Download Bio-chemistry(Enzymes)

Bio-chemistry(Enzymes)
Enzyme:
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Enzymes may be defined as biocatalysts synthesized by living cells. They are protein in
nature (exception - RNA acting as ribozyme), colloidal and thermo labile in character,
and specific in their action.
Holoenzyme is the functional enzyme.
Apoenzyme is protein part of holoenzyme.
Co-enzyme is non-protein part of holoenzyme.
Co-enzyme: The non-protein, organic, Iow molecular weight and dialysable substance
associated with enzyme function is known as coenzyme.
Ribozymes are a group of ribonucleic acids that function as biological catalysts and they
are regarded as non-protein enzymes
Isozymes: The multiple forms of an enzyme catalysing the same reaction are isoenzymes
or isozymes.
Classification of Enzyme: (OTHLIL)
1. Oxidoreductases: Enzymes involved in oxidation-reduction reactions.
E.g. Alcohol dehydrogenase (alcohol : NAD+ oxidoreductase), cytochrome oxidase
2. Transferases: Enzymes that catalyse the transfer of functional groups.
E.g. Hexokinase (ATP : D-hexose 6-phosphotransferase), transaminase, transmethylases,
phosphorylase.
3. Hydrolases: Enzymes that bring about hydrolysis of various compounds.
E.g. Lipase(triacylglycerol acyl hydrolase), esterase, acid and alkaline phosphatase
pepsin, urease.
4. Lyases: Enzymes specialized in the addition or removal of water, ammonia, CO2 etc.
E.g. Aldolase (ketose1 –phosphate aldehyde lyase), fumarase, histidase.
5. lsomerases: Enzymes involved isomerization reactions.
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Bio-chemistry(Enzymes)
E.g. Triose phosphate isomerase(De –glyceraldehyde 3-phosphate ketoisomerase),
retinol isomerase.
6. Ligases: Enzymes catalyzing the synthetic reactions (Greek : ligate-to bind) where two
molecules are joined together and ATP is used.
E.g. Glutamine synthetase(L- glutamate amino liagase), acetyl
thiokinase.
CoA carboxylase,
Michaelis menten equation:
Order of reaction: When the velocity of the reaction is almost proportional to the substrate
concentration (i.e. [S] is less than Km,) the rate of the reaction is said to be first order with
respect to substrate. When the [S] is much greater than Km, the rate of reaction is independent
of substrate concentration, and the reaction is said to be zero order.
Enzyme Inhibition:
1) Reversible inhibition:
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Inhibitor binds non-covalently with enzyme.
Enzyme inhibition can be reversed if the inhibitor is removed.
Divided into two types:
l. Competitive inhibition:
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Inhibitor (l) closely resembles the real substrate(S).
Inhibitor competes with substrate and binds at the active site of the enzyme but does
not undergo any catalysis.
As long as the competitive inhibitor holds the active site, the enzyme is not available for
the substrate to bind.
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Bio-chemistry(Enzymes)
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Relative concentration of the substrate and inhibitor and their respective affinity with
the enzyme determines the degree of competitive inhibition.
Inhibition could be overcome by a high substrate concentration.
In competitive inhibition, the K- value increases whereas Vmax remains unchanged.
ll. Non-competitive inhibition :
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Inhibitor binds at a site other than the active site on the enzyme surface.
This binding impairs the enzyme function.
The inhibitor has no structural resemblance with the substrate however there usually
exists a strong affinity for the inhibitor to bind at the second site.
Inhibitor does not interfere with the enzyme-substrate binding.
Catalysis is prevented possibly due to a distortion in the enzyme conformation.
Inhibitor generally binds with the enzyme as well as the ES complex.
For non-competitive inhibition, the Km value is unchanged while Vmax is lowered.
2) lrreversible inhibition:
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Inhibitors bind covalently with the enzymes and inactivate them, which is irreversible.
Inhibitors are usually toxic poisonous substances.
Katal
One kat denotes the conversion of one mole substrate per second (mol/sec). Activity may also
be expressed as millikatals (mkat), microkatals (kat) and so on.
International Units (lU)
One Sl unit or International Unit (lU) is defined as the amount of enzyme activity that catalyses the
conversion of one micromol of substrate per minute.
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Bio-chemistry(Enzymes)
Increase in plasma enzymes in the diagnosis of diseases
Enzymes
Disease in which increased
DIGESTIVE ENZYMES
Amylase
Acute pancreatitis, mumps, obstruction
in pancreatic duct, severe diabetic
ketoacidosis.
Lipase
Acute pancreatitis, moderate elevation
in carcinoma of pancreas.
TRANSAMINASES
Alanine transaminase(ALT) or
Acute hepatitis (viral or toxic), jaundice,
Serum
glutamate
pyruvate cirrhosis of liver.
transaminase(SGPT)
Aspartate transaminase(ASP) or
Myocardial infarction, liver diseases,
Serum
glutamate
oxaloacetate liver cancer, cirrhosis of liver
transaminase(SGOT)
PHOSPHATESES
Alkaline phosphatase (ALP)
( pH optimum 9-10)
Bone diseases (related to higher
osteoblastic activity)- rickets, panget’s
disease,
hyperparathyroidism,
carcinoma of bone.
Liver disesease-obstructive jaundice
(cholestasis),
infective
hepatitis,
cirrhosis of liver.
Acid phosphatase (ACP)
( pH optimum 4-6)
Prostatic carcinoma i.e. cancer of
prostate gland, panget’s disease.
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Bio-chemistry(Enzymes)
ENZYMES
OF
METABOLISM
CARBOHYDRATE
Isocitrate dehydrogenase(ICD)
Liver diseases(inflammatory toxic or
malignant)
Lactate dehydrogenase(LDH)
Myocardial infarction, acute infective
hepatitis,
muscular
dystrophy,
leukemia, pernicious anaemia.
Aldolase
Muscular dystrophy, liver diseases,
myocardial infarction, myasthenia
gravis, leukemia
MISCELLANEOUS ENZYMES
Creatine kinase (CK) or
Creatine phosphokinase(CPK)
Myocardial infarction (CK useful for
early detection), muscular dystrophy.
Hypothyroidism, alcoholism.
Cholinesterase (ChEI)
5’-nucleotidase or
Nephritic
infarction.
Nucleotide phosphatase (NTP)
Γ-glutamyl transpeptidase (GGT)
Hepatitis, obstructive jaundice, tumors.
Alcoholism,
infective
hepatitis,
obstructive jaundice.
Ceruloplasmin (ferrooxidase)
Bacterial infections, collagen diseases,
cirrhosis, pregnancy.
syndrome,
myocardial
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