Enzymes (If you don’t have the energy, we can help!)

Enzymes Biological catalysts Made primarily from proteins Bind to substrate Not used up Speed up reaction

Catalysis Uncatalyzed G time R P Transition EAEA time R P G Catalyzed

Catalysts Increase rate of reaction Provide activation energy –Energy is conserved –Activation energy has to come from somewhere

Sources of Activation energy Binding energy –Solvent released from active site when substrate binds increases ΔS surr Induced fit –Enzyme forces substrate into unstable transition state –Catalytic antibodies Binding substrate brings reactive groups together

Catalysis by an enzyme E + S  > ES  > EP  > E + P

Enzyme Example Serine Proteases u/courses/03231/Prot ease/SerPro.htmhttp:// u/courses/03231/Prot ease/SerPro.htm Active site and substrate –Note side chain interactions –Substrate diffuses into active site

Serine proteases Substrate binds to active site Chemical groups interact Force substrate into unstable intermediate

Serine Proteases Peptide bond is cleaved Serine in active site is bound to carboxyl side of peptide

Serine proteases Half of protein diffuses out Enzyme used up –Has to be regenerated

Serine proteases Water diffuses into active site Juxtaposes chemical groups –Similar reaction to first

Serine proteases New unstable intermediate generated

Serine Proteases Carboxyl end of peptide diffuses out Enzyme regenerated Ready for another round

Serine protease summary Enzyme stabilized by various side chain interactions Substrate binds to enzyme. –Fits pocket –Forms unstable intermediate –Chemical groups on enzyme do reaction Chemistry happens and product diffuses out Enzyme regenerated

Kinetics Study of reaction rates Why? –Used to determine mechanisms Michaelis Menton kinetics V = rate of reaction V max = maximum reaction rate [S] Substrate concentration Km = substrate concentration where rate is half maximal

Michaelis-Menton Plot

Enzyme inhibition, Competitive Both substrate and inhibitor bind to active site –Compete Inhibitor blocks substrate from binding E S I E I S

Michaelis- Menton Plot for competitive inhibition uninhib V max V max /2 KmKm inhibited K mi V [S]

Michaelis- Menton Plot for competitive inhibition V max is not changed K m increased –Since substrate has to outcompete the inhibitor for the active site, it takes more substrate to get to the same rate.

Noncompetitive Inhibition S I Inhibitor binds to an allosteric site on the enzyme Changes active site so substrate doesn’t bind I

Michaelis- Menton Plot for noncompetitive inhibition uninhib V max /2 KmKm V max uninhib V max inhib V [S]

Michaelis- Menton Plot for noncompetitive inhibition Inhibition lowers V max K m unchanged –Since inhibitor doesn’t bind to active site, changing amount of substrate will have no effect

Michaelis Menton Kinetics Summary Competitive inhibition –Inhibitor competes with substrate for active site V max unchanged K m increased Noncompetitive inhibition –Inhibitor binds to allosteric site changing active site V max lowered K m unchanged

Allosteric Activation Active site will not bind substrate Allosteric activator binds and changes shape of active site Now substrate binds S A A S