enzyme 221 allosteric enzyme

8/8/2019 Enzyme 221 Allosteric Enzyme

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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

enzyme 221 allosteric enzyme

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