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
• • 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
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 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
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
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 In enzymecatalyzed reactions, the specificity...
View Full Document
This document was uploaded on 09/17/2013.
- Fall '13