Fig. 6.

Electron density of cyanuric acid bound in the AtzD active site. Left structure obtained at pH 4.5. Right structure obtained at pH 6.6. In each case Ser85 has been labelled and the electron density obtained for the substrate is shown. In the crystal obtained at pH 4.5 (too low for hydrolysis) the density has a near symmetrical distribution. In the crystal obtained at pH 6.5 (i.e. with hydrolytically active enzyme) the density has become polarized in the vicinity of Ser85, suggesting that Ser85 is the catalytic serine.