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Allosteric Enzyme
Dr. Samina Hyder Haq
Dept of BiochemistryKing Saud University
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Enzyme regulation Metabolism is the right integration of
varous processes. There are four principles ways in which this is
achieved:
Allosteric control Multiple forms of enzymes
Reversible covalent modification Proteolytic activation
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Isozymes
Isozymes (isoenzymes) are enzymes that differ in
sequence but catalyze the same reaction They usually display different kinetic behavior,
have differing substrate affinities or are regulatedin different manners
The existence of isozymes allows the fine-tuning of processes (e.g. metabolism) by usingdifferent amounts of each isozyme
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Isozymes: Lactate
Dehydrogenase
Humans have two forms of lactatedehydrogenase H form found in heart
M form found in skeletal muscle The two forms are 75% identical and both exist
as homotetramers (H4 and M4)
The H4 form has a higher affinity for substrateCombinations are possible (e.g. H3M, H2M2)allowing for different affinities
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Isozymes: Lactate
Dehydrogenase
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Isoenzyme in Heart attack
The pattern of isoenzymes found in the
plasma serve as a means of identifyingthe site of tissue damage. For example,the plasma levels of creatine kinase (CK)are commonly determined in the diagnosisof myocardial infarction.
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Regulation via Covalent
Modification
The covalent attachment of a molecule to anenzyme (or other protein) can alter its activity
Most such covalent modifications are reversiblee.g. phosphorylation, acetylation Some are irreversible e.g. attachment of a lipid
group that localizes the protein to the membrane
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Phosphorylation
Many proteins regulated via
phosphorylation - addition of phosphoryl group to hydroxyl oxygen of serine, threonine or tyrosine
Terminal ( ) phosphoryl group from ATPtransferred to specific serine, threonineand tyrosine residues Catalyzed by
Protein kinases
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Phosphorylation
Under physiological condition,
phosphorylation (and dephosphorylation)is essentially irreversible
- kinases and phosphatases are requiredState of phosphorylation is then dependantupon the relative activities of kinases and
phosphatases
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Allosteric Regulation
Allosteric modulators bind at a site other
than the active site and cause activation or inhibition
Can include the substrate itself Protein has quaternary structure
Non-Michaelis-Menten kinetics
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Allosteric Enzyme Kinetics: Sigmoid Curve
instead of Hyperbola.
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Why Sigmoid Curve.Affinity for substrate increases with increasing substrateconcentration. A plot of product formation as a function ofsubstrate concentration produces a sigmoidal curve because
the binding of substrate to one active site favors theconversion of the entire enzyme into the R state, increasingthe activity at the other active sites. Thus, the active sites
show cooperativity .
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Allosteric Enzyme
Allosteric enzymes have two conformations:
active (R-state) and less active (T-state) 1. T-state: less active, stabilized by inhibitors 2. R-state: more active, stabilized by substrate
and activators 3. Allosteric enzymes have multiple subunits.
Cooperativity results from the R to T transition of subunits and the interaction of these subunits(quaternary structure)
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Concerted models: All subunits are either R or T (explains positive cooperativity)
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Sequential model
subunits convert from Rto T individually (pos. or neg. coop.)
Positive cooperativitymeans activity increasesas substrateconcentration increases.
B. Negative cooperativitymeans activity decreasesas substrateconcentration increases.
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Heterotropic effectors: The effector may be different from the substrate, in which
case the effect is said to be heterotropic. For example, thefeedback inhibition . The enzyme that converts D to E has
an allosteric site that binds the endproduct, G. If the concentration of G increases (for example, because itis not used as rapidly as it is synthesized), the firstirreversible step unique to the pathway is typically inhibited.Feedback inhibition provides the cell with a product it needsby regulating the flow of substrate molecules through thepathway that synthesizes that product. Heterotropiceffectors are commonly encountered, for example, theglycolytic enzyme phosphofructokinase-1 is allostericallyinhibited by citrate, which is not a substrate for the enzyme
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Feed back inhibition
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Examples of Zymogens
1. Digestive enzymes: pepsin, chymotrypsin,
trypsin, elastase, carboxypeptidase2. Blood clotting - activated by a cascade of
proteolytic activations3. Some hormones: insulin4. Collagen -Collagenase - enzyme that
breaks down collagen5. Caspases - proteolytic enzymes involved inapoptosis (programmed cell death
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Zymogen Active site
Trypsinogen
Chymotrypsinogen
Pepsinogen
Prothrombin
Zymogen provides protection to the body. As the active E maydestroy body substances if activated in absence of S.e.g. if thormbinis formed in the body, it will convert Fibrinogen to Fibrin.This will form clot in blood causing heart attack and Stroke.
TRypsin + peptideEnterokinas
Chymotrypsin + peptide
Pepsin + PeptideHCl
Thrombin + PeptideClottingFactor
Trypsin