Human erythrocyte pyruvate kinase in complex with substrate analog phosphoglycolate and the allosteric activator fructose-1,6-diphosphate. Ribbon representation of the human erythrocyte pyruvate kinase monomer (A) and tetramer (B). Phosphoglycolate and fructose-1,6-diphosphate are shown in stick representation and colored yellow and gray, respectively. Metal ions in the active site are shown as blue (potassium) and pink (manganese) spheres. (A) PK monomer with domains N, A, B, and C colored violet, cyan, orange, and lime, respectively. (B) PK tetramer with individual subunits colored lime, cyan, violet, and orange. (The figures were generated from the atomic coordinates of protein data bank entry 1LIU using the program PyMOL.)