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Enzymes
Enzymes are proteins that act as catalysts to lower the activation energy of biochemical reactions without being consumed in the process. They achieve this by creating a new reaction pathway or binding site that stabilizes the transition state of reactions. The specificity of enzymes is determined by their three-dimensional structures, which form active sites that allow only specific substrate molecules to bind. While the lock-and-key model describes substrate binding via exact fit, the induced fit model suggests substrates can cause small conformational changes in enzymes to better orient the reaction. Reaction rates depend on factors like substrate and enzyme concentration, temperature, and pH, with optimal conditions varying for each enzyme.
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Enzymes
- 1. ENZYMES Proteins with catalyticproperties © 2017 Paul Billiet ODWS
- 2. Chemical reactions • Chemicalreactions need an initial input of energy = THE ACTIVATION ENERGY • During this part of the reaction the molecules are said to be in a transition state. © 2017 Paul Billiet ODWS
- 3. Reaction pathway (exothermic) ©2017 Paul Billiet ODWS
- 4. An endothermic reaction Activation energy Reactants Products Free energy Reactioncoordinate Transition state © 2017 Paul Billiet ODWS
- 5. Making reactions gofaster • Increasing the temperature makes molecules move faster • Biological systems are very sensitive to temperature changes • Enzymes can increase the rate of reactions without the need to increase the temperature • They do this by lowering the activation energy • They create a new reaction pathway “a short cut” . © 2017 Paul Billiet ODWS
- 6. An enzyme controlledpathway Enzyme controlled reactions proceed 108 to 1011 times faster than corresponding non-enzymic reactions. © 2017 Paul Billiet ODWS
- 7. Enzyme structure • Enzymesare proteins • They have a globular shape • A complex 3-D structure http://www.chem.ubc.ca/personnel/faculty/withers/group/grou p/begum/Human%2520Pancreati%2520Amylase% Human pancreatic amylase© 2017 Paul Billiet ODWS
- 8. The active site Theshape and the chemical environment inside the active site permits a chemical reaction to proceed more easily. DNA ploymerase © 2017 Paul Billiet ODWS
- 9. Cofactors • An additionalnon-protein molecule needed by some enzymes to help the reaction • Tightly bound cofactors are called prosthetic groups • Cofactors that are bound and released easily are called coenzymes • Many vitamins are coenzymes. Nitrogenase enzyme with Fe, Mo and ADP cofactors© 2017 Paul Billiet ODWS
- 10. The substrate • Thesubstrates of enzymes are the reactants that are activated by the enzymes • Enzymes are specific to their substrates • The specificity is determined by the active site. © 2017 Paul Billiet ODWS
- 11. The Lock andKey Hypothesis Enzyme may be used again Enzyme- substrate complex E S E E P P Reaction E © 2017 Paul Billiet ODWS
- 12. The Lock andKey Hypothesis • Fit between the substrate and the active site of the enzyme is exact • Like a key fits into a lock very precisely • Enzyme-substrate complex formed • Products have a different shape from the substrate • Products are released from the active site • Leaving it free for another substrate molecule. © 2017 Paul Billiet ODWS
- 13. The Lock andKey Hypothesis • This explains enzyme specificity • This explains the loss of activity when enzymes denature. © 2017 Paul Billiet ODWS
- 14. The Induced FitHypothesis • Some proteins can change their shape (conformation) • Substrate + enzyme, induces a change in the enzyme’s conformation • Active site moulded to a precise conformation • The chemical environment is now suitable for the reaction • The bonds of the substrate are stretched to make reaction easier (lowers activation energy). © 2017 Paul Billiet ODWS
- 15. The Induced FitHypothesis This also how some enzymes can react with a range of substrates of similar types. Hexokinase (a) without (b) with glucose substrate © 2017 Paul Billiet ODWS
- 16. Factors affecting Enzymes •substrate concentration • pH • temperature • inhibitors. © 2017 Paul Billiet ODWS
- 17. Substrate concentration: Non-enzymic reactions Theincrease in velocity is proportional to the substrate concentration. Reaction velocity Substrate concentration © 2017 Paul Billiet ODWS
- 18. Substrate concentration: Enzymicreactions • Faster reaction but it reaches a saturation point when all the enzyme molecules are occupied • Alter the concentration of the enzyme then Vmax will change too. Reaction velocity Substrate concentration Vmax © 2017 Paul Billiet ODWS
- 19. The effect ofpH Optimum pH values Enzyme activity Trypsin Pepsin pH 1 3 5 7 9 11 © 2017 Paul Billiet ODWS
- 20. The effect ofpH • Extreme pH levels will produce denaturation • The structure of the enzyme is changed • The active site is distorted and the substrate molecules will no longer fit in it • At pH values slightly different from the enzyme’s optimum value, small changes in the charges on the enzyme and it’s substrate molecules will occur • This change will affect the binding of the substrate with the active site. © 2017 Paul Billiet ODWS
- 21. The effect oftemperature • Q10 (the temperature coefficient) = the increase in reaction rate with a 10°C rise in temperature • For chemical reactions the Q10 = 2 to 3 • Enzyme-controlled reactions follow this rule as they are chemical reactions • BUT at high temperatures proteins denature • The optimum temperature for an enzyme controlled reaction will be a balance between the Q10 and denaturation. © 2017 Paul Billiet ODWS
- 22. The effect oftemperature Temperature / °C Enzyme activity 0 10 20 30 40 50 Increasing number of collisions (Q10) Denaturation Optimum © 2017 Paul Billiet ODWS
- 23. The effect oftemperature • For most enzymes the optimum temperature is about 30°C • Many are a lot lower, cold water fish will die at 30°C because their enzymes denature • A few bacteria have enzymes that can withstand very high temperatures up to 100°C • Most enzymes however are fully denatured at 70°C. © 2017 Paul Billiet ODWS