enzyme reaction on nuclic acid and proteian reaction

enzyme are protein that denature the structure and adjust in it

1. Regulatory Enzymes Exhibit increased or decreased catalytic activity in response to certain signals. Generally catalyze the rate limiting step in a metabolic pathway. Allow the organism to adjust to change in environmental conditions and developmental stage.

2. Regulation of enzymes Allosteric Covalent Proteolytic cleavage Binding of regulatory proteins

3. Allosteric enzymes are the enzymes having ‘other’ sites. When a ligand modulator binds to its binding site on the enzyme, there is a change in conformation of the enzyme which is transmitted to the other part of the enzyme and changes the binding property of the ‘active site’ for the substrate molecule. The ligand modulator is known as allosteric modulator or allosteric effector . If ligand modulator and the substrate are the same molecule, the allosteric enzyme is said to be homotropic , while if modulator and substrate are different molecules, the enzyme is said to be heterotropic . Binding of ligand modulator may have negative or a positive affect on binding of the substrate molecule, thereby making the enzyme more or less active upon binding of modulator. Allosteric enzymes are generally multisubunit enzymes. Allosteric site and the active site are localized in separated subunits. Binding of a ligand modulator to one subunit results in a conformational change which is transmitted to the other subunit containing the active site for the substrate molecule. This phenomenon where binding of one ligand molecule facilitates the binding of other molecules to a protein is known as cooperativity . Allosteric enzymes

4. Allosteric enzymes

5. Allosteric enzymes are often have multiple subunits Aspartate transcarbamoylase (12 subunits, 6+6) Blue and purple subunits = Catalytic subunits Red and yellow subunits = Regulatory subunits

6. Aspartate transcarbamoylase

7. Aspartate transcarbamoylase CTP

8. Feedback Inhibition In multienzyme metabolic pathways, usually the first enzyme is the allosteric enzyme. The first enzyme is often inhibited by the end product by negative allosteric regulation. This phenomenon called ‘ end product inhibition ’ slows down the entire pathway when enough of the metabolite has been synthesized.

9. Allosteric enzymes do not exhibit Michaelis-Menton kinetic behavior Homotropic allosteric enzymes have a sigmoid substrate- saturation curve. Heterotropic allosteric enzymes have substrate saturation curves of different shapes

10. Allosteric enzymes do not exhibit Michaelis-Menton kinetic behavior Homotropic allosteric enzymes have a sigmoid substrate- saturation curve. Heterotropic allosteric enzymes have substrate saturation curves of different shapes

11. Homotropic allosteric enzymes have sigmoid substrate saturation curve Substrate concentration at which half the V max is achieved is called K 0.5

12. Substrate-saturation curve of heterotropic allosteric enzymes: Case I Positive modulation: K 0.5 decreases without a change in V max Negative modulation: K 0.5 increases without a change in V max

13. Substrate-saturation curve of heterotropic allosteric enzymes: Case II Positive modulation: V max increases without a change in K 0.5 Negative modulation: V max decreases without a change in K 0.5 (Case II is less common than case I)

14. Enzyme regulation by covalent modification Regulation of enzyme activity by reversible posttranslational modification of specific residues is common both in prokaryotes and eukaryotes. The covalent modifications that have been shown to regulate enzyme activity are phosphorylation, adenylation, uridylylation, ADP-ribosylation, methylation, acetylation and glycosylation. Each of these posttranslational modifications are performed by a a pair of enzymes.

15. Enzyme regulation by covalent modification

16. Phosphorylation Ser, Thr, Tyr, Lys are phosphorylated when ATP donates its γ-PO 3 2- . His and Asp phosphorylation occurs in prokaryotes. Phosphorylation imparts negative charge; this can result in conformational changes due to ionic interactions (with backbone N or Arg side chain) and hydrogen bonding. Adding a phosphate (PO 4 ) to a polar R group of an amino acid can increase the polar and hydrophilic character of the protein. In eukaryotes, protein phosphorylation can be an important regulatory event. Many enzymes and receptors are switched "on" or "off" by phosphorylation and dephosphorylation. Signals are amplified in protein kinase cascades. ATP ADP Kinase Phosphatase

17. An example of the important role that phosphorylation plays is the p53 tumor suppressor/ transcription factor gene, which—when active—stimulates transcription of genes that suppress the cell cycle, even to the extent that the cell undergoes apoptosis. However, this activity should be limited to situations where the cell is damaged or physiology is disturbed. To this end, the p53 protein is extensively regulated. In fact, p53 contains more than 18 different phosphorylation sites. Upon the deactivating signal, a protein becomes dephosphorylated and stops working. Enzyme activation by phosphorylation p53 p53 Pi Protein kinase Phosphatase Tumor suppressor

18. One common mechanism for phosphorylation-mediated enzyme inhibition was demonstrated in the tyrosine kinase called "src" (pronounced "sarc”). When src is phosphorylated on a particular tyrosine, it folds on itself, and thus masks its own kinase domain, and is shut "off". Enzyme inactivation by phosphorylation

19. Enzyme activation by phosphorylation Glycogen (Glucose n ) + Pi a Glycogen (Glucose n-1 ) + Glucose-1- phosphate Glycogen phosphorylase

20. Dinitrogen reductase (involved in nitrogen fixation in bacteria). Diphtheria toxin ADP- ribosylates and inactivates EF2. Cholera toxin ADP- ribosylates and inactivates G-proteins. Enzyme regulation by covalent modification

21. Enzyme regulation by covalent modification

22. Enzyme regulation by proteolytic cleavage Many enzymes are synthesized as inactive precursor. Activation of such enzymes involve proteolytic cleavage of the inactive precursor to produce fully active enzyme molecule. Inactive precursor of the enzyme is called zymogen . Chymotrypsinogen a zymogen of chymotrypsin. Trypsinogen a zymogen of trypsin. Unlike allosteric and covalent regulation, cleavage is an irreversible type of regulation.

23. Enzyme regulation by proteolytic cleavage

24. Proteolysis of trypsin Trypsin is secreted into the intestine, where it acts to hydrolyze proteins into smaller peptides or amino acids. Trypsin is produced in the pancreas in the form of inactive zymogen , trypsinogen . It is then secreted into the small intestine, where the enzyme enteropeptidase activates it into trypsin by proteolytic cleavage. The resulting trypsins themselves activate more trypsinogens (autocatalysis), so only a small amount of enteropeptidase is necessary to start the reaction. As extra insurance, the pancreas also makes a small protein, trypsin inhibitor , that binds to any traces of active trypsin that might be present before it is secreted into the intestine. It binds to the active site of trypsin , blocking its action but not itself being cut into tiny pieces. trypsinogen trypsin Trypsin inhibitor enteropeptidase

25. Enzyme regulation by binding of inhibitors Some enzymes are regulated by binding of inhibitor molecules. Pancreatic trypsin inhibitor binds to and inhibits trypsin. a 1-antiproteinase inhibits neutrophil elastase, a protease acting on elastin.