Mconcentrations ab cdenergyunderstandardconditions

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Unformatted text preview: reaction For the reaction A + B → C + D, ∆Go´ is calculated by subtracting the sum of the free energies of formation of the reactants (in this case A & B) from that of the products (C & D) ∆Go´ = [Gf(C) + Gf (D)] – [Gf (A) + Gf (B)] Catalysis Catalysis • • A free­energy calculation tells us only whether energy is released or required; it says nothing about the rate of the reaction Consider this reaction: • • • • H2 + 1/2 O2 ⇒ H2O, ∆G0´= ­237 kJ This reaction is very favorable but simply mixing H2 and O2 together would take eons to form H2O This is because the formation of water requires that the chemical bonds of the reactants be broken first The breaking of bonds requires energy, and this energy is referred to as activation energy Catalysis Catalysis o o o o A catalyst is a substance that lowers the activation energy of a reaction, thereby increasing the rate of reaction Catalysts facilitate reactions but are not consumed or transformed by the reaction Catalysts do not affect the energetics or the equilibrium of a reaction; catalysts affect only the speed at which reactions proceed HINT: You may see these concepts again. The Need for Speed The Need for Speed o o o Thermodynamically favored reactions that should go in the direction of product formation (ie. ∆ G=­), do not necessarily go fast. Velocity or speed of a reaction is determined by energy of activation. At high temperatures, reactions will go fast, but this is not feasible in living cells !! Important Point Important Point o Free energy determines direction for reactions as they proceed to equilibrium, while the speed at which those reactions occur is determined by activation energy that in cells is lowered by enzymes that result in increased reaction velocity Biological Catalysts: Enzymes Biological Catalysts: • • • • • The catalysts of biological systems are proteins called enzymes Enzymes are highly specific, catalyzing only a single type or class of reaction In enzyme catalyzed reactions, the enzyme (E) temporarily combines with the reactant or “substrate”(S) forming an enzyme­substrate complex (E­S) The substrate binds to the enzymes active site with weak bonds: hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces As the reaction proceeds, the product (P) is released and the enzyme (E) returns to its original form ES COMPLEX ES COMPLEX E + S [ES] E + P When E forms complex with S, certain bonds in the substrate [S] are weakened, more easily broken, and new bonds are more likely to form. Thus, not as much activation energy is needed for the reaction to proceed at significant rates. Precise fit of S in active site of E Lock and Key Induced Fit Molecular Surfaces Need Complementary Molecular Surfaces Need Complementary Surfaces to Maximize Weak Interactions Complementary structures Contrast Lock & Key with Induced Fit Enzyme Catalysis Enzyme Catalysis o Enzymes display enormous catalytic power, accelerating reactions rates between 108 to 1020 times faster than uncatalyzed reactions. o o This is far faster than any synthetic catalysts Specificity: o o o In enzyme­catalyzed reactions, the specificity...
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This document was uploaded on 09/17/2013.

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