FIGURE 5.
Substrate specificity of APA in the absence of calcium. A, detailed interactions between a glutamate side chain and the S1 pocket of APA. B, detailed interactions between an arginine side chain and the S1 pocket of APA. APA residues are in pink, and ligands are in green. Electron density maps of ligands correspond to Fo − Fc omit maps (contoured at 2.5σ) that were calculated in the absence of ligands. Models of APA-bound ligands were built based on these maps. Unit of distances is angstrom. C, relative catalytic activities of APA on different substrates (amino acid-pNA) in the absence or presence of calcium. The catalytic activity of APA on Glu-pNA in the absence of calcium was taken as 100%. Each bar shows the mean ± S.E. (n = 3). D, relative catalytic activities of APA mutants on substrates. The catalytic activity of wild-type APA on Glu-pNA was taken as 100%.