Sinh học - Chapter 5 (part 1) Enzymes: Introduction

study of reaction rate determines number of steps involved determines mechanism of reaction identifies “rate-limiting” step

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Chapter 5 (part 1)Enzymes: IntroductionCatalystsubstance that increase rates of a chemical reactiondoes not effect equilibriumremain unchanged in overall processreactants bind to catalyst, products are released Catalysts increase product formation by (1) lowering the energy barrier (activation energy) for the product to form (2) increases the favorable orientation of colliding reactant molecules for product formation to be successful (stabilize transition state intermediate)Catalytic PowerEnzymes can accelerate reactions as much as 1016 over uncatalyzed rates! Urease is a good example: Catalyzed rate: 3x104/sec Uncatalyzed rate: 3x10 -10/sec Ratio is 1x1014 !Specificity Enzymes selectively recognize proper substrates over other molecules Enzymes produce products in very high yields - often much greater than 95% Specificity is controlled by structure - the unique fit of substrate with enzyme controls the selectivity for substrate and the product yieldClasses of enzymesOxidoreductases = catalyze oxidation-reduction reactions (NADH)Transferases = catalyze transfer of functional groups from one molecule to another.Hydrolases = catalyze hydrolytic cleavageLyases = catalyze removal of a group from or addition of a group to a double bond, or other cleavages involving electron rearrangement.Isomerases = catalyze intramolecular rearrangement.Ligases = catalyze reactions in which two molecules are joined.Enzymes named for the substrates and type of reactionCo-enzymesNon-protein molecules that help enzymes functionAssociate with active site of enzymeEnzyme + Co-enzyme = holoenzymeEnzyme alone = apoenzymeOrganic co-enzymes – thiamin, riboflavin, niacin, biotinInorganic co-enzymes – Mg ++, Fe++, Zn++, Mn++Kineticsstudy of reaction ratedetermines number of steps involveddetermines mechanism of reactionidentifies “rate-limiting” step

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