Enzymes are essential for life. Acting as biological catalysts, they accelerate the rate
of virtually every chemical reaction that occurs in cells. Without them, life as we know
it wouldn’t exist. But for a subject so central to biology, especially for competitive
exams like NEET, understanding how enzymes are named is equally important. After
all, it’s not just about what enzymes do—but also how we systematically identify and
refer to them.
Imagine you’re asked to identify which class of enzyme “DNA ligase” belongs to.
Without understanding enzyme classes, you might just guess. But with a solid grasp
of the nomenclature and classification system, you’ll know it’s a Ligase—an enzyme
that joins two DNA strands together using energy from ATP.
Let’s explore the systematic and organized world of enzyme nomenclature with
clarity and simplicity so you can master this important topic for your NEET
preparation.
Historical Background of Enzyme Nomenclature
Before we had a standard system in place, enzyme names were all over the place.
Back in the early 1900s, scientists named enzymes based on the substrate they
acted upon or the type of reaction they catalysed. For example, the enzyme that
breaks down sucrose was called “sucrase.” Similarly, an enzyme that catalysed the
oxidation of alcohol was just known as “alcohol oxidase.” While this might sound
convenient, the lack of structure quickly became a problem.
As more and more enzymes were discovered, the naming conventions became
confusing. Different labs named the same enzyme differently, or worse, gave
different names to enzymes that did similar things. It was chaotic!
To solve this, the International Union of Biochemistry and Molecular Biology (IUBMB)
stepped in. They introduced a systematic approach to naming enzymes based on
the type of reaction they catalyse. This system was not only standardized but also
universally accepted, making it easier for researchers, students, and educators to
communicate without confusion.
By the 1960s, this system was formalized with the introduction of the Enzyme
Commission (EC) numbers—a numeric system to classify enzymes based on their
function. This move brought much-needed order and clarity to the field of
enzymology and has remained the gold standard to this day.
So when you come across an enzyme like “hexokinase,” you now know there’s a
reason behind that name—and probably an EC number too!
Basics of Enzyme Nomenclature
Understanding the difference between common names and systematic names is the
first step toward mastering enzyme nomenclature. Let’s break this down in a way
that’s easy to understand and memorize for NEET.
Common Names vs Systematic Names
Common names of enzymes are usually derived from the substrate they act on or
the type of reaction they catalyse. These names are often short, memorable, and
widely used. Examples include:
Lactase (breaks down lactose)
Lipase (acts on lipids)
DNA polymerase (helps in DNA replication)
While common names are useful for quick identification, they don’t provide detailed
information about the enzyme’s function.
Systematic names, on the other hand, follow a strict pattern and are designed to
describe the exact chemical reaction an enzyme catalyses. For example, the
systematic name for “lactate dehydrogenase” is (S)-lactate:NAD+ oxidoreductase.
Quite a mouthful, right? But it tells you exactly what the enzyme does—it removes
hydrogen from lactate and transfers it to NAD+.
So, in NEET, both names can appear in options. But understanding how to interpret
systematic names gives you a big edge.
Importance of EC Numbers
To bring even more clarity, the IUBMB developed a numerical system called the
Enzyme Commission (EC) number. Every enzyme gets a unique EC number that
describes its function in four levels:
What is EC Number?
An EC number is a four-part number separated by periods, like EC 1.1.1.1. Each
digit has a specific meaning.
Structure and Meaning of EC Number
Let’s decode this:
First Digit: Refers to the main enzyme class (e.g., EC 1 = Oxidoreductases)
Second Digit: Refers to the type of substrate or reaction
Third Digit: Refers to the acceptor involved in the reaction
Fourth Digit: It’s the serial number of the enzyme in that subgroup
Example: EC 2.7.1.1
2 = Transferase (transfers functional groups)
7 = Transfers phosphorus-containing groups
1 = Uses an alcohol group as acceptor
1 = First enzyme listed in this subgroup (Hexokinase)
By understanding EC numbers, you can instantly deduce the function and
classification of any enzyme in a question.
The Six Major Classes of Enzymes
This is the heart of enzyme nomenclature. Every enzyme in your syllabus and NEET
question paper will fall into one of these six major categories. These classes are
defined based on the type of reaction they catalyse.
Oxidoreductases (EC 1)
These enzymes are responsible for oxidation-reduction (redox) reactions, where
electrons are transferred from one molecule (the reductant) to another (the oxidant).
Function and Examples
Think of oxidoreductases as the electricians of the cell. They manage the electron
flow, critical for energy production. Common examples include:
Dehydrogenases – remove hydrogen atoms (e.g., Lactate
dehydrogenase)
Oxidases – transfer electrons to oxygen (e.g., Cytochrome oxidase)
Reductases – facilitate reduction reactions
In NEET, questions may ask:
“Which class of enzyme does alcohol dehydrogenase belong to?”
Correct answer: Oxidoreductase
Transferases (EC 2)
Transferases are enzymes that catalyse the transfer of a functional group (like
methyl, glycosyl, or phosphate) from one molecule to another. They’re essentially the
messengers, moving groups from donor to acceptor molecules, which is critical for
metabolism and biosynthesis.
Function and Examples
These enzymes are vital in biochemical pathways. For example, in glycolysis, the
enzyme hexokinase transfers a phosphate group from ATP to glucose, forming
glucose-6-phosphate. Here are some common transferases:
Kinases – transfer phosphate groups (e.g., glucokinase, creatine kinase)
Transaminases (aminotransferases) – transfer amino groups (e.g.,
alanine transaminase)
Methyltransferases – transfer methyl groups (e.g., DNA
methyltransferase)
NEET Tip: If the question involves ATP and phosphorylation, the answer often lies
within the transferase class. Students often mistake kinases for hydrolases, but since
the phosphate group is transferred—not removed—kinases belong to the transferase
Hydrolases (EC 3)
Hydrolases catalyse the hydrolysis of various bonds, including ester, glycosidic,
peptide, and others, using water as a reactant. Think of them as the
“breakers”—they use water to break larger molecules into smaller ones.
Function and Examples
Hydrolases play a huge role in digestion:
Proteases/Peptidases – break down proteins (e.g., trypsin, pepsin)
Nucleases – hydrolyse nucleic acids (e.g., DNase, RNase)
Lipases – hydrolyse fats (e.g., pancreatic lipase)
Amylases – break down starch into sugars (e.g., salivary amylase)
These enzymes are particularly important in NEET questions related to digestion and
human physiology.
Example NEET MCQ:
Which enzyme class does pancreatic amylase belong to?
Answer: Hydrolase
Lyases (EC 4)
Lyases catalyse the breaking of various chemical bonds by means other than
hydrolysis and oxidation. These enzymes often form new double bonds or ring
structures. Essentially, they break molecules apart without using water or redox
reactions.
Function and Examples
Lyases are not as intuitive to understand as hydrolases, but they perform some
fascinating reactions:
Decarboxylases – remove carboxyl groups (e.g., pyruvate
decarboxylase)
Aldolases – break C-C bonds in sugars (e.g., aldolase in glycolysis)
Synthases – add groups to double bonds (e.g., citrate synthase)
Quick NEET Hint: If a bond is broken without water or oxidation, and especially if
CO₂ is removed, think of lyases.
Isomerases (EC 5)
Isomerases are responsible for the rearrangement of atoms within a molecule. They
transform a molecule into its isomeric form—like flipping the mirror image or shifting
a group from one position to another.
Function and Examples
These enzymes are subtle but crucial for metabolism:
Mutases – move functional groups within the molecule (e.g.,
phosphoglucomutase)
Racemases and epimerases – convert stereoisomers (e.g., alanine
racemase)
Isomerases – rearrange molecules (e.g., triose phosphate isomerase)
NEET Example:
Which class does phosphohexose isomerase belong to?
Answer: Isomerase
This class is key for understanding the intermediate steps in biochemical cycles like
glycolysis and the Calvin cycle.
Ligases (EC 6)
Ligases catalyse the joining of two large molecules, usually with the help of ATP. You
can think of them as molecular glues—sticking molecules together using energy.
Function and Examples
Ligases are crucial in DNA replication and repair:
DNA ligase – joins Okazaki fragments during replication
Aminoacyl-tRNA synthetase – attaches amino acids to tRNA
Pyruvate carboxylase – catalyses addition of CO₂ to pyruvate
In NEET: This class often appears in molecular biology questions. The keyword
“ligase” almost always signals this class.
Special Focus on Enzyme Specificity and Classification
Let’s now go beyond just enzyme types and look at how enzymes exhibit
specificity—one of the most fascinating traits that makes them so effective and
accurate.
Substrate Specificity
Enzymes are highly specific. Some enzymes work on only one substrate (absolute
specificity), while others may act on a group of similar substrates (group specificity).
Absolute specificity: Urease acts only on urea.
Group specificity: Hexokinase can phosphorylate multiple hexoses (like
glucose and fructose).
Stereochemical specificity: Lactic acid dehydrogenase acts only on the
L-form of lactate, not the D-form.
In NEET: Understanding enzyme specificity helps in analysing complex metabolic
pathways and answering application-based questions.
Reaction Specificity
Some enzymes can act on various substrates but only catalyse one type of reaction.
For example:
Alcohol dehydrogenase always oxidizes alcohols, regardless of their
chain length.
Pepsin always hydrolyses peptide bonds, no matter which protein it acts
upon.
This type of specificity highlights how enzymes focus on “what” they do rather than
“where” they do it.
Cofactor Requirement in Enzyme Classification
Many enzymes require non-protein helpers known as cofactors. These are essential
for their activity and are often vitamins or metal ions.
Coenzymes: Organic molecules like NAD+, FAD, coenzyme A
Prosthetic groups: Tightly bound coenzymes (e.g., heme group in
cytochromes)
Metal ions: Zn²⁺ in carbonic anhydrase, Mg²⁺ in kinases
Relevance in NEET: Enzyme cofactor-related questions test your understanding
of enzyme function under physiological conditions.
Previous Year NEET Questions on Enzyme Nomenclature
Let’s look at some actual NEET-level questions that have appeared between 2010
and 2024, along with answers and explanations.
MCQ 1 (NEET 2016)
Which of the following enzymes belongs to the class of ligases?
A) Alcohol dehydrogenase
B) DNA ligase
C) Lipase
D) Lactase
Answer: B) DNA ligase
Explanation: DNA ligase joins two strands of DNA using ATP, which is the hallmark
of ligase enzymes.
MCQ 2 (NEET 2019)
Which enzyme is involved in the conversion of glucose to glucose-6-phosphate?
A) Amylase
B) Glucose oxidase
C) Hexokinase
D) Lactase
Answer: C) Hexokinase
Explanation: Hexokinase transfers a phosphate from ATP to glucose, classifying it
as a transferase.
MCQ 3 (NEET 2020)
Which of these is an isomerase enzyme?
A) DNA ligase
B) Triose phosphate isomerase
C) Urease
D) Trypsin
Answer: B) Triose phosphate isomerase
Explanation: Isomerases convert molecules into their isomers without adding or
removing atoms.
MCQ 4 (NEET 2023)
Lactate dehydrogenase catalyses which type of reaction?
A) Transfer
B) Oxidation-Reduction
C) Hydrolysis
D) Ligation
Answer: B) Oxidation-Reduction
Explanation: Dehydrogenases remove hydrogen atoms, classifying them as
oxidoreductases.
MCQ 5 (NEET 2024)
Which of the following is a hydrolase involved in digestion?
A) Pyruvate kinase
B) Trypsin
C) DNA ligase
D) Isomerase
Answer: B) Trypsin
Explanation: Trypsin breaks peptide bonds using water—characteristic of
hydrolases.
Summary and Quick Revision Table for NEET
To help you revise faster before the exam, here’s a handy summary table of all six
enzyme classes, their function, and key examples:
Class (EC No.) Function Examples
Oxidoreductases
(1) Redox reactions Alcohol dehydrogenase,
Catalase
Transferases (2) Transfer of functional groups Hexokinase, Transaminase
Hydrolases (3) Hydrolysis (break bonds with
water) Amylase, Trypsin, Lipase
Lyases (4) Break bonds without water or
oxidation Aldolase, Decarboxylase
Isomerases (5) Rearrangement within a molecule Isomerase, Racemase
Ligases (6) Join molecules using ATP DNA Ligase, Carboxylase
Conclusion
Enzymes are fundamental to life, and understanding their nomenclature is essential
for success in NEET. While the system may seem complex at first, breaking it down
by classes, examples, and reaction types makes it much more approachable. Focus
on learning through function, not just memorizing names. Use mnemonics, visualize
the reactions, and keep practicing with NEET-style questions to reinforce your
learning.
By mastering enzyme nomenclature, you not only prepare for NEET but also build a
strong foundation in biochemistry that will benefit you throughout your medical
journey.
FAQs
- What is the easiest way to remember enzyme classes?
Use mnemonics like “Old Tigers Hunt Lions In Lakes” to recall Oxidoreductase,
Transferase, Hydrolase, Lyase, Isomerase, Ligase. - How are enzymes named in the NEET syllabus?
They are referred to by both their common and systematic names. Focus is given on
enzyme class, reaction type, and examples. - Are common names accepted in NEET answers?
Yes, but you should also understand their function and class for multiple-choice
accuracy. - What are EC numbers and why are they important?
EC numbers classify enzymes based on the reactions they catalyse. They help in
identifying the enzyme function precisely. - Can one enzyme belong to multiple classes?
No. Each enzyme is assigned to a specific class based on its primary catalytic
function.
