Biochemistry/metabolic regulation question

[Ann Menendez]

C Cantina wrote:
> 
> Hi,
> 
> I am a final year BSc student in Biochemistry, Microbiology, and Botany.
> In the course of enzyme mechanisms I encountered a problem, and do not
> seem to find the answers in the books I have consulted. Even the
> lecturer could not answer my question, so I hope that somebody out there
> might have the answer. (This question goes beyond the scope of the
> course, so I so not seek an answer to any Test/Exam question, but for my
> personal gain only!)
> 
> The enzyme phosphoglucomutase catalyzes the following reversible
> reaction:
> 
> Glucose-1-phosphate  to  Glucose-6-phosphate.
> 
> This reaction proceeds only in the presence of Magnesium and
> Glucose-1,6-bisPhosphate; a phosphoryl intermediate is formed in which a
> phosphate group on a single serine residue of the enzyme is exchanged
> with substrate and Glucose-1,6-bisPhosphate.
> 
> 1. Enz-OH + Glucose-1,6-bisPhoshate   to   Enz-O-Phosphate +
> Glucose-1(or -6)-Phosphate
> 
> 2. Enz-O-Phosphate + Glucose-1-Phosphate   to   Glucose-1,6-bisPhosphate
> + Enz-OH
> 
> 3. Enz-OH + Glucose-1,6-bisPhosphate   to   Glucose-6-Phosphate +
> Enz-O-Phosphate.
> 
> Here is the question:
> 
> 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?
> 
> Please if any one knows where I can find the answer, or could direct me
> to it would be greatly appreciated. Please send an email to
> ccantina at iafrica.com
> 
> One idea I have is that a sort of steric hinderance could be involved.
> As said before I cannot find it in the text books.

I am no expert in enzyme reactions but I had a thought.  You mentioned
the reaction will only take place in the presence of Magnesium, which is
positively charged.Lets say that in the second  reaction, when
Enz-O-Phosphate reacts with Glucose-1-phoshate to make end products
Glucose-1,6-bisphoshate and Enz-OH again, either the Glucose-1,6
bisphosphate or Enz-OH have picked up a positive charge in the
reaction.  Picking up a charge from the magnesium will cause the Enz-OH
to react with only one area of the Glucose-1,6-bisphospate molecule.  Do
you think this could be the possible solution? 
Ann Menendez MT(ASCP)