ENZYMES

M.Prasad NaiduMSc Medical Biochemistry, Ph.D,.

Based on the type of reaction enzymes can be classified into SIX MAJOR TYPES

Classification of enzymes

1. Oxidoreductases- lactate dehydrogenase

2. Transferases- glucokinase

3. Hydrolases- chymotrypsin, G6Pase

4. Lyases- fumarase

5. Isomerases- phosphoglucoisomerase

6. Ligases- Acyl CoA synthetase

Transfer of electrons (Hydride ions or H atoms)

1) Dehydrogenases2) Oxidases3) Oxygenases

There are several subclasses

OXIDOREDUCTASE

alcohol aldehyde

Oxidation of Ethanol by Alcohol Dehydrogenase

Remove electrons and/or hydrogen atoms

Dehydrogenases

Catalyze incorporation of oxygen into a substrate

Hydroxylation of Progesterone by a Monooxygenase

Oxygenases

Transfer functional groups in group transfer reactions

1) Amino transferases 2) Kinases3) Glycosyl transferases

Types of transferases

TRANSFERASES

Transfer an amino group from one amino acid to a new keto acid

Amino Transferases (or Transaminase)

Catalyze transfer of the phosphoryl group

Kinases

Transfer an activated glycosyl residue to a glycogen primer. This is a key enzyme in glycogen synthesis

Glycosyl Transferases

Hydrolysis reactions

The generalized reaction involves hydrolytic cleavage of C-O, C-N, O-P, and C-S bonds

HYDROLASES

R1 C NH

O

R2 R1 C O-O

NH3 R2+ H2O +

Cleavage of a peptide bond by peptidases

Peptidase

Addition or removal of elements of water, ammonia, or carbon dioxide

LYASES

Removal of an element of CO2 from -a and b-keto acids or amino acids

R C C

O

O- R C H

O

CO2+ H+ +

O

Decarboxylases

The substrates bicarbonate and pyruvate are ligated to form a four-carbon (C4) oxaloacetate

Pyruvate Decarboxylase

Fumarase reversibly converts fumarate to malate. It removes H2O in a dehydration reaction converting malate to fumarate

Dehydratases

Transfer of groups within molecules to yield isomeric form

Isomerases

Isomerases that catalyze inversion at asymmetric carbon atoms

Epimerases

Involved in synthetic reactions where two molecules are joined at the expense of a high-energy phosphate bond of ATP

LIGASES

6.Ligases

BiotinAcetyl CoA + CO2 Malonyl CoA

ATP ADP + Pi Enzyme is Acetyl CoA carboxylase or

Acetyl CoA-CO2 ligase (systematic) code number EC.6.4.1.2ese enzymes link two substrates together, usually.

Significance of Km

km indicates the affinity of enzyme to the substrate

Smaller the km larger the affinity

Larger the km smaller the affinity

Substrate conc affects reaction velocity (rate)

As substrate concentration is increased, Initial

velocity vi, increases until it reaches

maximum Vmax. further increase in substrate concentration

does not increase velocity the enzyme is said to be saturated.

Note that the shape of the graphic curve relating to catalytic activity and substrate concentration is hyperbolic

Michaelis – menten equation illustrates in

mathematical terms the relationship between initial reaction velocity , vi, and substrate concentration.

Km = Michaelis constant Vmax = maximal velocity Vi = initial velocity[s] = substrate concentration

Vmax [s]Vi =

Km + [s]

Km value or Michaelis – menten constant is defined as substrate concentration (expressed in moles / liter) to produce half maximal velocity in an enzyme catalysed reaction at a particular enzyme concentration.

It indicates that 50% of enzyme molecules are bound with the substrate molecules.

It’s a characteristic of an enzyme or signature of an enzyme.

Lower the Km value Higher the affinity of the enzyme for substrate.

Higher the Km value, the affinity of the enzyme for the substrate is low.

HexokinaseGlucose Glucose-6-

Phosphate ATP ADP

Glucokinase

HK Km value 10-2 mmol / LGK Km value 20 mmol / L

Enzyme Patterns (Enzyme profiles) in diseses

1.Hepatic diseases 1.Alanine amino transferase (ALT)

Marked increase in parenchymal liver disease

2. Alkaline phsophatase (ALP)Marked increase in obstructive liver disease

3.Nucleotide phsophatase (NTP)Elevated in liver dysfunction with cholestasis

4. Gamma glutamyl transferase (GGT) Increase in obstructive and alcoholic liver disease

II.Myocardial Infarction:

1.Cratine Kinase (CK – MB)First enzyme to rise following infarction, CK-MB isoenzyme

is specific.

2.Aspartate amino transferase (AST)Rises after the rise in CK and returns to normal in 4-5 days

3.Lactate dehydrogenase (LDH)Last enzyme to rise. LDH-1 becomes more than 2 (Flipped

pattern)

Enzyme pattern in Myocardial infraction

AST

Normal pattern of LDH in serum

Flipped pattern of LDH in MI

III.Muscle disease1.Creatine Kinase (CK-MM)

Marked increase in muscle disease. CK-MM fraction is elevated

2. Aspartate amino transferase (AST)

Shows an increase in different types of muscle diseas; not specific

3. Aldolase

Earliest enzyme to rise. But not specific

IV. Bone diseases1.Alkanine phosphatase (ALP)

Marked elevationin osteoblastic bone activity as in rickets. Heat labile bone isoenzyme is elevated. Also in Paget’s disease.

V.Prostate cancer1.Prostate specific antigen (PSA)

Marker for prostate cancer. Mild increase in benign prostate enlargement.

2. Acid phosphatase (ACP)Marker for prostate cancer . Metastatic bone disease especially from a

primary from prostate.

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