04. Regulatory Enzymes.pptx

Regulatory Enzymes in Biochemistry

1. REGULATORY ENZYMES 1 Dr. Muhammad Anees Ur Rehman Assistant Professor/ Head of Department Ruth Pfau College of Nutrition Sciences

2. REGULATORY ENZYMES 2 A regulatory enzyme is an enzyme in a biochemical pathway which, through its responses to the presence of certain other biomolecules, regulates the pathway activity. This is usually done for pathways whose products may be needed in different amounts at different times, such as hormone production. Some examples are: Pyruvate dehydrogenase kinase Phosphofructokinase

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4. ENZYMES REGULATION 4 Regulate: to control or direct according to a rule, principle or law Enzyme regulation : is the control of the rate of a reaction catalyzed by an enzyme by some effector (e.g., inhibitors or activators) or by alteration of some condition (e.g., pH or ionic strength). Constitutive enzymes: Enzymes needed at the same level all of the time Regulated enzymes: Enzymes needed under some conditions but not others For example enzymes of the Lac Operon Enzymes are made to break down lactose only if lactose is present

5. TYPES OF REGULATION 5 Regulation of amount of enzyme made: At the level of transcription = is RNA made? At the level of translation = is protein made? Regulation of enzyme activity : Allosteric regulation Covalent modification Isoenzymes Proteolytic cleavage of proenzyme Slower process (minutes) Very rapid process (seconds or less than a second)

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7. ALLOSTERIC REGULATION 7 Some enzymes possess a site in addition to the substrate site , This site is known as the allosteric site . Such enzymes having allosteric sites are known as allosteric enzymes. Allosteric site is meant for binding of an allosteric molecule Binding of allosteric molecule changes the conformation of substrate site.

8. ALLOSTERIC REGULATION 8 The allosteric molecule is also known as: Allosteric effector Allosteric modifier Allosteric regulator Some allosteric molecules facilitate the conformational change required for substrate binding They are known as allosteric activators (positive modifiers ) and responsible for the activation of the enzyme

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10. ALLOSTERIC REGULATION 10 Allosteric enzymes have 2 binding site: Active site binds substrate Allosteric site (Effectors or modifiers)

11. ALLOSTERIC ENZYMES, PHOSPHOFRUCTOKINASE 11 PFK-1 catalyzes an early step in glycolysis Phosphoenol pyruvate (PEP), an intermediate near the end of the pathway is an allosteric inhibitor of PFK-1, Some other examples of allosteric effect produced in response to certain hormones and Ca + ions are 3, 5 cyclic AMP ( cAMP ), 3,5 cyclic GMP (cGMP). cAMP and cGMP comes from ATP and GTP under the influence of enzymes adenylyl cyclase and guaninyl cyclase respectively.

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13. ALLOSTERIC INHIBITION 13 It appears when allosteric inhibitor combines with allosteric site of the enzyme. The enzyme in the presence of inhibitor is said to exist in tight (T) state. The enzyme in the presence of substrate is said to exist in relaxed (R) state . In some cases, even the true substrate of an allosteric enzyme can inhibit it, the effect is known as “ Homotrophic effect”. Situations where, not the substrate, some other chemical compounds can inhibit the enzyme is known as “Heterotrophic effect”.

14. FEED-BACK INHIBITION OF ENZYMES 14 In certain cases an allosteric enzyme is inhibited by the products of its own catalytic action. When the product is formed in excess, it brings about the inhibition of enzyme activity, resulting the decrease of its own production. This mechanism is known as Feed back or end product inhibition. The same phenomenon is observed in multi-enzyme systems. In addition to simple ways of feed back inhibition, branched biosynthetic pathways may show more complex manners of feed back inhibition.

15. ALLOSTERIC ACTIVATION 15 It is just opposite of allosteric inhibition. Presence of certain reagents called positive effectors which combine with enzyme at allosteric site results in increased activity of these enzymes. Certain enzymes are present which can bind simultaneously with both positive and negative effectors. The regulation of such enzymes is complex.

16. COVALENT MODIFICATION 16 Several enzymes are first present in an inactive form. They become activated when phosphate bonds present are added to their molecules forming covalent bond. Addition or loss of the these bond change the quaternary structure of enzymes. It leads to structural changes in their catalytic site. Certain chemical groups are covalently linked to regulatory enzymes in a reversible manner to change their activity by changing their conformation .

17. COVALENT MODIFICATION 17 Glycogen phosphorylase, a muscle enzyme is an example of covalent modification regulation. The enzyme is present in two forms A & B representing its active and inactive forms respectively.

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19. PHOSPHORYLATION/DE-PHOSORYLATION 19 most common covalent modification involve protein kinases/phosphatase PDK inactivated by phosphorylation Amino acids with –OH groups are targets for phosphorylation (usually serine, tyrosine or threonine) Phosphates are bulky (-) charged groups which effect conformation

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21. ENZYME REGULATED BY Ca 2+ IONS & CALMODULIN 21 Many enzymes are dependent on Ca 2 + ions and a protein called calmodulin . By combining with Ca 2 + its binding sites for these ions undergoes a conformational change. In this changed form, calmodulin can activate or inhibit a large number of enzymes.

22. ISO-ENZYMES (ISOZYMES) 22 Some metabolic processes are regulated by enzymes that exist in different molecular forms isoenzymes Isoenzymes : multiple forms of an enzyme which differ in amino acid sequence but catalyze the same reaction Isoenzymes can differ in: kinetics, regulatory properties, the form of coenzyme they prefer and distribution in cell and tissues Isoenzymes are coded by different genes

23. ISO-ENZYMES (ISOZYMES) 23 Lactate dehydrogenase – tetramer (four subunits) composed of two types of polypeptide chains, M and H There are 5 Isozymes of LDG: H 4 : heart HM 3 : reticuloendothelial system H 2 M 2 : lung H 3 M: kidney , pancreas M 4 : liver , muscle Example : lactate dehydrogenase (LDG) Lactate + NAD + pyruvate + NADH + H +

24. ACTIVATION BY PROTEOLYTIC CLEAVAGE 24 Many enzymes are synthesized as inactive precursors (zymogens) that are activated by proteolytic cleavage Proteolytic activation only occurs once in the life of an enzyme molecule. Examples of specific proteolysis Digestive enzymes Synthesized as zymogens in stomach and pancreas Blood clotting enzymes Cascade of proteolytic activations Protein hormones Proinsulin to insulin by removal of a peptide

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