Figure 4.

Active-site region in HsvA. The side chains of selected amino acids in the active-site region are shown, with carbon shown in gray, oxygen in red, nitrogen in blue, and sulfur in yellow. Selected regions of secondary structure surrounding the active site and the substrate access channel are shown as well. In this cross-sectional view, the active-site region is oriented so that the channel is being viewed from the side, with the opening of the channel at the top of the diagram and the base of the channel near the bottom. The three amino acids forming the catalytic triad near the base of the channel (Cys³⁵¹, His²⁴⁹, and Asp²⁰⁰) are labeled in boldface type. Glu²⁴⁴, which is believed to be a primary determinant of substrate specificity, is located just below the entrance of the channel, where the carboxylate is positioned to form an electrostatic interaction with the α-amino group in arginine or with a protonated amine in the polyamine acceptor substrate. Glu¹⁸⁵ is located on the exterior of HsvA near the channel opening and, together with Glu⁵³ and Glu⁵⁴, confers a significantly negative electrostatic potential to the region surrounding the channel entrance.