Enzymes Chapter 6

Important Group of Proteins Catalytic power can incr rates of rxn > Specific Often regulated to control catalysis Coupling  biological pathway

Catalysis Happens… Enzymes use many intermolecular forces –At enzyme active site –From atoms making up R grps of aa’s Substrates brought together –Optimal orientation Making/breaking bonds facilitated –Transition state stabilization –Allows high energy transition state Enzyme native conform’n crucial

Additional Chemical Components Prosthetic Groups –Cofactors (Table 6-1) –Coenzymes (Table 6-2) Bound to apoenzyme (apoprotein)  Holoenzyme

Rxns Occur at Enzyme Active Sites Physical clefts “Lined” w/ atoms that make up aa R grps Stabilize transition state S  P Complex ES forms (reversible)

 G Calc’d for Any Rxn S  P  G = Diff in free energy between S, P REMEMBER:  G =  H _ T  S –What are these terms??

Energetics  G =  H _ T  S  G: –If negative –If = 0 –If positive  G: –Depends on free energy prod’s – free energy reactants –Independent of path of rxn Catalysis doesn’t alter –No info on rate of rxn

S* = Transition State = High Energy Intermediate Must add energy for S   S* Common rxn intermediate “Fleeting molecular moment” Can go to S or P  G*(S  P) = Activation Energy –Diff in energy S to S* –Enzymes lower  G*

ES* = Enzyme Substrate Complex Must add energy for E + S  ES* BUT less energy So lower rxn pathway Can go to E + S or E + P Note: E is always regenerated  G*(cat’d) –Diff in energy S to ES* –So rxn more energetically favorable in presence of catalyst

For S  P at Equilibrium Keq = [P] / [S]  G =  G’ o + RT ln [P] / [S], and  G = 0, so  G’ o = - RT ln [P] / [S]  G’ o = - RT ln Keq ’ –So Keq directly related to  G for rxn

 G’ o = Diff in Free Energy between S, P Enzymes do NOT effect Keq ’,  G’ o Enzymes impt when energy must be added for rxn to proceed

Enzymes Effect Rxn Rate Use rate constant (k) to describe rate S  P Velocity (V) of rxn dependent on [S], k –V = k [S] –First order rxn Can relate k to  G* –Eq’n 6-6 –Relationship between k and  G* is inverse and exponential

Summary Enzymes don’t change overall energy difference (S  P), equilibrium (Keq) Enzymes do lower EA Enzymes do increase rate (k)

Source of Energy from within Enz to Facilitate Rxn S  P Most impt: ES complex ES proven experimentally, theoretically Enzyme active site –Aa’s directly participate (catalytic grps) –Only small part of total volume –Catalytic grps may be far apart in primary structure Folding impt!

Substr Binding to Enz Active Site Multiple weak interactions –What are these? –  Binding energy (  G B ) Stabilizes ES* Must have proper orientation between atoms Substrate, active site have complementary shapes

Commonly crevice nonpolar –If polar aa’s, often participate –Water excluded unless participates in rxn So: microenvironment w/ aa funct’l grps that have partic prop’s essential for catalysis of rxn

Binding Specificity DNA evolution  protein w/ optimal aa sequence  optimal E/S interactions  lowering energy nec for rxn So, depends on precisely arranged atoms in active site

Two Theories of E/S “Match” Lock & key (Fisher, 1894) –If precise match to S, why  S* or P? Complementarity to S* –Enz active site complementary to transition state –So weak interactions encourage S*, then stabilize it

Best energetically when S* fits best into enz active site –Must expend energy for rxn to take place –BUT overall many weak interactions lower net act’n energy E/S “match” also confers specificity

Other Factors that Reduce Act’n Energy Besides multiple, weak, atom-atom interactions Physical, thermodynamic factors influence energy, rate of catalyzed rxn –Entropy reduction S held in proper orientation Random, productive collisions not nec

–Desolvation H-bonds between S and solvent decr’d Incr’s productive collisions –Induced fit Enzyme conform’n changes when S binds Brings impt funct’l grps to proper sites Now has enhanced catalytic abilities