# Why does glycogen phosphorylase only act on the non-reducing end of the glycogen chain?

I read that glycogen phosphorylase only acts on the non-reducing end of the glycogen chain. This enzyme requires an inorganic phosphate molecule and PLP as a cofactor. The mechanism of the enzyme is as follows:

I attempted to workout the mechanism for the reducing end of the molecule and concluded that perhaps it results in a ring opening reaction.

Is this correct? If not what is the reason?

Glycogen phosphorylase does indeed phosphorylates the non-reducing end of a glycogen chain. It is unclear from the crystal structures how come this is an exo type enzyme. The prevailing catalytic mechanism is almost as you illustrated, involving the protonation of the $$\pu{\alpha}$$-1,4 link leading to the formation of a oxocarbenium ion and release of the long chain; nucleophilic attack by an inorganic phosphate to the anomeric carbon $$\ce{C1}$$ of the intermediate, leading to the final product. [1] Overall, the $$\pu{\alpha}$$ anomeric configuration is maintained from substrate to product, instead of a product with $$\pu{\beta}$$ configuration as shown on your drawing.

a) How come there is little $$\pu{\beta}$$-D-glucose that is produced as a side-product? This is quite unexpected for such an unstable intermediate as an oxocarbenium ion.

b) How does the above mechanism account for the loss of 500-fold activity upon mutation of Glu672? [2,3] This is a huge decrease! I'm wondering whether this residue may act as a nucleophile forming a covalent intermediate with the residual saccharide?

Regarding the second question from the OP: I wouldn't think there is ring opening at the chain anomeric center as there is no evidence that this reducing end of the chain is presented in glycogen phosphorylase catalytic site. It is the non-reducing end that is presented in the catalytic site and the $$\pu{\alpha}$$-1,4 link that is presented at the catalytic center.

Sources

1. Duke, E.M., Hadfield, A., Martin, J.L., Clifton, I.J., Hajdu, J., Johnson, L.N., Reid, G.P., Trentham, D.R., Bruce, I. and Fleet, G.W. (1991) "Towards time-resolved diffraction studies with glycogen phosphorylase" Ciba Found. Symp. 161:75-86. DOI: 10.1098/rsta.1992.0064

2. Schinzel, R. and Palm, D. (1990) "Escherichia coli maltodextrin phosphorylase: contribution of active site residues Glutamate-637 and Tyrosine-538 to the phosphorolytic cleavage of $$\pu{\alpha}$$-glucans", Biochem. 29:9956-9962. DOI: 10.1021/bi00494a028

3. Schinzel, R. and Drueckes, P. (1991) "The phosphate recognition sites of Escherichia coli maltodextrin phosphorylase", FEBS Lett. 286:125-128. DOI: 10.1016/0014-5793(91)80956-4