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Enzymes Classification, Properties - NEET PG Biochemistry

Feb 16, 2023

Enzymes NEET PG Biochemistry

Enzymes play a crucial role in various biological processes. They are responsible for catalyzing the chemical reactions that occur in living organisms, including those involved in metabolism. Moreover defects in enzyme function can lead to various medical conditions, including genetic disorders and diseases caused by environmental factors. 

Understanding the role of enzymes in metabolic pathways is critical for understanding how the body functions at a molecular level.

Let’s learn some more about this important topic of biochemistry through this blog below.


Enzymes are proteins except ribozymes in which RNA acts as enzyme.

Enzymes - NEET PG Biochemistry
  • Active site: Binding Site + Catalytic site
  • serine proteases are
    • Protein breaking enzyme
    • Aspartate & Histidine are present at binding site for substrate
    • Serine is present at catalytic site and is responsible for cutting the substrate
    • e.g. Chymotrypsin, Trypsin, elastase, plasmin, thrombin, clotting factor 10 & 11 and prostate specific Antigen
    • These serine proteases have a role in tumor cell metastasis.

Important Information

  • Chymotrypsin cuts at C- terminal of large hydrophobic amino acids like Phe, Tyr and Trp
  • Trypsin cuts at carboxy terminal of basic amino acids like Lys and Arg
  • Elastase cuts at carboxy terminal of small neutral amino acids e.g. glycine, serine.

Michaelis Menten Graph (Most Enzymes)

  • For simple or most of the enzymes.
Michaelis Menten Graph (Most Enzymes)
  • Michaelis-Menton constant [Km]
    • Km is defined as that substrate concentration at which velocity of reaction is half of Vmax
    • Km can't be equal to Vmax /2 (as the units of two parameters are different)
    • Km is signature of Enzyme as It is constant value for a particular enzyme
    • Km does not change with change in either enzyme or substrate concentration
    • Km 1affinity , So, in case of competitive inhibition affinity↓ so Km

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Graph for Allosteric or regulatory Enzymes (Few Enzymes)

  • Few Enzymes
  • Have active site + regulatory/allosteric site 
    • regulator (activator or inhibitor) binds to allosteric site 
    • V vs [S] is Sigmoidal/S shape graph
Graph for Allosteric or regulatory Enzymes (Few Enzymes)

Temperature & pH Graph: Bell Shaped Graph

Temperature & pH Graph: Bell Shaped Graph
  • At extremes of temp/ pH, proteins/enzymes are denatured, so, velocity is negligible
  • Velocity is max at optimum temp and pH only
  • Optimum temp is 37o C and optimum pH is 5-9 for human enzymes


Enzyme Class (EC no.)

Distinguishing Feature

1. Oxidoreductases

  • Oxidases
  • Dehydrogenase

  • Peroxidase
  • Oxygenase
  • Reductase

  • Use O2 as an electron acceptor like cyt C Oxidase
  • Use molecules other than O2 as electron acceptor (NAD, FAD, NADP); e.g. PDH in link reaction.
  • Use H2O2 as electron acceptor; e.g., Glutathione peroxidase
  • Incorporate O2 into the substrate; 2 types: 
    • Dioxygenase: incorporates 2 atoms of molecular o2 into the substrate e.g. homogentisate dioxygenase
    • Monooxygenases/Hydroxylases/mixed function oxidases: incorporates 1 atom of molecular O2 into the substrate. e.g. phenylalanine hydroxylase (converts Phe to Tyr)
  • Example: Glutathione reductase

2. Transferases

    • Methyltransferase
    • Aminotransferase
    • Kinase
    • Phosphorylase

  • Transfer one carbon units
  • Transfer amino groups
  • Transfer phosphate from ATP
  • Transfer phosphate Pi

3. Hydrolases

    • Phosphatase
    • All digestive enzymes 

  • Remove phosphate from a substrate using water
  • any enzyme that breaks macromolecule e.g. amylase, maltase etc.

4. Lyases

    • Synthases
    • Aldolase A & B
    • Simple Decarboxylases
    • Hydratase

  • Link 2 molecules without using ATP
  • Produce aldehydes via elimination reactions
  • Produce CO2 via elimination reactions
  • Add or remove water but do not break bond e.g. Enolase, aconitase, fumarase, PEPCK

5. Isomerases

    • Racemase
    • Mutase
    • Epimerase

  • Interconvert L & D stereoisomers
  • Transfer group b/w atoms within a molecule
  • Interconvert epimers

6. Ligase

    • Synthase
    • Carboxylase

  • Link 2 molecule via an ATP-dependent
  • Use ATP, Biotin and CO2 (Mn- ABC) and also uses Mg2+


Important Information

  • Oxidases which do not require copper: Xanthine oxidase and Sulfite oxidase


Type of InhibitionKmVmax

Also Read:

Amino Acid Disorders - NEET PG Biochemistry

An easy approach to controversial questions in Biochemistry

Important Topics in Biochemistry for AIIMS-PG by Dr. Smily Pruthi

Biochemistry FAQs answered by Dr. Smily Pruthi

Important Topics in Biochemistry for NEET-PG by Dr. Smily Pruthi

How to Prepare Biochemistry for Medical PG Entrance | PrepLadder


  • Isozyme number is inversely related to mobility. i.e.
    • Least number moves maximum 
    • Highest number moves least
      • In five LDH isoenzymes, LDH-1 moves max and LDH-5 moves least
      • Out of 3 isoenzymes of CK, CK-1 moves max and CK-3 moves least
active enzymes


  • ↑ velocity/rate of reaction
  • ↓ Activation energy
  • Do not change the equilibrium of reaction
  • Do not change the free energy of substrates/products 

Previous Year’s Question

Q. Identify the type of inhibitor in the graph?(INICET May 2022)

Identify the type of inhibitor in the graph
  1. Competitive inhibitor
  2. Non-competitive inhibitor
  3. Allosteric inhibitor
  4. Uncompetitive inhibitor

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