7 min readConcept sheetOften a 5- or 10-mark answer

Enzymes: definition, types and mechanism

Enzymes are biological catalysts, mostly proteins, that speed up reactions without being used up. Substrate binds at the active site to form an enzyme–substrate complex.

DefinitionProtein catalysts that speed up reactions in cells

IUB classesSix (EC 1–6); a 7th, translocases, added in 2018

How they workLower the activation energy

HoloenzymeApoenzyme + cofactor

IUB classification: six classes

EC no.	Class	Reaction type	Example
1	Oxidoreductases	Oxidation–reduction (move hydrogen or electrons)	Lactate dehydrogenase, cytochrome oxidase
2	Transferases	Move a group (amino, phosphate, methyl) from one molecule to another	Hexokinase, alanine transaminase (ALT)
3	Hydrolases	Break bonds by adding water	Amylase, pepsin, lipase
4	Lyases	Remove or add groups without water, often forming double bonds	Aldolase, fumarase
5	Isomerases	Rearrange atoms within one molecule	Phosphohexose isomerase, triose phosphate isomerase
6	Ligases (synthetases)	Join two molecules using energy from ATP	Pyruvate carboxylase, glutamine synthetase

Mechanism of action

Every reaction must cross an energy barrier, the activation energy. An enzyme lowers this barrier, so the reaction runs fast at body temperature. It does not change the final equilibrium.

1
E + S → ES complex
substrate (S) binds the active site of the enzyme (E)
2★
ES → EP
bonds are strained and the reaction happens with lower activation energy
3
EP → E + P
product (P) leaves; the enzyme is free to act again

★ one-way, controls the pace

Two models of the active site

Lock-and-key (Emil Fischer, 1894)The active site is rigid and already shaped to fit the substrate, like a key in a lock. It explains specificity but not how the enzyme adjusts.

Induced fit (Daniel Koshland, 1958)The active site is flexible. Binding of the substrate changes the enzyme's shape so it closes around the substrate, like a glove on a hand. This is the more accepted model.

Both models lead to the same sequence: E + S → ES → E + P.

Key terms

Substrate: the molecule an enzyme acts on.
Active site: a small cleft on the enzyme where the substrate binds and the reaction takes place.
Apoenzyme: the protein part alone; inactive by itself.
Cofactor: the non-protein part needed for activity, either a metal ion (Zn2+, Mg2+, Fe2+) or an organic coenzyme (NAD+, FAD, TPP).
Holoenzyme: apoenzyme + cofactor, the complete active enzyme.
Prosthetic group: a cofactor bound tightly to the enzyme, e.g. FAD in succinate dehydrogenase.
Zymogen (proenzyme): an inactive form switched on later, e.g. pepsinogen → pepsin, trypsinogen → trypsin.
Isoenzymes: different forms of one enzyme in different tissues, e.g. the five forms of lactate dehydrogenase (LDH).

Nutrition link

Many cofactors come from food: B vitamins form coenzymes, and minerals such as zinc (carbonic anhydrase), magnesium (kinases) and iron (cytochromes, catalase) activate enzymes. Digestive enzymes such as pepsin and trypsin are made as inactive zymogens, so they do not digest the cells that make them.

Exam angle

10 marks: “Define enzymes. Classify them according to the IUB system and explain their mechanism of action.” Definition, six IUB classes with one example each, active site, E + S → ES → E + P, an activation-energy graph, and both binding models.

5 marks: “Explain the lock-and-key and induced-fit models of enzyme action.” Fischer: rigid active site fits the substrate like a key in a lock. Koshland: flexible site changes shape on binding. Draw both.

2 marks: “What is a holoenzyme?” The complete active enzyme, made of the protein part (apoenzyme) plus its non-protein cofactor or coenzyme.

Remember it

Read the six classes as OTHLIL: Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases. This is also their EC order, 1 to 6.