Q.Describe in detail the six major classes of enzymes recognised by the IUBMB classification system with one example of each, and explain the mechanism of enzyme action with reference to the active site and the enzyme-substrate complex.

Enzyme Classification — The Big Idea

Imagine a factory floor with thousands of workers, each doing a different job. Some workers cut metal, some weld pieces together, some rearrange parts, and others carry materials from one station to the next. If you had to organise them, you wouldn't group them by their names or the colour of their uniforms — you'd group them by what they actually do.

That is exactly the logic behind enzyme classification.

Enzymes are biological catalysts, and there are thousands of them. Their names (like hexokinase or lactate dehydrogenase) often hint at what they do, but the system can feel chaotic. The International Union of Biochemistry (IUB) solved this by classifying every enzyme into one of six major classes based solely on the type of chemical reaction it catalyses. The reaction type is the only thing that matters — not the enzyme's structure, not where it comes from, not its size.

The Six Classes — What Each One Does

Let's walk through each one with intuition.

1. Oxidoreductases — The Electron Movers

If a reaction involves transfer of electrons (which often means hydrogen atoms or oxygen atoms are being added or removed), the enzyme is an oxidoreductase. These are the enzymes of respiration, photosynthesis, and many metabolic pathways.

Example: Alcohol dehydrogenase removes two hydrogens from ethanol to make acetaldehyde. Electrons move — it's an oxidoreductase.

2. Transferases — The Group Passers

These enzymes take a functional group (a phosphate, a methyl group, an amino group, a sugar unit) from one molecule and stick it onto another. They don't break the group down — they just move it.

Example: Hexokinase takes a phosphate from ATP and puts it onto glucose. That's a transferase (specifically a kinase, which is a subclass of transferases).

3. Hydrolases — The Water Users

Hydrolases break bonds by adding water. The water molecule splits — its H⁺ goes to one fragment and its OH⁻ goes to the other. Digestion is full of these: proteases break proteins, lipases break fats, nucleases break DNA.

Example: Pepsin in your stomach breaks peptide bonds in proteins. Water is consumed in the process — it's a hydrolase.

4. Lyases — The Bond Breakers (Without Water)

Lyases break bonds by means other than hydrolysis or oxidation. They often remove a group to leave a double bond, or add a group to a double bond. They can also join two molecules without using ATP (that's the ligase's job).

Example: Fumarase removes water from malate to form fumarate (a double bond forms). No water is consumed — water is produced. That's a lyase.

5. Isomerases — The Rearrangers

Isomerases take one molecule and change its shape — turning a cis form into trans, an aldose into a ketose, an L-isomer into a D-isomer. No atoms are added or removed; they just get shuffled.

Example: Triose phosphate isomerase converts dihydroxyacetone phosphate into glyceraldehyde-3-phosphate during glycolysis. Same atoms, different connectivity.

6. Ligases — The Gluers

Ligases join two molecules together, and they need energy from ATP (or a similar nucleotide) to do it. The ATP is hydrolysed to ADP and phosphate, and that energy drives the bond formation.

Example: DNA ligase seals nicks in the DNA backbone by joining a 3'-OH to a 5'-phosphate. ATP is consumed — it's a ligase.

The EC Number — A Deeper Level …