Q.What are different classes of enzymes? Explain any two with the type of reaction they catalyse.
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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
The six IUB enzyme classes
- Oxidoreductases — transfer electrons (oxidation-reduction)
- Transferases — transfer a functional group (e.g., methyl, phosphate, amino)
- Hydrolases — break bonds using water (hydrolysis)
- Lyases — add or remove groups without hydrolysis or oxidation (often forming or breaking C–C, C–O, C–N bonds)
- Isomerases — rearrange atoms within a molecule (geometric or structural change)
- Ligases — join two molecules using ATP (bond formation coupled to ATP hydrolysis)
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.
If you see NAD⁺, NADP⁺, FAD, or a metal ion like Fe²⁺/Fe³⁺ involved, the enzyme is almost certainly 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.
Students often confuse hydrolases with lyases because both break bonds. The key difference: hydrolases use water; lyases do not.
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.
| Enzyme class | Bond broken? | Uses water? | Uses ATP? |
|--------------|--------------|-------------|-----------|
| Hydrolase | Yes | Yes | No |
| Lyase | Yes | No | No |
| Ligase | No (joins) | No | Yes |
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 …
- Enzymes are grouped into six broad classes based on the type of chemical reaction each one catalyses, with each class further divided into subclasses and every enzyme given a characteristic four-digit identifying number.
- The six classes are oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases.
- Oxidoreductases (dehydrogenases) catalyse oxidation-reduction reactions between two substrates, with one substrate reduced while the other is oxidised, effectively transferring electrons or hydrogen between them. …
Enzymes are sorted into six classes by reaction type - oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases - each with its own characteristic chemistry.
With thousands of enzymes known, order is brought to the variety by classifying them according to the kind of chemical reaction each one drives. Under this system, every enzyme falls into one of six broad classes, each further divided into subclasses, with a characteristic four-digit number locating it precisely.
The six classes are:
- Oxidoreductases (dehydrogenases) - oxidation-reduction reactions between two substrates.
- Transferases - transfer of a chemical group (other than hydrogen) from one substrate to another.
- Hydrolases - hydrolysis reactions, splitting bonds with the addition of water.
- Lyases - removal of groups from substrates by routes other than hydrolysis, typically leaving a double bond behind.
- Isomerases - interconversion of isomers of a molecule (optical, geometric or positional).
- Ligases - joining two compounds together to form a new bond. …
Method: Sort Enzymes by Reaction Type, Then Detail Two
When a question asks for a classification scheme AND detail on a subset of it, answer in two clearly separated stages rather than blending them. First, lay out the full classification cleanly — here, list all six enzyme classes together, since naming five out of six or leaving one vague is an easy way to lose marks even though the "explain two" part is answered well.
The fastest way to remember all six is to attach each class name to the single verb that describes what it does to a substrate: oxidoreductases move electrons/hydrogens (oxidation-reduction), transferases move a chemical group from one molecule to another, hydrolases break a bond by adding water, lyases remove a group without using water (usually leaving a double bond), isomerases rearrange a molecule into an isomer of itself, and ligases join two molecules together to form a new bond. Once you have that one-verb-per-class list, picking any two to explain in more depth is just a matter of restating that verb with a labelled exa …
- AP EAPCET 2026Set ap-2026-05-19-AN1 markMCQQ.Match the following lists List-I (Enzyme): I - Transferases; II - Hydrolases; III - Isomerases; IV - Ligases List-II (Mechanism of reaction): A - Removal of group from other than hydrolysis; B - Catalyses linkage of C-O; C - Catalysing inter-conversion of optical isomers; D - Catylysing transfer of a group; E - Catylising hydrolysis of ester (A) I-D, II-E, III-A, IV-B (B) I-D, II-A, III-E, IV-B (C) I-D, II-C, III-E, IV-B (D) I-D, II-E, III-C, IV-B
›Reveal solutionSolution
Transferases transfer groups, hydrolases hydrolyse (e.g., esters), isomerases interconvert isomers, and ligases join molecules via bond formation — giving I-D, II-E, III-C, IV-B.
Concept and Intuition
Enzymes are classified into six major categories based on the type of reaction they catalyse. Four of these appear here: transferases move a chemical group between molecules; hydrolases break bonds using water (hydrolysis); isomerases rearrange a molecule into an isomeric form without changing its formula; ligases join two molecules together, typically using energy from ATP hydrolysis.
Step-by-Step Solution
- Transferases (I): catalyse transfer of a functional group other than hydrogen from a donor to acceptor molecule → I-D.
- Hydrolases (II): catalyse hydrolysis reactions, such as ester hydrolysis by esterases → II-E.
- Isomerases (III): catalyse interconversion of isomeric forms, e.g., optical isomers → III-C. …
- AP EAPCET 2025Set ap-2025-05-19-AN1 markMCQQ.Choose incorrect statement from the following (A) Ligases linking of C – O, C – S, C – N bonds (B) Hydrolases catalysing C – C, C – halide P – N bonds (C) Isomerases catalysing transfer of a group between a pair of substrates (D) Oxido reductase catalyse oxidation reduction reactions
›Reveal solutionSolution
This tests the six IUB functional classes of enzymes. The odd one out is (C), which describes a Transferase's job but labels it Isomerase.
Concept and Intuition
Enzymes are named/classified by the type of reaction they catalyse, not by their substrate alone. The six IUB classes are: Oxidoreductases (redox reactions), Transferases (group transfer between substrates), Hydrolases (hydrolysis of bonds using water), Lyases (non-hydrolytic removal of groups leaving double bonds), Isomerases (rearrangement within one molecule — interconversion of isomers), and Ligases (joining two molecules using energy, e.g. ATP).
Step-by-Step Solution
- Ligases: catalyse the joining of two compounds, forming bonds such as C–O, C–S, C–N (e.g., DNA ligase, using ATP). Statement (A) matches this exactly — correct.
- Hydrolases: catalyse hydrolysis of ester, ether, peptide, glycosidic bonds and also C–C, C-halide, or P–N bonds. Statement (B) lists a valid subset of these bonds — correct, not the false one.
- Isomerases: catalyse interconversion of optical, geometric, or positional isomers of a single substrate — they do NOT transfer a group between two different substrates. That job belongs to Transferases. Statement (C) describes Transferases while calling them Isomerases — this is the incorrect statement. …
- AP EAPCET 2025Set ap-2025-05-20-AN1 markMCQQ.Match the following List - A: A. Hydrolases B. Lyase C. Ligase D. Transferase List - B: I. Argino succinase II. Fructose 1,6 bisphosphatase III. Hexokinase IV. Ribose-5-phosphoisomerase V. Glutamine synthetase (A) A-II, B-V, C-I, D-III (B) A-II, B-I, C-V, D-III (C) A-I, B-II, C-III, D-IV (D) A-II, B-III, C-I, D-V
›Reveal solutionSolution
Each enzyme class is matched to a named enzyme by its catalytic action.
Concept and Intuition
Hydrolases catalyse hydrolysis; fructose-1,6-bisphosphatase removes a phosphate by hydrolysis. Lyases remove groups without hydrolysis (often named '...ase' from 'lyase' directly) — argininosuccinase (argininosuccinate lyase) IS a lyase by name. Ligases join two molecules using ATP energy — glutamine synthetase joins glutamate and ammonia using ATP, a ligase reaction. Transferases move a functional group from one molecule to another — hexokinase transfers a phosphate from ATP to glucose, a classic transferase (kinase).
Step-by-Step Solution
- Hydrolases → Fructose 1,6-bisphosphatase (II): hydrolytic removal of phosphate.
- Lyase → Argininosuccinase (I): the enzyme's own name flags its class.
- Ligase → Glutamine synthetase (V): ATP-dependent bond formation joining two substrates.
- Transferase → Hexokinase (III): phosphoryl-group transfer from ATP to glucose. …
- AP EAPCET 2024Set ap-2024-05-17-FN1 markMCQQ.Find the incorrect statement regarding Enzymes A. Enzyme catalysing the linking together of 2 compounds are lyases B. Glutamic acid is converted as glutamine in the presence of glutamine synthetase C. The average content of 'S' and that of transition state is called activation energy D. Inorganic catalysts work similar to enzymes at high temperature (A) A, D (B) A, B (C) C, D (D) B, D
›Reveal solutionSolution
Of the four statements, the enzyme-classification one (ligases mislabelled as lyases) and the inorganic-catalyst comparison are the flawed ones; the glutamine-synthetase fact and the activation-energy definition are both standard and correct.
Concept and Intuition
Enzymes are classified into six functional classes based on the type of reaction catalysed (oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases). Ligases specifically catalyse the linking together of two compounds, usually coupled to the breakdown of a high-energy phosphate bond (e.g., ATP) — forming C–O, C–S, or C–N bonds. Lyases, by contrast, catalyse the removal (or addition) of groups to form (or saturate) double bonds, by mechanisms other than hydrolysis — a fundamentally different job from "joining two compounds."
Separately, enzymes are prized for achieving enormous rate acceleration under mild conditions (physiological temperature and pH), which is precisely what sets them apart from inorganic catalysts, which typically require high temperature and/or pressure to catalyse comparable reactions — and even then don't achieve the same specificity or efficiency.
Step-by-Step Solution
- Statement A: "Enzymes catalysing the linking together of 2 compounds are lyases" — incorrect; this is the definition of ligases. A is wrong.
- Statement B: "Glutamic acid is converted to glutamine in the presence of glutamine synthetase" — correct; this is exactly what the enzyme glutamine synthetase does (Glutamate + NH₃ + ATP → Glutamine). B is correct.
- Statement C: activation energy as the energy difference between the substrate and the transition state — this matches the standard definition of activation energy. C is correct. …
- AP EAPCET 2022Set ap-2022-07-11-FN1 markMCQQ.______ enzyme converts sucrose into glucose and fructose (A) Zymase (B) Invertase (C) Isomerase (D) Maltase
›Reveal solutionSolution
This tests knowledge of the enzyme that hydrolyses sucrose; the answer is Invertase.
Concept and Intuition
Sucrose is a non-reducing disaccharide made of glucose + fructose joined by a glycosidic bond between their anomeric carbons. Hydrolytic enzymes act on specific glycosidic bonds — the enzyme name "invertase" comes from the fact that hydrolysis inverts the optical rotation of the sugar solution (sucrose is dextrorotatory, the glucose+fructose mixture is levorotatory because fructose's strong left rotation dominates).
Step-by-Step Solution
- Sucrose + H2O --(enzyme)--> Glucose + Fructose.
- The specific hydrolase that catalyses this reaction is invertase (also called sucrase or β-fructofuranosidase). …
- AP EAPCET 2022Set ap-2022-07-12-FN1 markMCQQ.The term Enzyme was proposed by ___________ (A) W F Khune (B) E Fisher (C) D Koshland (D) E Buchner
›Reveal solutionSolution
The word 'enzyme' (meaning 'in yeast') was coined by Wilhelm Friedrich Kühne in 1878.
Concept and Intuition
The term 'enzyme' (from Greek, meaning 'in yeast') was coined by W. F. Kühne in 1878 to describe the catalytic activity observed in yeast extracts. Eduard Buchner later (1897) demonstrated that cell-free yeast extract could still ferment sugar, proving enzymes act independent of living cells (earning him the 1907 Nobel Prize), but he did not coin the term. Emil Fischer proposed the lock-and-key model of enzyme action, and Koshland proposed the induced-fit model — both about mechanism, not naming.
Step-by-Step Solution
- 'Enzyme' literally means 'in yeast' (en-zyme).
- W. F. Kühne first proposed/coined this term in 1878. …
- AP EAPCET 2022Set ap-2022-07-12-FN1 markMCQQ.Enzymes are mainly classified into _________ groups (A) 5 (B) 6 (C) 7 (D) 8
›Reveal solutionSolution
The International Union of Biochemistry and Molecular Biology (IUBMB) classifies enzymes into six major classes based on the type of reaction catalysed.
Concept and Intuition
Enzymes are systematically classified by the reaction type they catalyse into six major classes: (1) Oxidoreductases (oxidation-reduction), (2) Transferases (group transfer), (3) Hydrolases (hydrolysis), (4) Lyases (addition/removal without hydrolysis), (5) Isomerases (isomerisation), and (6) Ligases (joining two molecules using ATP energy). Each of these six classes is further divided into sub-classes and sub-sub-classes, giving each enzyme a unique four-digit EC (Enzyme Commission) number.
Step-by-Step Solution
- Enzyme classification (IUBMB) is based on reaction type catalysed. …
- AP EAPCET 2021Set ap-2021-09-03-AN1 markMCQQ.Match the following? Enzymes:(a) Lyases(b) Isomerases(c) Dehydrogenases(d) Ligases Functions:(i) Oxido reduction(ii) Link compounds(iii) Transfer of group(iv) Interconversion(v) Removal of groups (A) (a-v), (b-iv), (c-i), (d-ii) (B) (a-iv), (b-v), (c-ii), (d-iii) (C) (a-iii), (b-ii), (c-iv), (d-i) (D) (a-i), (b-iii), (c-v), (d-iv)
›Reveal solutionSolution
This tests the standard six-class enzyme nomenclature (IUBMB) mapped to specific enzyme names. The answer is (A).
Concept and Intuition
Enzymes are classified into six major classes by reaction type: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.
- Lyases catalyze the removal of groups from substrates by means other than hydrolysis, often creating a double bond (or adding a group across one) — 'removal of groups'.
- Isomerases catalyze the rearrangement of atoms within a molecule, i.e., interconversion of isomeric forms — 'interconversion'.
- Dehydrogenases are a type of oxidoreductase, catalyzing transfer of electrons/hydrogen — i.e., 'oxido-reduction'.
- Ligases (synthetases) join two molecules together, typically coupled with ATP hydrolysis — 'link compounds'. Note 'transfer of group' (which belongs to transferases) is a distractor since transferases aren't among the four enzymes listed here.
Step-by-Step Solution …
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