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12 Cards in this Set
- Front
- Back
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True facts about enzymes
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-rxns occur at the active site, usually a crevice on the surface of the enzyme
-substrate must bind to the enzyme before an rxn can occur -enzyme yields a specific produce, nonbiological catalyst may produce more than one product and side rxns may occur -generally, enzymes are specific for a substrate -most are proteins, exceptions are catalytic RNAs |
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What do enzymes promote?
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-formation of a transition site, lower activation energy, increase magnitude of the rate constant for both the forward and reverse rxns
-activation energy is the difference between the transition state and reactants |
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Enzymes cannot change the value of
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delta G,
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Acid-base catalysis
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-a proton is transferred between enzyme and substrate
-may use a.a. such as aspartate and lysine -lowers the energy or stabilizes the transition state or intermediate -catalyst retains its original for after rxn occurs |
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Covalent catalysis
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-temp. covalent bond forms btwn the nucleophile functional group of an enzyme and the substrate, yielding the intermediate
-two-part process involving the formation of the intermediate and the breakdown of the intermediate -lowers the energy or stabilizes the transition state or intermediate -catalyst retains its original for after rxn occurs |
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Metal ion catalysis
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-mediating oxidation-reduction rxns
-properly orienting the substrate in the enzyme active site through ionic interactions -stabilizing a negatively charged intermediate -lowers the energy or stabilizes the transition state or intermediate -catalyst retains its original for after rxn occurs |
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Enzyme labeling can be used to map the structure of the active site of an enzyme using an affinity label, a molecule that is structurally similar to the substrate.
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-labeling involves irreversible covalent modification of the enzyme as the analog binds to the active site
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Enzyme cofactors
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-needed for catalyzing oxidative-reductive rxns and some group-transfer rxns
-metal ions -coenzymes, small organic molecules >transiently associated coenzymes, NADH, FADH, serve as "co-substrates" >tightly associated prosthetic groups -apoenzyme (inactive) + cofactor = holoenzyme (active) |
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Free energy of activation DGǂ
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free energy of transition state - free energy of reactants
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(kB * T/h) * e^(-deltaGǂ /RT)
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kB = boltzman constant
h = planck's constant R = gas constant T = kelvin (273.15 + C) |
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deltadeltaGcat=
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deltaGǂuncat - deltaGǂcat
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Rate enhancement (ratio of the rates of the catalyzed and uncatalyzed reactions) is given by
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e^deltadeltaGcat/RT
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