C Cantina asked, in relation to phosphoglucomutase:
"Why can the enzyme in the first reaction either cleave the phosphate
from position C1 or position C6, but in the last step of the reaction
(3.) it only cleaves the phosphate from position C1?"
Where did you get your information? I think it is either incorrect, or
at least expressed in a misleading way. Whatever the mechanism, I think
it has to be symmetrical in glucose 1-phosphate and glucose 6-phosphate.
Probably what your source meant was that starting from glucose
1,6-phosphate the first product can be either glucose 1-phosphate or
glucose 6-phosphate, but once the decision is made the identity of the
second product is decided.
This enzyme is discussed briefly on p. 172 of my book "Fundamentals of
Enzyme Kinetics" (2nd edn., Portland Press 1995;
http://ir2lcb.cnrs-mrs.fr/lcbpage/athel/fek.html), but not in enough
detail to be much help for your question. There is much more detail in
my earlier book "Principles of Enzyme Kinetics" (Butterworths 1976; pp.
97-100), but you may have difficulty in finding that.
The mechanism (for the rabbit muscle enzyme) was studied by Ray &
Roscelli [J. Biol. Chem. 239, 3935-3941 (1964)], but my belief is that
the somewhat different mechanism proposed by Britton & Clarke [Biochem.
J. 110, 161-179 (1968)] is more likely to be correct, as it accounts not
only for the observations of Ray and Roscelli but also some later
isotope exchange data.
Athel
Home page: http://ir2lcb.cnrs-mrs.fr/lcbpage/athel/home_page.html
