it catalyses | Diagnostic clue |
|---|---|---|
| 1. Oxidoreductases | Redox reactions | Electron / reducing-equivalent transfer |
| 2. Transferases | Transfer of functional groups | Group moves donor → acceptor |
| 3. Hydrolases | Bond cleavage with water | Hydrolysis |
| 4. Lyases | Non-hydrolytic removal/addition | Often double-bond changes |
| 5. Isomerases | Rearrangement within one compound | Isomer conversion |
| 6. Ligases | Energy-coupled joining | New bonds between compounds |
| 7. Translocases (modern) | Energy-linked transmembrane movement | Membrane transport reaction |
Some enzymes require non-protein components
A protein's amino-acid side chains supply many catalytic chemical groups, but certain reactions need additional capabilities. A small organic helper may transfer electrons, or a metal ion may coordinate the substrate and stabilise charge. These non-protein helpers are called cofactors. If a required cofactor is removed, the protein enzyme may remain structurally present yet lose much of its activity. Supplying more substrate will not always replace a missing helper.
A required non-protein component that contributes to enzyme activity.
The protein portion of a cofactor-dependent enzyme considered without its required helper.
The complete catalytically functional enzyme system with its required cofactor or cofactors.
Three major cofactor types
A non-protein component tightly attached to an enzyme, often persisting over many catalytic cycles.
Haem serves as a tightly bound prosthetic group in catalase and peroxidase, contributing iron-centred redox chemistry. It is not itself the entire enzyme; surrounding protein provides an appropriate environment and specificity. The source names haem as part of the enzyme's catalytic apparatus.
A small organic helper that participates in catalysis, often associating transiently with an enzyme and carrying electrons or functional groups.
NAD and NADP are coenzymes containing chemical components derived from niacin, a vitamin. They can carry reducing equivalents among reactions. A coenzyme can be regenerated by other metabolic reactions; a single molecule may be used by different enzymes at different stages. This contrasts with the usually tighter binding of a prosthetic group.
Metal ions can also serve as cofactors. The source gives zinc in carboxypeptidase, where the metal participates in substrate binding and catalytic chemistry. Zinc is an inorganic ion, not an organic vitamin derivative. Its removal can reduce or abolish function even while the polypeptide backbone remains intact.
| Type | Association | Representative |
|---|---|---|
| Prosthetic group | Tightly associated helper | Haem in catalase and peroxidase |
| Coenzyme | Organic helper, often transient | NAD / NADP containing niacin-derived components |
| Metal cofactor | Coordinated inorganic ion | Zn²⁺ in carboxypeptidase |
Apply the two classifications independently
An enzyme's reaction class and cofactor requirement answer different questions. A dehydrogenase may be an oxidoreductase and use NAD as a coenzyme. A protease may be a hydrolase and use zinc to assist catalysis. Thus the same enzyme can have an EC reaction category and a distinct description of the chemical helpers required for activity.
To classify an unknown reaction, first identify whether electrons are transferred, a group is moved, a bond is cleaved with water, an internal rearrangement occurs or an energy-coupled bond is formed. Only then ask what kinds of protein and cofactor structures are responsible for the transformation. A mechanistic answer is stronger than matching a suffix alone.
Problem
An enzyme transfers electrons using NAD as a transient helper. Give its broad reaction class and cofactor type.
- 1.Electron-transfer chemistry places it among oxidoreductases.
- 2.NAD is a small organic coenzyme, not a tightly bound structural peptide.
- 3.The class describes reaction chemistry; the cofactor describes one required molecular participant.
Quick Quiz
Quiz
Which class catalyses oxidation–reduction?
Which class catalyses transfer of a functional group?
Which enzyme class uses water to split bonds?
Which class catalyses internal isomer conversion?
Which seventh class occurs in modern EC nomenclature?
What is the apoenzyme?
What is haem in catalase?
Which compound contributes to NAD and NADP?
Practice Problems
- Match six classical classes with reaction types.
- What is the modern seventh enzyme class and what does it describe?
- Distinguish apoenzyme and holoenzyme.
- Compare prosthetic groups, coenzymes and metal ions.
- Explain why removal of zinc can impair a carboxypeptidase without destroying its chain.
Oxidoreductases—redox; transferases—functional-group transfer; hydrolases—bond hydrolysis; lyases—nonhydrolytic removal/addition; isomerases—isomer conversion; ligases—energy-coupled bond joining.
Key Takeaways
• The classical six classes are oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases. • Current enzyme nomenclature additionally recognises translocases as class 7. • Reaction class depends on chemistry, not the location or organism where the enzyme occurs. • Some enzymes require non-protein cofactors to function. • The apoenzyme is the protein component; the holoenzyme includes the required helper. • Haem is a prosthetic group; NAD/NADP are organic coenzymes associated with niacin. • Zinc is an inorganic cofactor required by enzymes such as carboxypeptidase.