The glycine-rich loop of adenylate kinase forms a giant anion hole

The conformation of the glycine-rich loop of adenylate kinase is described in detail. It forms a giant anion hole for a sulfate ion, which presumably mimicks a nucleotide phosphoryl group. This loop had been called flexible, because at pH values of 6 or below it is displaced in the crystal. In the region of this loop the adenylate kinases are probably homologous to the p21 proteins. Is is known that a mutation in this loop at residue 12 of p21 causes cell transformation and therefore cancer. Other potentially homologous proteins are indicated.