On 28 Jan 1998, Paul M. Schlosser wrote:
> >Now since the localized enzyme "senses" LOCAL substrate concentration,
> >it behaves accordingly (slower rate at steady state, not only due to
> >relatively lower substrate but sometimes to higher product
> >concentrations, as well different pH, ionic strength or cofactor
> >concentrations occuring locally).
>> In chemical engineering the ratio:
>>> actual rate
> ------------------------------------------------------
> rate that would occur without mass transfer resistance
>>> is referred to as an effectiveness factor. It is (by definition) less
> than one, since there will always be some resistance. But it can be very
> close to one.
Right, but the difficult part is to get an expression of the effectiveness
factor in terms of the intrinsic parameters of the enzyme (e.g., kcat,
Km), its inherent parameters (surface or local concentration, dimensions
of the support), and the mass transfer parameters (Diffusion coeff.,
stirring/flow, etc.).
My paper in JTB 1992 provides that (eqs. 30-33) as well as its dependence
on effective parameters ([total enzymes]). It is also shown (assuming
identical intrinsic kinetic parameters for the localized and soluble
enzyme) that this effectiveness factor represents the ratio of average
product concentration to local product concentration.
The bottom line is that in a moderately slow reconstituted biological
system (mitochondria even in plain aqueous suspension) the diffusional
restrictions are most significant, as seen by a local observer.
The conclusion, as pointed out by Michael Kohn, is that there is a marked
advantage for enzymes catalyzing sequential steps to be colocalized.
Moreover, spatial segregation of different isozymes even within the same
topological compartment (e.g., the so-called cytosol or even mitochondrial
matrix) may lead to distinct different metabolic paths for the same
metabolite in this compartment.
> More often, one is stuck with fitting effective parameters to data which
> include whatever diffusional resistance exists. Which brings us back to
> the question: what equation should be used? I'd still suggest using the
> same equation as for bulk solution as a first attempt, and only turning
> to more complex analyses which include diffusional resistance if that
> fails.
The point is that fitting effective behaviors to homogeneous catalysis
equations never fails! (at least if one is not too fussy about perfect
fit).
I would say that a proper diagnosis of diffusional resistances is always
pertinent. A simple experimental test is measurements +/- stirring
(in the case of particles the size of yeast and above), but as
pointed out above, one can be surprised...
Who said there's no "action" in this group?
Claude
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