1. Biology 102 Lecture 11: Energy Flow in Cells (Part 2)

2. Lecture outline 1. Brief review of chemical reactions and energy flow (from previous lecture) 2. Activation energy and the rate of chemical reactions 3. Enzymes: biological catalysts Enzyme structure and function Enzyme structure and function Regulation of enzyme levels Regulation of enzyme levels

3. What is a chemical reaction? What is a chemical reaction?  Making and breaking of chemical bonds Implications of the second law: Implications of the second law:  Orderly reactants to less ordered products releases energy (EXERGONIC)  Examples… 1. Review of chemical reactions

4. Implications of the second law (cont.) Implications of the second law (cont.)  Less ordered reactants to more orderly products requires energy (ENDERGONIC)  Examples…

5. Energy carriers ATP is the principle energy carrier that allows coupling of reactions ATP is the principle energy carrier that allows coupling of reactions  ATP produced during an exergonic reaction provides energy to for endergonic reactions  Bonds easily formed for easy energy transfer from a high energy compound to ATP  Bonds are unstable to easily give up energy in a reaction Electron carriers such as NADH also carry energy along with electrons Electron carriers such as NADH also carry energy along with electrons 

6. Coupled reactions within living cells ATP is the principle energy carrier that allows coupling of reactions

7. Electron carriers Electron carriers such as NADH also transfer energy, along with electrons, between exergonic and endergonic reactions

8. 2. Activation energy and the rate of chemical reactions Initial input of energy required to start reactions (both exergonic and endergonic) Initial input of energy required to start reactions (both exergonic and endergonic)

9. Will a reaction occur? Low activation energy: reaction will occur spontaneously Low activation energy: reaction will occur spontaneously High activation energy: reaction requires something else to get it over the activation energy hump… High activation energy: reaction requires something else to get it over the activation energy hump…  Temperature increase  Enzymes  NOTE: Understand how these work in different ways!

10. 3. Enzymes: Biological catalysts Example of a metabolic pathway Example of a metabolic pathway

11. What does a catalyst do? Decreases activation energy Decreases activation energy

12. Principles of all catalysts Speed up reactions Speed up reactions Speed up only reactions that would occur anyway, but at a slower rate Speed up only reactions that would occur anyway, but at a slower rate Not consumed or permanently changed in the reaction Not consumed or permanently changed in the reaction

13. Enzymes: Biological catalysts Have all the properties of other catalysts Have all the properties of other catalysts Are proteins! Are proteins! Are highly specific for particular “substrates” (reactants) Are highly specific for particular “substrates” (reactants)  Examples

14. Structure of an enzyme 3-dimensional structure 3-dimensional structure  Think about versatility of proteins Active site Active site

15. Cycle of enzyme-substrate reactions

16. Key points Active site highly specific for the particular reactants Active site highly specific for the particular reactants Conformational change brings together reactants into the correct position to bond Conformational change brings together reactants into the correct position to bond Enzyme allows for the capture of energy in exothermic reactions, and use of energy in endothermic reaction Enzyme allows for the capture of energy in exothermic reactions, and use of energy in endothermic reaction One enzyme can catalyze many reactions One enzyme can catalyze many reactions

17. Regulation of enzymes Importance of regulation Importance of regulation Types of regulation Types of regulation  Regulation of synthesis  Regulation of protein activation  Feedback inhibition

18. Mechanisms of inhibition Allosteric inhibition: Allosteric inhibition:  Bind at a different site than the substrates  Negative feedback can work this way Competitive inhibitors: Competitive inhibitors:  Bind directly at the active site  Some poisons act this way

19. Impact of the environment on enzymes pH pH  Can activate or inactivate  Example: Pepsin (pH=2 in stomach) Salt Salt  Disruption of 3-D structure Temperature Temperature  Direct effects on reaction rates  Denaturation of proteins (2-D, 3-D or 4-D structure) Coenzymes Coenzymes