regulation of enzyme activity

04/12/2023 Dr. Ashok Kumar J; Professor; Department of Biochemistry.

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Regulation of enzyme activityDr. Ashok Kumar. J.

International Medical SchoolManagement and Science University

Malaysia


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OBJECTIVES: To learn……

• Enzyme specificity• Importance of regulation of enzyme activity• Different modes of regulation of enzyme activity

• Allosteric regulation• Covalent modification• Induction and repression• Compartmentalization• Isoenzymes


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Enzyme Specificity

Types of Specificity1. Absolute specificityAct on only one substrate and catalyze

one reaction

Urea AmmoniaUREASE


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2. Stereospecificity

Show specificity towards one steroisomeric form of the substrate

e. g. : L- Lactate dehydrogenase can act only on L-lactateD- Amino acid oxidase can act only on D- amino acid not on L- amino acid


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3. Bond specificity Group SpecificityProteolytic enzymes show bond specificity Hydrolyze specific bond with a specific side chain group

Proteolytic enzymes :- Enzymes involved in hydrolyzing peptide bond

e.g.: Trypsin : hydrolyze peptide bond formed by carboxyl group of arginine or lysine Chemotrypsin : hydrolyze peptide bond

formed by the carboxyl group of aromatic amino acids


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4. Group specificity

Same enzyme catalyze the same reaction on a group of structurally similar compounds

e.g: HexokinaseCatalyzes phosphorylation of glucose, galactose, mannose


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Importance regulation of enzyme activityRegulation of by

Allosteric regulationCovalent Modification (reversible and irreversible)Induction and repressioncompartmentalizationIsoenzymes

Regulation of enzyme activity


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Importance

• Helps to use the substrate economically• Regulate the metabolic pathways and their interrelations


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ALLOSTERIC REGULATION

Allostearic “occupying another space”Allosteric site was first proposed by

Jacques Monod

Substances which bind to allostearic site can modify their activity – Allosteric effector

Allosteric effectorsSubstances that bind to allosteric site and

modifies the activity of the protein

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Binding of an allosteric effector induces conformational change in the enzyme

Allosteric activator and inhibitor exhibit positive and negative cooperativities with the substrate

Allosteric activator :-on binding to the allosteric site promote

binding of substrate to the acive site or the catalytic actionAllosteric inhibitor:- has opposite action

Dr. Ashok Kumar J; Professor; Department of Biochemistry.


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Allosteric enzymes possessing more than one substrate binding site on its subunits can bind many substrates

Binding of one substrate to active site increases the affinity of other active site to substrate

Homotropic effect

Allosteric modulator is different from the substrateHeterotropic allosteric effect


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According to heterotropic effect of allosteric modulators

Allosteric enzymes are classified into

1. K series enzymes2. V series (M series) enzymes


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E.g.: Phosphofructokinase

K – class of allosteric enzymes• Allosteric effector changes the Km of the

enzyme, Vmax is not altered• Double reciprocal plot is similar to

competitive inhibition


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E.g.: Acetyl CoA carboxylase

V – class of allosteric enzymes• Allosteric effector changes the Vmax of the enzyme, Km is not altered• Double reciprocal plot is similar to

non-competitive inhibition


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According to the model used (concerted model) to study allosteric enzymes –

they exists in two conformational states:

T (tense) and R (relaxed) state


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In the absence of the allosteric modulator an allosteric enzyme follows the hyperbolic kinetics

• Allosteric activator favour ‘R’ state of the enzyme

• Allosteric inhibitors favour ‘T’ state of the enzymeR

S

TX

R = Relax(active)

T = Tense(inactive)


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In the presence of the allosteric inhibitor an allosteric enzyme follows the sigmoid kinetics

Sigmoidal curve

Cooperative(Sigmoidal)

Noncooperative(Hyperbolic)


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An intermediate or a product of a metabolic pathway allosterically inhibits an enzyme

catalyzing earlier stepFeed back Allosteric Inhibition

E1 E2 E3 E4 E5 E6A B C D E F G


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Carbamoyl phosphate

Carbamoyl aspartic acid

CO2 + Glutamine + ATP

Carbamoyl phosphate synthetase II

Aspartate Trans

carbamoylase

CTP

Aspartate Transcarbamoylase

Initial step of pathway which synthesize CTPCTP acts as allosteric

inhibitor


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Feedback inhibition

04/12/2023 21Dr. Ashok Kumar J; Professor; Department of

Biochemistry.

Glycine + Succinyl CoA

δ aminolevulonic acid (ALA)

ALA Synthase

HEME

-ALA Synthase

catalyzes fist step of heme biosynthesis

Heme end product of the pathway act as allosteric inhibitor


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Feed forward allosteric activationAn intermediate of the pathway act as allosteric activator of the enzyme catalyzing later step in

that pathway


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COVALENT MODIFICATION

Addition of a group or removal of a group from enzyme protein forming covalent bond

Phosphorylation, dephosphorylation methylation, ADP ribosylation etc

Zymogen activation

Phosphorylation of enzyme proteins can activate or inactivate the enzyme

Phosphate group is attached to Serine, Threonine or tyrosine residues


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e.g. : enzymes involved in the metabolism of glycogen

Metabolism of glycogen takes place in cytosol


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Glycogen synthase - Active (Glycogen Synthesis)

Glycogen Phosphorylase - Inactive (Glycogen breakdown)

Glycogen Phosphorylase - Active (Glycogen breakdown) P

Glycogen synthase - Inactive (Glycogen Synthesis) P

ATPADP


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Zymogen activation (Activation of latent enzyme)

Pepsinogen

Pepsin

Trypsinogen

Trypsin

Irreversible covalent modification


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INDUCTION AND REPRESSIONAffect the amount of enzyme present

Increase in synthesis – InductionDecrease in synthesis - Repression

InducerRepressor

Amount of enzyme directly controls the velocity of the reaction


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Constitutive enzymes : Level of which is fairly constant

Adaptive enzymes : Concentration increases or decreases as per the need of the body

Alteration in enzyme levels as a result of induction and repression of enzyme protein

synthesis are slow (Hours to days)


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Hormone insulin:Induces synthesis of –

Glucokinase, phosphofructokinase, Pyruvate kinase

Represses Synthesis of – Pyruvate carboxylase, Phosphoenol pyruvate

carboxykinase (PEPCK), Glucose 6 phosphatase


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Hormone Glucagon, Glucocorticoids and epinephrin Induces synthesis of –

Pyruvate carboxylase, Phosphoenol pyruvate carboxykinase (PEPCK), Glucose 6 phosphatase

Glucagon Represses Synthesis of –

Glucokinase, phosphofructokinase, Pyruvate kinase


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Compartmentalization

Fatty acid synthesis takesplace in cytosol

Fatty acid oxidation takes place in mitochondria

Synthetic and catabolic pathways located in different subcellular sites to achieve maximum economy


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Isoenzymes

Multienzyme complexes

Increases the efficiency of the metabolic pathway


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Thank you

regulation of enzyme activity

Health & Medicine

allosteric enzymefollows

allosteric site andmodifies

allostearic site

group specificitysame

acive site

spaceallosteric site

substratebinding site

vmax ofthe enzyme