ENZYMES
Enzymes are grouped into 6 classes:
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Class 1: OXIDOREDUCTASES: catalyze oxidation & reduction of substrates. Ex: Alcohol dehydrogenase
Class II: TRANSFERASES: Transfers one group (other than H2) from 1 substrate to another.
Ex: Hexose + ATP Hexokinase Hexose-6-PO4
Class III: HYDROLASES: Hydrolyzes esters, ether, peptide/glycosidic bonds by adding H2O to then breaking the bond.
Acetylcholine + H2O Acetylcholine esterase Choline + Acetate
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Class IV: LYASES: can remove groups from substrates or break bonds by mechanisms other than hydrolysis.
Fructose-1,6-bisphosphate Aldolase Glyceraldehyde-3-PO4 + Dihydroxyacetone-PO4
Class V: ISOMERASES: produce optical, geometric/positional isomers of substrates.
Ex: cis trans isomerase
Many acids are trans to isomers.
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Class VI: LIGASES: Link 2 substrates together with simultaneous splitting of ATP.
Acetyl CoA + CO2 + ATP Acetyl CoA Carboxylase Malonyl-CoA + AMP + CP
COENZYMES
Enzymes may be simple proteins/complex enzymes, containing non-protein part, called prosthetic group. The prosthetic group is called coenzyme.
Enzyme (Holoenzyme)
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Protein part (Apoenzyme) + Coenzyme
Taking part in rxn catalyzed by oxidoreductases by donating/accepting H+ or e- Enzyme + Cosubstrate/Substrates
Taking part in rxn catalyzed by transferases or isomerases using group other than H2
Ex: NAD, FAD, FMN, CoA
CoA-essential for biological activity of enzyme. Low organic substrate, heat stable, combines loosely with enzyme molecules.
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Molecules of coenzymes is able to convert a large no. of substrate molecules with the help of ATP.
Ex: Glyceraldehyde-3-P Glyceraldehyde-3-P dehydrogenase 1,3-BPG
Lactate Lactate dehydrogenase Pyruvate
MODE OF ENZYME ACTION
Lowering of Activation Energy:
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Enzymes lowers the energy of activation.
Activation is the energy required to convert substrate molecules to transition state.
Acid-Base catalysis: Ex. The action of ribonuclease
Histidine 12 + Histidine 119 Active site of Ribonuclease function
The ionizable ty of prosthetic group act as acid/base.
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Substrate Strain:
Binding of substrate to preformed site on enzyme can induce strain in substrate.
The energy level of substrate is raised, like transition state intermediate.
The resulting strain stretches/distorts the targeted bond, weakening it & making it more vulnerable to cleavage.
Ex: Lysozyme-acid base catalysis
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Substrate strain
Substrate + Enzyme Lysozyme E-S complex strained conformation
Lysozyme N-Acetyl glucosamine + Muramic acid Bacterial cell wall
Covalent Catalysis:
Nu Enzyme + Substrate Covalent bond E' Enzyme
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Regenerate (transitions) E + P
Modified E'
1. New rxn pathway
2. Faster rxn
Covalent catalysis often follows ping pong mechanism.
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Ex: Transaminase
Substrate 1 + Enzyme Substrate 2 + Product
Isoenzymes
Properties coded by diff gene
Located in diff organs/diff compartments of same cell
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May have diff optimal pH
Diff mobility in electrophoresis
Isoenzymes are distinct enzyme forms that catalyze the same rxn.
These are diff molecular forms of same enzyme synthesized from various tissues.
Creatine Kinase (CK)/Creatine phosphokinase
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Creatine PO4 + ADP CK Creatine + ATP
CK has 3 isoenzymes:
CK-MM-skeletal muscle-most in blood
CK-BB-brain-min in blood
CK-MB-brain & skeletal muscle
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Useful diagnostic funct
Marker for MI appears within 4-6 hrs of MI
Cardiac diseases
Peaks at (24 hrs) (1 day)
Returns to baseline by (48-72 hrs) (2-3 days)
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Individual CK isoenzymes-separated by electrophoresis, facilitating detection
Today in most clinical lab, CK is supplemented by Troponin for diagnosis of MI.
Isoenzymes have diff pI
Separated by electrophoresis
Lactate Dehydrogenase
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Tetrameric enzyme-2 monomer types
H (for heart)
M (for muscle)
4 H/M monomers combine to yields 5 isoenzymes
I1-HHHH-predominates in heart tissue
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I2-HHHM
I3-HHMM
I4-HMMM
I5-MMMM-predominates in liver
Tissue specific expression of H/M isomers determines relative proportion in diff tissues.
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Tissue injury
Sp. LDH isoenzymes
Separated
Electrophoresis
Increased substrate conversion coupled assay
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Marker of MI
Alkaline Phosphatase (ALP)
Essential for rxn at pH
Studied for diagnosis of liver disease
H2O + Aliphatic/Aromatic/Heterocyclic compound ALP
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Mg, Mn
Zn constituent of ALP
Produced by osteoblasts of bone, associated with calcification process.
Moderate ? 2-3 times
Very ? level of ALP (10-12 times of upper limit)
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Drastically high (10-25 times)
Hepatic disease
Infective hepatitis
Infective alcohol, hepatitis
Extrahepatic
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Obstruction
Obstructive jaundice
Caused by gall stones/P in bile duct by carcinoma of head of pancreas
Bone disease
Osteoblastic activity ?
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Rickets
Osteomalacia
Metastatic carcinoma of bones
Hyperthyroidism
6 isoenzymes of ALP
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Major fractions-liver isoenzymes, bone, intestinal
a-2 isoenzymes-heat stable
PAA
Intestinal (placental origin)
Heat stable placental isoenzyme-blood of normal pregnancy
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Bone Alkaline phosphatase (BAP):-Identified by heat inactivation
Marker of metabolic bone disease
Catalytic site/Active site/Active center of enzyme
Catalysis occurs at active site of enzyme.
The region of the enzyme where substrate binding and catalysis occurs is called substrate active site or active centre of the enzyme.
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Although all parts are required for maintaining the exact 3D-structure of the enzyme, the rxn is taking place at the active site. The active site occupies only a small portion in the whole enzyme molecule.
Generally, active site is situated in a crevice/cleft in the enzyme molecule.
To the active site, the sp. substrate is bound. The binding of substrate to the active site depends on the alignment of atoms/specific groups at the active site.
The active site contains substrate binding site & catalytic site, sometime these 2 may be diss.
Proteolytic enzymes having serine residue at the active site are called serine proteases.
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Ex-Pancreatic Proteases P, coagulation factors.
The substrate binds to AS by non covalent bonds. These forces are hydrophobic in nature.
During the binding, the sp. groups on active site may realign themselves to provide unique conformational orientation so as to promote exact fitting of substrate to the active site.
The AA/groups that directly participates in making/breaking of bonds at the active site are called catalytic residue/groups.
Factors Affecting the Rate of Enzyme Catalyzed Rxn
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Inhibition
Reversible-temporary
Irreversible-permanently destroy & activity
Competitive
Non competitive
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Suicide Inhibition/Mechanism learned inhibition-
Ex-Allopurinol
Allosteric Modifier
+ve Modulator
-ve Modulator
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This download link is referred from the post: MBBS Lecture Notes for all subjects (updated for 2021 syllabus) - All universities
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