• The secret ingredient in living organism is Catalysis, a process performed by protein enzymes. • Their three-dimensional architecture gives them exquisite specificity to select the substrate molecules to which they will bind and on which they will operate. • The scene of operation called “active site” is usually a groove ,cleft or cavity on the surface of the protein. • Enzyme function frequently occurs many times, and in some cases many thousands of times per second.
• The miracle of life: a myriad chemical reactions in the cell occur simultaneously with great accuracy and at astonishing speed. • Without the proper enzymes to process the food you eat, it might take you ...view middle of the document...
•The Activation energy profile show the intermediate stages of a reaction between initial and final states. •The catalyst speed up a reaction changing the mechanism and thus lowering activation energy ΔG°‡(the amount of free energy required to bring the reactants to the transition state).
•The most important effect of catalyst is lowering ΔG°‡ , but not free energy change ΔG° . •Enzymes lowers ΔG°‡ needed for substrate molecule to reach the transition state. •Conversion of H2O2 to H2O+O2 provide an example of effect of catalysts on activation energy.
Temperature Effect :
• It increase the energy available to the reactants to reach the transition state, thus increasing the rate of chemical reaction. • More temperature increasing (>56° C ) enzyme denaturation. slowing down the reaction.
Enzymes are biological catalysts. They increase the rates of reactions by lowering the free energy of activation, but they don’t affect the thermodynamic aspects of reactions.
3) Description of Enzyme Kinetics in Mathematical Terms
A + B P Rate of disappearance of A = - Δ[A]/Δt = = = = B = - Δ[B]/Δt = = appearance of P = Δ[P]/Δt -Δ[A] -Δ[B] Δ[P] Rate=-------- =--------- =-------= k [A]f[B]g Δt Δt Δt K = rate constant, f & g must be determined experimentally. They are usually small whole numbers, such as 1 or 2 (0 in some cases).
The values of the exponents are related to the number of molecules involved in the detailed steps that constitute the mechanism: 1) A P, Rate=k [A]1 :first order reaction. 2) A+B C+D, Rate=k [A]1[B]1:2nd order reaction. 3) A B , Rate=k [A]0=k : zero order.
(enzyme-catalyzed reactions can exhibit zeroorder kinetics when the concentrations of reactants are so high that the enzyme is completely saturated with reactant molecules).
4)Binding of Substrates to Enzymes
• Enzyme binds to the substrate to form a complex ES; this leads to the transition-state species which then forms the product. • A substrate binds noncovalently to a small portion of the enzyme called active site (cleft-crevice) consisting of certain amino acids that are essential for enzymatic activity.
• The first step is the binding of substrate to the enzyme, which occurs because of highly specific interactions between the substrate and the side chains and backbone groups of the amino acids making up the active site. • Two important models have been developed to describe the binding process :
Lock-and-key model :assumes a high degree of similarity between the shape of the substrate and the geometry of the binding site on the enzyme.Substrate binds to a site whose shape complements its own, like a key in a lock.
Induced-fit model : The substrate binding induces a conformational change in the enzyme results in a complementary fit after the substrate is bound, taking in account the enzyme 3-dimensional shape flexibility.
•If binding was perfect attraction between E and S, which will cause ES complex to be lower on energy diagram than...