weber uiuc edu 05 September 2007 MCB150 Lecture 06 Lecture 06 05 September 2007 Announcements Today is deadline for adding changing sections Weekly review assignment is due 5PM Friday o Will not cover material in current week o Always on content we have already covered WebCrossing is having the roster uploaded today o Login Our Name o Password 150valine Change this immediately after logging in o Talk to him if there are any problems not Alejandra o From this point on use WebCrossing for all questions not email LIGASE student tutoring o Tuesday 6 7PM 422 NHB o Wednesday 6 7PM 422 NHB o This is free peer level tutoring Spontaneous Reactions The typical definition Happens without external provocation Biological definitions When it runs to completion it releases significant energy o Note the difference in meanings o Fortunately spontaneous does not happen spontaneously Energy is lost as heat when this happens uncontrollably o Want to control the reaction to use that energy for a purpose Activation Energy Ea A little energy in more energy out Catabolic reactions are those that release energy Energy is released when bonds are broken o Could heat up the cell if not handled in a controlled fashion Uncontrolled heating is a bad idea it can denature proteins and damage the cell At that point the energy is of no use to the cell and is wasted It becomes a non functional entity o Enzymes control the reaction rates Most enzymes are proteins RNA enzymes are not proteins they are ribozymes Not all proteins are enzymes But all enzymes except ribozymes are proteins Standard Energy of Activation Energy is stored in the substrate labeled S o Potential energy of the substrate Energy level of the products is labeled P o Spontaneous reactions will release energy The energy of activation labeled Ea is typically a small hump assuming the reaction is exergonic Page 1 of 4 weber uiuc edu 05 September 2007 MCB150 Lecture 06 o However Ea is usually high enough to inhibit spontaneous reactions Enzymes will lower the energy of activation o Note no difference in energy levels of S or P Only in Ea o Enzymes do not let reactions proceed that wouldn t naturally occur o They simply raise the rate of the reaction The rate at which equilibrium between substrate and product is reached o Enzymes change the rate to a biologically relevant scale Example o 2H2O2 2H2O O2 o Add platinum as a catalyst This decreases the Ea by 1 3 33 o Add catalase instead This decreases the Ea by 90 Catalase is very efficient Very important in cleaning up peroxides out of cells How Do Enzymes Decrease Ea 1 Bind reactants with high specificity due to 3D shape o Substrate fits exactly into the active site It must fit or the enzyme cannot work o Will catalyze only with specific molecules o Perfect fit lock key model o Enzyme shape distorts induced fit model strained o Consequence not dependent on random collisions any longer i No more finding the other substrate in a crowd and orienting them correctly ii Makes the reaction more productive o The enzyme can put a strain on the substrate i Put a bit of energy in to make it favorable ii Now in a transition state for a ready reaction 2 Provide acidic basic R groups for transfer of H back and forth o Negative positive charges influence the substrates in the active site o Acid base catalysis transfer of protons 3 Accept give up other chemical groups o This is always a temporary situation Important Points About Enzymes Accepted chemical groups are given back at the end of the reaction enzymes emerge unchanged This is the definition of an enzyme Without this key factor enzymes cannot be re used The equilibrium does not change it only reaches that equilibrium faster o Enzymes decrease Ea for both forward and reverse reactions Free Energy G The change in free energy can be positive or negative G releases energy exergonic G requires energy endergonic Conditions Affect Enzymes Because most enzymes are proteins conditions affect their ability to operate Page 2 of 4 weber uiuc edu 05 September 2007 MCB150 Lecture 06 Enzymes have an optimal pH and temperature Most are optimal at pH 7 and 37oC body temp for humans Deviations can result in rapid cessation of function o Ex Pepsin salivary amylase arginase o Pepsin tangent here in old days old names for enzymes end in in Today enzymes end in ase Pepsin an old name is active in the stomach where a low pH is better for activity Pepsin doesn t work at all at higher pH o Arginase optimally active in a basic environment Temperature can cause similar drop offs in effectivity o The point is actively based on changes in temperature o More likely to have steep drop offs on the hot side o Hotter temps push towards denaturation at which point it will be useless Keep in mind the consequences of these things Enzyme Inhibition This is the essence of regulation Cells have a constant need to regulate expression o E S ES E P o E I EI Inhibition can be reversible and irreversible in nature Reversible has two flavors o Competitive and non competitive inhibition o For these two things focus on what we talk about in class Irreversible inhibition is the end of the line The cell must produce more unaltered enzyme to replace that which is irreversibly inhibited Irreversible Permanently bind to or modify the enzyme It will no longer be able to work Cannot do anything to get it back cells must make more o Increase in substrate or inhibitors nothing happens Nerve gas sarin works this way an irreversible inhibitor for acetylcholinesterase which catalyzes transmission of nerve impulses Aspirin is an irreversible inhibitor too Works by inhibiting pain responses The body has time to make more enzyme which is why aspirin s effect wears off Irreversible inhibitors tend to be molecules not typically encountered by that particular cell o Cells rarely do something in an irreversible fashion Reversible Competitive Competitive inhibitors bind to active site The inhibitor has to match the 3D shape of the active site No products are formed It is reversible pop it out and the enzyme resumes activity Page 3 of 4 weber uiuc edu 05 September 2007 MCB150 Lecture 06 Know what happens statistically here Competitive inhibitors can be flooded out o Increase the amount of the substrate o Lower the amount of the inhibitor o In either case more of the substrate will have access to the enzyme and fewer inhibitors will bind to the active site just out of competition Examples methanol poisoning o Alcohol