FIGURE 1.

A: a ribbon plot of the arginase trimer (liver arginase 1). The binuclear manganese cluster is represented by a pair of spheres in each monomer. B: the binuclear manganese cluster. Metal coordination interactions are indicated by green dotted lines, and the hydrogen bond between the metal-bridging hydroxide ion (red sphere) and Asp-128 is indicated by a white dotted line. Mn²⁺ is coordinated with square pyramidal geometry, leaving a vacant coordination site that permits octahedral coordination geometry as a means of transition state stabilization in catalysis. Mn²⁺ is coordinated with octahedral geometry. [From Kanyo et al. (104), with permission from Nature Publishing Group.]