Pentcho Valev wrote:
> Guy Brown wrote:>>>>>>>>>>>>>>
> Why is it that if a reaction has a second order rate constant of
> anything from 10^7 to 10^12 (s^-1) it is described as
> "diffusion controlled"? Surely if collisions can occur at 10^12
> then a rate constant of 10^7 can not be diffusion controlled.
> Presumably steric factors are invoked: the molecules collide
> without reacting. But how can we distinguish steric factors
> from the "binding" of the substrate by the enzyme?
> How low could a rate constant go and still be diffusion
> controlled?<<<<<<<<<<<<<<<<<<
>> I am not competent, but here is a suggestion: not steric factors are
> essential but rather the effectiveness of conversion of ES into
> E + Product. In the limit it is absolutely effective so the
> concentration of ES is zero and the Michaelis-Menten curve is linear
> everywhere.
How large the diffusion limit is depends on what the interacting
molecules are. The diffusion-limited rate constant depends on the
diffusion constants of the two reactants. For very small reactants with
large diffusion constants, we might get rate constants of up to 10^{12}
M^{-1}s^{-1}. For reactions involving enzymes or other large molecules,
10^7 M^{-1}s^{-1} is a closer guesstimate of the diffusion-limited rate
constant.
Marc R. Roussel
(roussel at uleth.ca)
Department of Chemistry and Biochemistry
University of Lethbridge
