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BIOD60H3 (23)
Lecture 9

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Biological Sciences
Shelley A.Brunt

Lecture 9 Mechanisms of Enzyme Action ContinuedProgress of the reaction transition state and intermediates during catalysis The progress of a reaction can be represented by an energy diagram or energy profile Figure 61 slide 2 is an example that shows the conversion of a substrate reactant to a product in a single step The y axis shows the free energies of the reacting species The x axis called the reaction coordinate measures the progress of the reaction beginning with the substrate on the left and proceeding to the product on the right This axis is not time but rather the progress of bond breaking and bond formation of a particular molecule The transition state occurs at the peak of the activation barrierthis is the energy level that must be exceeded for the reaction to proceed The lower the barrier the more stable the transition state and the more often the reaction precede Catalysts create reaction pathways that have lower activation energies than those of uncatalyzed reactions Catalysts participate directly in reactions by stabilizing the transition states along the reaction pathwaysEnzymes are catalysts that accelerate reactions by lowering the overall activation energyCatalysts accelerate reactions by lowering the overall activation energyEnergy diagram for a reaction with intermediates Energy diagram for a reaction with an intermediateThe intermediate occurs in the trough between the two transition states The rate determining step in the forward direction is formation of the first transition state the step with the higher energy transition state S represents the substrate and P represents the productThe slowest step the ratedetermining or ratelimiting step is the step with the highest energy transition stateCatalytic mechanisms Acidbase catalysisCovalent catalysisMetal ion catalysisProximity and orientation effectsPreferential binding of transition stateChemical Modes of Enzymatic Catalysis The formation of an ES complex places reactants in proximity to reactive amino acid residues in the enzyme active site Ionizable side chains participate in two kinds of chemical catalysis acidbase catalysis and covalent catalysisA Polar Amino Acid Residues in Active Sites
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