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2Alecture+8 - Ac>va>on energies do not determine the...

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1/24/10 1 Lecture 8:Enzymes Inorganic catalysts Enzymes (and other proteins) Protein structure and folding Measuring enzyme ac>vity Costs associated with quality control Average energy state (A + B) Transition state A+B->C+D Average energy (C + D) Progress of the reaction Ac>va>on energies do not determine the direc>on of the reac>on, but they do affect its rate. A+B > C+D C+D > A+B Average energy state (A + B) Transition state A+B->C+D Average energy (C + D) Progress of the reaction Distribution of energy among molecules If the average energy of the reactants is too low, very few molecules will make it over the “ac>va>on energy barrier” Energy state (A + B) Transition state A+B->C+D (this stays the same) Average energy (C + D) (this will go up too) Progress of the reaction Activation energy shrinks Hea>ng up a reac>on will speed it up, as heat increases the G of all the molecules involved Energy of input compounds (A and B) RAISED Progress of a reaction Energy of products (C and D) RAISED Δ G Transition state is SAME A,B -> C,D Thus AT HIGHER TEMP, the KINETIC BARRIER DROPS (for both forward and back reactions) BUT the DIFFERENCE (the G) stays the same. But this is not a viable solu>on for living things But just hea>ng everything up increases the rate of all reac>ons-­૒ it’ll just shorten the >me to equilibrium Enzymes (catalysts), in contrast, provide specificity They also can be flipped on and off -­૒ allowing the cell to express just those biochemical pathways that it needs at the moment Life is ALL ABOUT regula>on of metabolism-­૒ and choosing the right pathway for the job.
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1/24/10 2 Fermenta>on Pyruvate oxida>on Respira>on glucose -­૒> pyruvate (glycolysis) ? Stay in cytoplasm Move to mitochondrion Citric acid cycle For example… This is a major decision-­૒ the cell must interpret signals from the environment, and then appropriately tune enzymes and transporters up and down. Catalysts Catalysts take part in a reac>on, and speed it up, but emerge from the reac>on with no permanent chemical change. An inorganic example is solid MnO 2 , which will very quickly change to H 2 O 2 to H 2 O and O 2 . A li_le chunk of MnO 2 can last you a life>me-­૒ it never gets “used up”. Transi>on metals (metals that are very flexible about their oxida>on state) oaen catalyze redox reac>ons-­૒ this is true for biological reac>ons, too. The metal provides a shortcut for the electron to travel from reductant to oxidant. So do all catalysts work through metals? No, they have other tricks, too… 1. Specific features -- specific binding– SUBSTRATE SPECIFICITY 2. Binding restricts motion/vibration/flex 3. Same average energy distributed between fewer states 4. Helps keep reactants close (less energy required to provide collisions) 5. Binds stretched form of reactants (less energy required to complete reaction) REACTANTS BIND TO A SURFACE Inorganic Catalysts For example, Clay and pyrite (iron sulfide) provide a surface that can orient organic molecules, causing them to organize into chains (polymerize)
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