ENZYMES A protein that allows chemical reactions to occur faster by lowering the activation energy. © 2007 Paul Billiet ODWS

Chemical reactions Chemical reactions need an initial input of energy = THE ACTIVATION ENERGY © 2007 Paul Billiet ODWS

Reaction pathway © 2007 Paul Billiet ODWS

Making reactions go faster Increasing the temperature make molecules move faster Enzymes can increase the rate of reactions without increasing the temperature. They do this by lowering the activation energy. © 2007 Paul Billiet ODWS

An enzyme controlled pathway © 2007 Paul Billiet ODWS

Enzyme structure Enzymes are proteins They have a globular shape A complex 3-D structure Human pancreatic amylase © Dr. Anjuman Begum © 2007 Paul Billiet ODWS

The active site One part of an enzyme, the active site, is particularly important The shape and the chemical environment inside the active site permits a chemical reaction to proceed more easily © H.PELLETIER, M.R.SAWAYA ProNuC Database http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html © 2007 Paul Billiet ODWS

The substrate The substrate of an enzyme are the reactants that are activated by the enzyme Enzymes are specific to their substrates The specificity is determined by the active site © 2007 Paul Billiet ODWS

Enzyme may be used again Enzyme-substrate complex E S Reaction coordinate © 2007 Paul Billiet ODWS

The Induced Fit Hypothesis Some proteins can change their shape (conformation) When a substrate combines with an enzyme, it induces a change in the enzyme’s conformation The active site is then moulded into a precise conformation Making the chemical environment suitable for the reaction The bonds of the substrate are stretched to make the reaction easier (lowers activation energy) © 2007 Paul Billiet ODWS

The Induced Fit Hypothesis Hexokinase (a) without (b) with glucose substrate http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/ENZYMES/enzyme_mechanism.html This explains the enzymes that can react with a range of substrates of similar types © 2007 Paul Billiet ODWS

Factors affecting Enzymes substrate concentration pH temperature inhibitors © 2007 Paul Billiet ODWS

Substrate concentration: Non-enzymic reactions Reaction velocity Substrate concentration The increase in velocity is proportional to the substrate concentration © 2007 Paul Billiet ODWS

Substrate concentration: Enzymic reactions Reaction velocity Substrate concentration Vmax Faster reaction but it reaches a saturation point when all the enzyme molecules are occupied. If you alter the concentration of the enzyme then Vmax will change too. © 2007 Paul Billiet ODWS

The effect of pH Optimum pH values Enzyme activity Trypsin Pepsin pH 1 3 5 7 9 11 © 2007 Paul Billiet ODWS

The effect of pH 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 © 2007 Paul Billiet ODWS

The effect of temperature The rate of enzyme-controlled reactions increases as the temperature increases BUT at high temperatures proteins denature © 2007 Paul Billiet ODWS

The effect of temperature Temperature / °C Enzyme activity 10 20 30 40 50 Q10 Denaturation © 2007 Paul Billiet ODWS

The effect of temperature 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 © 2007 Paul Billiet ODWS

Inhibitors Inhibitors are chemicals that reduce the rate of enzymic reactions. The are usually specific and they work at low concentrations. They block the enzyme but they do not usually destroy it. Many drugs and poisons are inhibitors of enzymes in the nervous system. © 2007 Paul Billiet ODWS

The effect of enzyme inhibition Irreversible inhibitors: Combine with the functional groups of the amino acids in the active site, irreversibly. Examples: nerve gases and pesticides, containing organophosphorus, combine with serine residues in the enzyme acetylcholine esterase. © 2007 Paul Billiet ODWS

The effect of enzyme inhibition Reversible inhibitors: These can be washed out of the solution of enzyme by dialysis. There are two categories. © 2007 Paul Billiet ODWS

The effect of enzyme inhibition Competitive: These compete with the substrate molecules for the active site. The inhibitor’s action is proportional to its concentration. Resembles the substrate’s structure closely. Enzyme inhibitor complex Reversible reaction E + I EI © 2007 Paul Billiet ODWS

The effect of enzyme inhibition Succinate Fumarate + 2H++ 2e- Succinate dehydrogenase CH2COOH CHCOOH COOH CH2 Malonate © 2007 Paul Billiet ODWS

The effect of enzyme inhibition Non-competitive: These are not influenced by the concentration of the substrate. It inhibits by binding irreversibly to the enzyme but not at the active site. Examples Cyanide combines with the Iron in the enzymes cytochrome oxidase. Heavy metals, Ag or Hg, combine with –SH groups. These can be removed by using a chelating agent such as EDTA. © 2007 Paul Billiet ODWS

Applications of inhibitors Negative feedback: end point or end product inhibition Poisons snake bite, plant alkaloids and nerve gases. Medicine antibiotics, sulphonamides, sedatives and stimulants © 2007 Paul Billiet ODWS