1. Lecture 7 Reactions and Enzymes

2. Chemical Reactions in a Cell The Terminology The sum total of all the chemical conversions in a cell is called metabolism. Organic compounds participating in metabolism are called metabolites. Two types of metabolic reactions: anabolic and catabolic reactions. Anabolic reactions are those that link simple molecules together to make complex ones. These are energy-storing reactions (endergonic). Catabolic reactions are those that break down complex molecules into simpler ones. Some of these reactions provide the energy for anabolic reactions. These are energy releasing reactions (exergonic).

3. Thermodynamics All chemical changes are governed by the laws of thermodynamics. 1 st Law – Conservation/ transformation of energy 2 nd Law – Tendency toward disorder/randomness Ordered systems need energy to maintain their order or they will deteriorate. Energy changes in molecules occur when bonds are broken and new ones formed. Organic compounds are a store of potential energy

4. Activation Energy The energy required to initiate a chemical reaction. Once this barrier is overcome the reaction will proceed, generating enough energy to occur spontaneously. E.g Petrol + air The activation energy is important in living organisms as it prevents breakdown of components in the presence of air/oxygen. The biological alternative of overcoming the activation energy is less lethal.

5. Sun (source of energy) Photosynthesis Electrons excited to a higher energy level Anabolic (endergonic) reaction Sugar (source of potential energy) Respiration Catabolism (exergonic) reaction Free energy for endergonic reactions

6. Enzymes Globular proteins Encoded in DNA sequence Enzymes speed up the rate of metabolic reactions in cells, they are biological catalysts. Enzymes are not used up by the reaction, and they cannot change the basic nature/equilibrium of a reaction. Specific Activity is affected by pH, temperature, substrate concentration and enzyme concentration. Efficient

7. Example of a Catalytic Reaction. 2H 2 O 2 2H 2 O + O 2 Hydrogen peroxide water + oxygen Heat to 150 o C Rapid decomposition/explosive reaction clearly not suitable for living organisms 2H 2 O 2 2H 2 O + O 2 Hydrogen peroxide water + oxygen Catalase 37 o C AB Breakdown of a toxic compound into less harmful components at a temperature better suited for living organisms

8. Components of Enzyme Catalysed Reactions E + SES EPE + P

9. Enzyme Structure Substrate E S Active site 5 4 3 Amino acids

10. Enzyme Action- Mechanism of Catalysis Enzymes are thought to accomplish this by reducing the activation energy required for a reaction to occur. The activation energy is reduced when the substrate attaches (by weak ionic attractions and hydrogen bonds) to the active site of the enzyme. The enzyme-substrate complex is known as the transition state. In the transition state, internal bonds of the substrate molecule become distorted and strained. Hence less energy is required to break the internal covalent bonds of the substrate.

11. Enzyme Action- Mechanism of Catalysis

12. Rate of Enzyme Reaction Affected by:- – A) Enzyme concentration-more active sites for substrates increases reaction. – B) Substrate concentration-rate will increase however eventually all active sites will be occupied so rate will reach plateau (saturation). – C) Temperature-increase in collision of enzyme and substrate so rate will inc as temp is inc. However above optimum temperature enzyme denatures. – D) pH

13. Rate of Enzyme Reaction Conc. of Enzyme Rate A Conc. of Substrate Rate B Temperature Rate C pH Rate D optimum temp optimum pH denaturation

14. Enzyme activity can be inhibited Competitive Inhibition – action of antibiotics such as penicillin. Prevents cell wall synthesis by competing with amino acid for active site. Bacterial cell bursts because of an incomplete cell wall. Non competitive reversible inhibition- rate of reaction decreases as inhibitor conc. increases, at saturation rate of reaction will be nil. Non competitive irreversible inhibition e.g heavy metals, nerve gas contains a compound that combines with serine in active site of acetylcholinesterase, and stops acetylcholine from binding. Causes accumulation of actelycholine. Nerve impulses cannot be stopped. Prolonged muscle contraction.

15. Enzymes are also able to regulation Metabolic Pathways Commonest form of regulation in cells. Allosteric Enzymes Regulated by compounds that act as non competitive inhibitors. Bind well away from the active site but causes a change in active site such that it no longer binds substrate.

16. Assignment 3 Read pp 125-127 Q1. Give an example of an enzyme co- factor. Q2. How are metabolic pathways in a cell regulated (controlled)?

17. Term Paper Apply scientific method Introduction Data Analysis Conclusion Word limit excl. diagrams 2000