New Lecture_15_DW.pdf

BIOL or CHEM 3361 Biochemistry I Enzyme Kinetics Reading: Chapter 13 Km values for some enzymes and their substrates Summary Only one substrate is varied ES  E + P is irreversible and [P]0=0 [S] > [E]total and [E]total is constant All other conditions...

BIOL or CHEM 3361 Biochemistry I Enzyme Kinetics Reading: Chapter 13 Km values for some enzymes and their substrates Summary Only one substrate is varied ES  E + P is irreversible and [P]0=0 [S] > [E]total and [E]total is constant All other conditions are identical Kinetic Measures For an enzyme-catalyzed reaction: k1 k2 S + E  ES  E + P k-1 Steady state or rapid equilibrium? Vmax = kcat n[E]T (n is no. of substrate binding sites) The Turnover Number or kcat A measure of catalytic activity kcat, the turnover number, is the number of substrate molecules converted to product per enzyme molecule per unit of time, when E is saturated with substrate. If the model fits, k2 = kcat Values of kcat range from less than 1/sec to many millions per sec k1 k2 S + E  ES  E + P k-1 kcat The Ratio kcat/Km is Catalytic Efficiency The catalytic efficiency: kcat/Km An estimate of "how perfect" the enzyme is kcat/Km is an apparent second-order rate constant It measures how well the enzyme performs when S is low The upper limit for kcat/Km is the diffusion limit - the rate at which E and S diffuse together An enzyme with a catalytic efficiency around 108-109 M- 1sec-1 is a perfect enzyme kcat/Km Kinetic Measures - Review For an enzyme-catalyzed reaction: k1 k2 S + E  ES  E + P k-1 Ks is the dissociation constant of E and S Km is the substrate concentration at ½ Vmax Kcat is k2 when M-M applies Kcat/Km is a measure of catalytic efficiency with an upper limit of 109 M-1sec-1 When [S] >> Km v approaches Vmax When [S]