1. Chapter 3 Enzymes

2. Chemical Reactions Chemical reactions: – Involve breaking of chemical bonds in reactants Requires activation energy – Making new chemical bonds to form products – Usually involve changes in potential (chemical) energy levels of atoms

5. Enzymes are Catalysts Speed up chemical reactions Not used up (not a reactant) Naming system: – Prefix is based on the reactant it catalyzes – Suffix is -ase

6. Enzymes are Globular Proteins Roughly round in shape Tertiary and Quaternary protein structure is maintained by: – Hydrophobic R groups on the inside – Hydrophilic R groups on the outside Water soluble

7. Enzymes Have an Active Site Shape and charges of active site allow for induced fit with substrate Substrate specific Enzyme-substrate complex stresses bonds in substrate Facilitates reaction, with less activation energy

8. The active site – Is the region on the enzyme where the substrate binds Figure 8.16 Substate Active site Enzyme (a) Figure 8.16 (b) Enzyme- substrate complex

9. Induced fit of a substrate – Brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction Figure 8.16 (b) Enzyme- substrate complex

10. The catalytic cycle of an enzyme Substrates Products Enzyme Enzyme-substrate complex 1 Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). 2 Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. 3 Active site (and R groups of its amino acids) can lower E A and speed up a reaction by acting as a template for substrate orientation, stressing the substrates and stabilizing the transition state, providing a favorable microenvironment, participating directly in the catalytic reaction. 4 Substrates are Converted into Products. 5 Products are Released. 6 Active site Is available for two new substrate Mole. Figure 8.17

12. The Course of a Reaction Initial rate of reaction – steepest (fastest) rate of reaction Effect of enzyme concentration: – Direct relationship Effect of substrate concentration: – Direct relationship up to the point where every enzyme molecule is bound in an enzyme-substrate complex

13. Effect of Enzyme Concentration on Initial Rate of Reaction

14. Effect of Substrate Concentration on Initial Rate of Reaction

15. Temperature and Enzyme Activity In general, rate of reaction is proportional to temperature High temperature disrupts protein secondary, tertiary and quaternary structure, causing it to denature Different enzymes have different optimum temperatures at which they are most efficient

17. pH and Enzyme activity Different enzymes have a different optimum pH H+ and OH- ions can disrupt secondary, tertiary and quaternary structure, denaturing an enzyme

19. Enzyme Inhibitors Irreversible Reversible – Competitive – Noncompetitive

20. Irreversible Inhibitors Nonspecific inhibitors – Acids & bases – Heavy metals (disrupt disulfide bonds between R groups) – Temperature Specific inhibitors – Covalently bond to enzyme, alter structure

21. Reversible Inhibitors Competitive inhibitors – Makes temporary bond with active site Noncompetitive inhibitors – Bonds at allosteric site – Feedback (end-product) inhibition – inhibitor may be a product of same metabolic pathway

22. Enzyme Inhibitors Competitive inhibitors – Bind to the active site of an enzyme, competing with the substrate Figure 8.19 (b) Competitive inhibition A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme (a) Normal binding

23. Competitive Reversible Inhibitors & Substrate Concentration Since competitive inhibitors compete with substrate molecules for the active site, their effects can be overcome by increasing substrate concentration.

24. Noncompetitive inhibitors – Bind to another part of an enzyme, changing the function Figure 8.19 A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions. Noncompetitive inhibitor (c) Noncompetitive inhibition

25. End-Produce (Feedback) Inhibition in isoleucine synthesis Active site available Isoleucine used up by cell Feedback inhibition Isoleucine binds to allosteric site Active site of enzyme 1 no longer binds threonine; pathway is switched off Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Intermediate B Intermediate C Intermediate D Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 End product (isoleucine)

27. Relationship of Temperature to Reaction Rate Temperature Reaction Rate Enzyme Denatures