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Enzymes
Chapter 6
Important Group of Proteins
• Catalytic power can incr rates of rxn > 1017
• 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*(SP) = 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],
andG = 0, soG’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 (GB)•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
http://www.blobs.org/science/enzyme/imgs/active2.gif
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