※ 引述《elelith (trapped)》之铭言： : ※ 引述《AppleAmily ()》之铭言： : : first of all, in part a) you need to eliminate the term [ES] by substituting : : the expression [ES] = K1[E][S]/(K'+K2) into d[ED]/dt = K2 [ES] since ES is : : an intermediate of which the actual concentration cannot be measured. : : the rate of formation for the product [ED] then becomes: : : d[ED]/dt = K1K2[E][S]/(K'+K2) : Well, for the first question, there was a hint I forgot to type out, it says : the expression will contain the term {[Et]-[ED]} instead of the usual [Et] : how and which term should I substitute with {[Et]-[ED]}? Oh....then I guess I didn't solve your problem in the right way. Sorry. So let's do this again..... Since {[Et]-[ED]} = [E] + [ES] as stated in the problem, and [ES] = K1[E][S]/(K'+K2) therefore, [E] = {[Et]-[ED]} - [ES] = {[Et]-[ED]} - K1[E][S]/(K'+K2) {1 + K1[S]/(K'+K2)}*[E] = {[Et]-[ED]} [E] = {[Et]-[ED]}/{1 + K1[S]/(K'+K2)} Substituting back to the rate expression for the formation of ED: d[ED]/dt = K1K2[E][S]/(K'+K2) = K1K2{[Et]-[ED]}/{1 + K1[S]/(K'+K2)}[S]/(K'+K2) Rearranging the above equation: d[ED]/dt = K1K2{[Et]-[ED]}[S]/{(K'+K2) + K1[S]} In this case, the rate of formation for ED will be first order with respect to [S] if K1 is small comparing to K' and K2 (K1 << K'+K2, thus the term K1[S] in the denominator will be insignificant). If we divide both the numerator and the denominator of the rate expression by [S], it becomes: d[ED]/dt = K1K2{[Et]-[ED]}/{(K'+K2)/[S] + K1} By looking at this expression we can see that if (K'+K2) is small comparing to K1 (K'+K2 << K1), the term (K'+K2)/[S] can be assumed negligible and the resulting rate expression will have no dependency on the substrate concentration, thus will be zeroth order with respect to [S]. : : when the rate is zeroth order with respect to [S], the expression becomes : : d[ED]/dt = K1K2[E]/(K'+K2) : : for the second question....I am not really sure if your answer is right or : : not since you didn't provide with any justification, but, generally, I would : : assume the rate of the enzyme catalyzed reaction to be zeroth order with : : respect to [S] if the concentration of the substrate is large comparing to : : [E] (thus can be assumed constant throughout the reaction); the rate should : : be 1st order with respect to [S] in all other cases according to the kinetics. : thx for your reply -- 我的网志： http://www.wretch.cc/blog/appleamily --

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