Enzymes
DEFINITION
They are highly specialized proteins that act as catalysts in
biological systems (biocatalyst)
Heat labile and water soluble
Word meaning : ‘ in yeast ’
CLASSIFICATION
International Union of Biochemistry and Molecular Biology
(IUBMB)
Each enzyme is characterised by a code number - E. C. number –
comprising of 4 digits, separated by points
IUBMB CLASSIFICATION (1964)
E.C. Number (enzyme commission number)
1.Class
2.Subclass
3.Subgroup (sub – sub class)
4.Serial number of the particular enzyme
CLASS
Oxidoreductases
Transferases
Hydrolases
Lyases
Isomerases
Ligases
OXIDOREDUCTASES
• Dehydrogenase
• Oxidases
• Oxygenases
• Peroxidases
SUBCLASSES: • Reductases
• Catalase
1.OXIDOREDUCTASES
They catalyze oxidation of one substrate with reduction of another substrate
AH₂+ B A + BH₂
I. DEHYDROGENASE
• Transfer of hydrogen from one substrate to another
• NAD , NADP, FAD acts as hydrogen carrier
• NAD : Nictoniamide adenine dinucleotide
• FAD : Flavin adenine dinucleotide
Examples :
Lactate dehydrogenase
• Lactate + NAD Pyruvate + NADH + H⁺
Alcohol dehydrogenase
Alcohol + NAD+ Aldehyde +NADH +H⁺
IUB name : Alcohol – NAD – Oxidoreductase
E C .1.1.1.1
II. OXIDASE
Removal of hydrogen from a substrate with oxygen as the
hydrogen acceptor
Examples:
Monoamino oxidase
Xanthine oxidase
III. OXYGENASE
• Substrate oxidation by incorporation of oxygen directly in to the
substrate
• Requires presence of metal ion such as Fe or Cu
Dioxygenase
Monooxygenase
Dioxygenase
Incorporate both atoms of molecular oxygen in to the substrate.
A+O₂ AO₂
• Homogentisate oxidase -Fe
• Tryptophan pyrrolase-Fe
• Monooxygenase (Hydroxylase/Mixed function oxidase)
• Incorporate only one atom of molecular oxygen in to the substrate
A-H + O₂ + ZH₂ A-OH + H₂O + Z
• Phenyl alanine hydroxylase
• Tyrosine hydroxylase
• Tryptophan hydroxylase
2.TRANSFERASES
Catalyse the transfer of a group other than hydrogen from one substrate to
another
A-R + B A + B–R
Subclasses
All kinases
Transmethylases
Transaminases
Transaldolases &Transketolases
KINASES
Transfer gamma phosphoryl group from ATP or another nucleotide triphosphate
to alcohol or amino group acceptors
Hexokinase
Hexose + ATP Hexose -6-PO4 + ADP
IUB name - ATP : D-hexose -6- phosphotransferase
E .C .No : 2.7.1.1
3.HYDROLASES
These enzymes can Hydrolyse ester, ether, peptide or glycosidic
bonds by adding water and then breaking the bond
1.All digestive enzymes are hydrolases
Pepsin ,trypsin chymotrypsin ,lipase
2.Acetyl choline + H20 choline + acetate
ACETYL CHOLINE ESTERASE
4.LYASES
• Remove groups from substrates or break bonds by mechanisms other than
hydrolysis
• Examples : Aldolase
HMG CoA lyase
ATP Citrate lyase
Argininosuccinase
5.ISOMERASES
• Catalyse geometric or structural changes within a single
molecule
• Produce optical , geometric or positional isomers of substrates
SUBCLASSES:
1. Racemases & Epimerases
2. Cis – Trans Isomerase
3. Intra molecular oxidoreductases
1. RACEMASES
Helps in interconversion of levo and dextro rotatory isomers
L – Alanine D – Alanine
EPIMERASES
UDP Glucose UDP Galactose
Ribulose 5 PO4 Xylulose 5 PO4
Intra molecular oxidoreductases – Aldose ketose isomerase
Glucose phosphate isomerase
D–Glucose 6 PO4 D–Fructose 6 PO4
Triose phosphate isomerase
Glyceraldehyde -3-PO4 Dihydroxy acetone phosphate
6. LIGASES
• These enzymes link 2 substrates together
• ATP dependent condensation of two molecules
SYNTHETASES & SYNTHASES
Synthetases : ATP dependent enzymes, belong to class ligases
• Eg: Carbamoyl phosphate synthetase
• Glutamine synthetase
Synthases : ATP independent enzymes, belong to class other than
ligases
• Energy required for bond formation is present in one of its reactants
• Eg: ALA synthase
Acetyl CoA carboxylase
ATP + Acetyl CoA + CO2 ADP+Pi+ MalonylCoA
TRANSLOCASES
• This newly added group of enzymes transport molecules across biological
membranes
• Na+ - K+ pump ( Intestinal wall and kidney tubules)
• ATP – ADP translocase ( Mitochondrial membrane)
ENZYME STRUCTURE
• Active site
The small cleft like portion of an enzyme where the
substrate binds and where catalysis occurs.
• Characteristics of an active site
1. In most enzymes, the active site contains a substrate binding site
(substrate binds) and a catalytic site (reaction occurs).
2. The coenzymes or cofactors are bound to the catalytic site.
3. The amino acids occurring at the active site participate directly in catalysis
- catalytic residues.
4. Serine is the most commonly occurring amino acid. Others are cysteine,
aspartate, glutamate, lysine, histidine, arginine, threonine and tyrosine
5. The active site is not rigid in conformation and shape— it is flexible,
to ensure effective binding of the substrate to the enzyme.
6. It is largely responsible for the substrate-specificity of an
enzyme.
7. Substrates are bound to enzymes at active sites by weak non-covalent
bonds such as hydrogen bonds and hydrophobic interactions or van der
Waals forces( enzyme–substrate binding is usually reversible.)
• Some enzymes possess additional binding sites other than active site
known as regulatory sites -allosteric regulation
Type Example
Serine proteases Trypsin ,chymotrypsin, clotting factors
Aspartyl proteases Pepsin
Cysteinyl aspartic proteases Caspases in apoptosis
Name of enzyme Important amino acid at catalytic site
Trypsin Serine ,Histidine
Chymotrypsin His, asp, Ser
Alkaline Phosphatase Serine
Hexokinase Histidine
Cofactor and coenzyme
• Enzymes contain a non protein prosthetic group apart from
the protein part.
• The protein part is called apoenzyme
• The non protein prosthetic group is called cofactor.
• Together they are called Holoenzymes
Co factors are of two types
• Metal ions (Inorganic)
• Coenzymes (Organic)
• Metal ions
When the prosthetic group in an enzyme is an inorganic trace
metal such as Zn, Mg, and Cu, the enzyme is called a
metalloenzyme.
Pyruvate kinase
Glycosyl transferase
carboxypeptidase
Mo Xanthine oxidase
• Some metal ions do not occur as the real constituents of the
holoenzyme structure, they are loosely held by the enzyme.
They are called as metal activators of those enzymes
• Amylase requires chloride ions
