Enzymes (Ch. 6) Intro Basics of catalysis General types of catalysis Quantification of catalysis –enzyme kinetics and inhibition Specific examples Allostery and enzyme regulation

ENERGY (G°) REACTION PROGRESS  G < 0 Reaction should be spontaneous Equil should favor products Biological reaction: sugar + oxygen ↔ CO 2 + water Reactants (R) Activation energy E A Kinetic barrier to reaction High energy “Transition state” Intermediate between R & P Products (P)

The energy barrier is critical for life Potentially deleterious reactions are blocked by E A –Complex molecule degrading to simpler constituents DNA nucleotide

How do enzymes speed up reactions? New reaction pathway Lower activation energy Decreased energy barrier 2H 2 O 2 → 2H 2 O + O 2 Isolated: E A ~ 86 kJ/mol In the presence of catalase:E A ~ 1kJ/mol Hydrogen peroxide

Binding of substrate to enzyme creates a new reaction pathway An enzyme changes E A NOT  G Affects RATE, not EQUILIBRIUM Without enzyme With enzyme E A =  G ‡

How is E A lowered? Enzyme’s ‘goal’ is to reduce  G ‡ Two ways enzymes can affect  G –Improve  H –Improve  S E A =  G ‡ =  H - T  S  G ‡ = G trans.state – G reactants Enzymes alter the free energy of the transition state enthalpy entropy

- Example: More favorable  H A B AOH BH A BH + + H2OH2O +OH - + Charge unfavorable Unstable transition st. A BH + Ionic interaction stabilizes the positive charge OH -

Example: More favorable  S Two molecules More ‘freedom’ Higher disorder (high S) One molecule Lower disorder (low S) Unfavorable entropically

ENZYME Example: More favorable  S Enzyme/Reactant COMPLEX Essentially a single molecule ENZYME Enzyme/Transition state complex Still a single molecule Not much difference entropically

Remember 1.Enzymes lower the energy barrier 2.Decrease E A (  G ‡ ) 3.Provide an environment where: Transition state is stabilized (lower enthalpy) Change of disorder (entropy) is minimized

Enzymes create a new reaction pathway  G o vs.  G ‡ transition state vs. reaction intermediates rate limiting step

Factors contributing to enzyme catalysis Weak interactions between enzyme and transition state Transient covalent bonds between S and E Entropy optimization in ES complex formation Solvation shell surrounding S & E (entropy/hydrophobic interactions) Substrate distortion upon binding to noncomplementary E Proper alignment of catalytic functional groups

Common catalytic mechanisms General acid/base catalysis –Proton transfer –Reactions with charged intermediates/AAs Fumarase –Precise positioning of acid/base: reaction occurs faster than specific acid/base reactions Free H + /OH -

Common catalytic mechanisms Covalent catalysis –Covalent bond formation between E and S –Reaction path is altered and new path has lower E a –Chymotrypsin (combination)

Common catalytic mechanisms Metal catalysis (metalloenzymes) –Ionic interactions Stabilize charged TS or orient charged substrate for reaction Carboxypeptidase –Oxidation/reduction Reversible changes in oxidation state of the metal Electron transfer reactions Transition metals Catalase ezample