The binding to glycogen phosphorylase b of glucose 6-phosphate and inorganic phosphate (respectively allosteric inhibitor and substrate/activator of the enzyme) were studied in the crystal at 0.3 nm (3A) resolution. Glucose 6-phosphate binds in the alpha-configuration at a site that is close to the AMP allosteric effector site at the subunit-subunit interface and promotes several conformational changes. The phosphate-binding site of the enzyme for glucose 6-phosphate involves contacts to two cationic residues, Arg-309 and Lys-247. This site is also occupied in the inorganic-phosphate-binding studies and is therefore identified as a high-affinity phosphate-binding site. It is distinct from the weaker phosphate-binding site of the enzyme for AMP, which is 0.27 nm (2.7A) away. The glucose moiety of glucose 6-phosphate and the adenosine moiety of AMP do not overlap. The results provide a structural explanation for the kinetic observations that glucose 6-phosphate inhibition of AMP activation of phosphorylase b is partially competitive and highly co-operative. The results suggest that the transmission of allosteric conformational changes involves an increase in affinity at phosphate-binding sites and relative movements of alpha-helices. In order to study glucose 6-phosphate and phosphate binding it was necessary to cross-link the crystals. The use of dimethyl malondi-imidate as a new cross-linking reagent in protein crystallography is discussed.
Allosteric interactions of glycogen phosphorylase b. A crystallographic study of glucose 6-phosphate and inorganic phosphate binding to di-imidate-cross-linked phosphorylase b
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A Lorek, K S Wilson, M S P Sansom, D I Stuart, E A Stura, J A Jenkins, G Zanotti, J Hajdu, L N Johnson; Allosteric interactions of glycogen phosphorylase b. A crystallographic study of glucose 6-phosphate and inorganic phosphate binding to di-imidate-cross-linked phosphorylase b. Biochem J 15 February 1984; 218 (1): 45–60. doi: https://doi.org/10.1042/bj2180045
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