Date post: | 11-May-2015 |
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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.
THANK YOU