physophorylation of monosaccharides

Raman barani at deft.cc.purdue.edu
Mon Jul 29 21:05:11 EST 1996

>Article: 630 of bionet.glycosci
>From: cpung at BANANA.WUSTL.EDU (Chris Pung)
>Subject: physophorylation of monosaccharides
>Can anyone help me with my understanding of what the 
>importance of phosphorylation of glucose (or other sugars) 
>in the cell is.

Metabolic importance of phosphorus was known nearly a century ago.
Alcohol fermentation was found to occur only in the presence of
inorganic phosphates and this lead to a discovery of several
phosphate metabolites. Phosphate esters are transferred to acceptors
thus providing high energy sources for cellular activity. For example
ADP is converted to ATP. These phosphate transfers are carried out by
enzymes popularly known as 'phosphotransferases' which include the
enzymes in glycose-degradation-metabolic pathways. Some of the enzymes
in this metabolic pathways are 

hexokinase, phosphoglucomutase
phosphohexose isomerase, phosphohexokinase
aldolase, phosphotriose isomerase
phosphotriose dehydrogenase
enolase, phosphomannomutase
pyruvate kinase, glucose-bisphosphate synthase

>Is there a significant chemical difference between 
>glucose-1-phosphate and glucose-6-phosphate?
The schematic structure of G-1-6-P is given below.
The phosphate group in site 1 is transferred to site 6
and this reaction is catalysed by phosphoglucomutase:


             O3            O4                O
               \          /                 /
                \        /                 /
                 C3-----C4          O6----P----O
                /         \        /       \
               /           \      /         \
         O2---C2           C5----C6          O
               \          /
                \        /
             / \
            /   \
           O     O

As you can see, site 1 is  close to C1 and site 6 is  close to C6
which is NOT in the ring. The main difference is most likely
in the conformational energy and structural suitability for
further catalysis after the PO3 fragment is transferred.  
G-6-P is subsequently converted into Fructose-6-P. 

>What I have been able to gather from reading is that 
>1) phosphorylation helps keep the monosaccarides in the cell
>by making them anions
>2) phosphorylated sugars are the forms that participate in 
>biochemical reactions
>But, does phosphorylation of glucose to glucose-1-phosphate 
>stabilize the hemiacetal form in the same way that making an 
>acetal at the 1 position would?  Does phosphorylation at the 
>6 position also stabilize the ring form of glucose? Or, is 
>the difference between G-1-P and G-6-P that G-6-P is still a 
>hemiacetal and therefore more reactive or able to linearize?
>And then, is it possible that one role of phosphorylation to 
>G-1-P is to keep the sugar from being a reactive molecule by 
>stbilizing it in the non-aldehyde conformation, protecting 
>the cell from unwanted reactions?

It is not likely that stability of the ring is a critical factor
for these reactions. Interconversion of G-1-P and G-6-P is a 
mechanism by which the demand and supply of energy can be met 
depending on the requirement. This is indirectly inferred from 
the fact that the enzymes involved in the PO3 transfer are 
usually bifunctional. 

>Chris Pung

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