ENZYMES A protein with catalytic properties due to its
ENZYMES A protein with catalytic properties due to its power of specific activation © 2007 Paul Billiet ODWS Chemical reactions Chemical reactions 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. © 2007 Paul Billiet ODWS Reaction pathway © 2007 Paul Billiet ODWS Making reactions go faster Increasing the temperature make molecules move faster Biological systems are very sensitive to temperature changes. Enzymes can increase the rate of reactions without increasing the temperature. They do this by lowering the activation energy. They create a new reaction pathway “a short cut” © 2007 Paul Billiet ODWS An enzyme controlled pathway Enzyme controlled reactions proceed 108 to 1011 times faster than corresponding non-enzymic reactions. © 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 © H.PELLETIER, M.R.SAWAYA ProNuC Database © 2007 Paul Billiet ODWS 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 Cofactors An additional nonprotein molecule that is 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 Jmol from a RCSB PDB file © 2007 Steve Cook © 2007 Paul Billiet ODWS H.SCHINDELIN, C.KISKER, J.L.SCHLESSMAN, J.B.HOWARD, D.C.REES STRUCTURE OF ADP X ALF4(-)-STABILIZED NITROGENASE COMPLEX AND ITS IMPLICATIONS FOR SIGNAL TRANSDUCTION; NATURE 387:370 (1997) 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 The Lock and Key Hypothesis Fit between the substrate and the active site of the enzyme is exact Like a key fits into a lock very precisely The key is analogous to the enzyme and the substrate analogous to the lock. Temporary structure called the enzyme-substrate complex formed Products have a different shape from the substrate Once formed, they are released from the active site Leaving it free to become attached to another substrate © 2007 Paul Billiet ODWS The Lock and Key Hypothesis S E E E Enzymesubstrate complex Enzyme may be used again P P Reaction coordinate © 2007 Paul Billiet ODWS The Lock and Key Hypothesis This explains enzyme specificity This explains the loss of activity when enzymes denature © 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 Vmax Reaction velocity Substrate concentration 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 1 © 2007 Paul Billiet ODWS 3 5 7 pH 9 11 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 At pH values slightly different from the enzyme’s optimum value, small changes in the charges of the enzyme and it’s substrate molecules will occur This change in ionisation will affect the binding of the substrate with the active site. © 2007 Paul Billiet ODWS The effect of temperature Q10 (the temperature coefficient) = the increase in reaction rate with a 10°C rise in temperature. For chemical reactions the Q10 = 2 to 3 (the rate of the reaction doubles or triples with every 10°C rise in temperature) 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. © 2007 Paul Billiet ODWS The effect of temperature Q10 Enzyme activity 0 © 2007 Paul Billiet ODWS 10 20 30 40 Temperature / °C Denaturation 50 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 1. 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. © 2007 Paul Billiet ODWS E+I Reversible reaction EI Enzyme inhibitor complex The effect of enzyme inhibition Fumarate + 2H++ 2e- Succinate Succinate dehydrogenase CH2COOH COOH CHCOOH CH2 CH2COOH COOH Malonate © 2007 Paul Billiet ODWS CHCOOH The effect of enzyme inhibition 2. 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
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