Question

kcat/Km is alternatively referred to as a second order rate constant and as a specificity constant. a) Why is this? Hint: consider [S] << Km b) After testing your favorite enzyme — despacitoase — with two substrates at an enzyme concentration of 10 nM, you obtain Vmax values of 100 µM•s-1 and 10 µM•s-1 and Km values of 100 µM and 20 µM for substrates Fonsi and Bieber, respectively. For which of these substrates is despacitoase more specific?

Answer 1

answer a)

Kcat/Km (product formation rate/enzyme-substrate binding affinity) called as a second order rate constant because here the rate of the reaction E + S ---> ES-----> E + P is dependent on both concentration of enzyme and substrate as well as the binding affinity of substrate to enzyme and dissociation of ES to E+P. Substrate concentration must be close to Km to apply M-M kinetics because if S << Km (substrate concenentration is very low) then the reaction will become first order reaction and there will be no saturation of enzymes which cause no value for Km and Vmax.

asnwer b)    substrate with Km= 20uM is more specific for despacitoase.

Km is the dissociation constant for ES and hence it is inversely proportional to the binding affinity of substrate to enzyme. higher the Km lesser is the binding affinity and vice versa.