Biochemistry/metabolic regulation question

C Cantina ccantina at
Tue Feb 25 13:19:57 EST 1997


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

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 +

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 

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.

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