Enzymes Just because a reaction is spontaneous, that does not mean it will happen rapidly

1. Enzymes Just because a reaction is spontaneous, that does not mean it will happen rapidly Enzymes catalyze reactions by lowering activation energy of the reaction

2. Enzymes lower activation energy of reactions

3. Where does activation energy come from? This energy makes chemical bonds unstable enough to break Can come from heat; but not in a physiological environment Enzymes lower activation energy so that a reaction can proceed They cannot change the direction of a reaction (cannot make an exergonic reaction endergonic- how DO those occur?)

4. Essential features of enzymes Specificity for substrate Defined by active site actually a small part of the enzyme “induced fit” Is not itself consumed by the reaction Is a protein, with protein properties

5. Modeling enzyme activity

6. What happens in “induced fit”? Substrate activation conformational change enzyme may donate or accept protons affecting reactivity of substrate; may be why pH is so important enzyme may donate or accept electrons forming (temporary bonds between enzyme and substrate) “Catalytic event” results in formation of product Product is released; enzyme may be used again.

7. Active site and catalysis

8. Environmental effects on enzyme function • Heat tends to increase enzyme activity- up to a point • Enzymes in our bodies are optimized for physiological conditions • Optimal range may be narrow or broad • Cofactors or coenzymes may also be required

9. Enzyme activity can be saturated

10. Regulation of enzyme activity Temperature and pH Substrate, enzyme concentration Availability of cofactors, coenzymes Inhibitors competitive allosteric

11. Cofactors and coenzymes • Cofactors are metallic ions • Might bind to active site • Coenzymes are often derived from vitamins • Usually act as carriers of electrons, protons, or other molecules

12. Inhibition of enzyme activity • Competitive inhibitors bind to the active site • Allosteric- bind to another site on the enzyme and change its conformation

13. Competitive inhibition can be reversible or irreversible can be overcome by adding more substrate irreversible inhibitors bind covalently example: neurotoxins bind to acetylcholinesterase and inhibit transmission of nervous impulses (many insecticides work this way) reversible inhibitors bind noncovalently

14. Noncompetitive inhibitors do NOT bind at the active sight (but do bind the enzyme) Allosteric regulation Covalent modification

15. Note: allosteric and active sites are different

16. Many biological reactions occur in pathways E A B C D E is the end product of the reaction Feedback inhibition prevents excess accumulation of E, or depletion of A

17. Pathways and their regulation (usually allosteric)

18. Enzymes can be regulated by covalent modification Chemical groups are added or removed from enzymes which affects their activity Phosphorylation and dephosphorylation are among the most ubiquitous Kinases and phosphorylases do just that Proteolytic cleavage can convert inactive “proenzymes” to active forms

19. Enzymes with related functions may be collected into specific parts of the cell Mitochondrion- respiration Endoplasmic reticulum- protein modification Lysosome- degradation of waste products

20. Ribozymes- not all enzymes are proteins! Cech et al., 1980’s (Nobel Prize, 1989) RNA is processed by removal of segments called introns RNA was shown to excise the introns and behave like enzymes, i.e., Michaelis- Menton kinetics RNA may have been the first catalyst, proteins later

21. Summary • Enzymes are proteins • Enzymes interact with specific substrates • Enzyme activity can be affected by environmental factors • Enzyme activity can be inhibited • Enzymatic reactions are organized into pathways in the cell