Enzymes, con't.

Substrate Activation (catalytic mechanisms) Strain on substrate –Weakens bonds –Makes more accessible for reaction Acid/base catalysis Covalent (nucleophilic/electrophilic) catalysis

Enzyme kinetics Study of reaction rates—can tell lots about reaction mechanisms

Michaelis-Menton Kinetics (saturation kinetics)

Leonor Michaelis and Maud Menton--1913

Enzyme Action

Simplifying assumptions No back reaction k 3 is rate limiting [ES] is constant (steady state assumption)

Km and Vmax

Km –Measure of binding affinity (roughly) –The lower the Km, the tighter the binding Vmax –Maximum rate of enzyme –Determined by turnover number (k cat ) How best to calculate them?

Double-reciprocal plot (Lineweaver-Burk)

Problems 7a-d,8a,b

Regulation

Irreversible inhibitors—generally not natural part of cell –Drugs and toxins –Covalent modification –Aspirin Reversible –Substrate level regulation –Competitive inhibitors –Noncompetitive inhibitors –Allosteric regulation (activators and inhibitors) –Covalent modification (reversible) –Proteolytic cleavage

Competitive inhibition

Noncompetitive inhibition

Regulation Reversible –Substrate level regulation –Competitive inhibitors –Noncompetitive inhibitors –Allosteric regulation (activators and inhibitors) –Covalent modification (reversible) –Proteolytic cleavage

Reversible covalent modification Phosphorylation Dephosphorylation

Proteolytic cleavage Only extracellular