The structure of rat liver arginase

The work presented in this thesis describes the structural characterization of arginase from rat liver. Each individual excretes $\approx$10 kilograms of urea per year resulting from the hydrolysis of arginine in the final cytosolic step of the urea cycle. This reaction provides for the disposal of nitrogenous waste from protein catabolism and is catalyzed by liver arginase. In tissues lacking a complete urea cycle, arginase regulates cellular arginine and ornithine concentrations for biosynthetic reactions, including nitric oxide synthesis. In macrophages, arginase activity is reciprocally coordinated with that of NO synthase to modulate NO-dependent cytotoxicity. The bioinorganic chemistry of arginase is particularly rich because this enzyme is one of only a handful that specifically requires a spin-coupled $\rm Mn\sp{2+}+{-}Mn\sp{2+}$ cluster for catalytic activity in vitro and in vivo. The 2.1 A resolution crystal structure of trimeric rat liver arginase reveals that this unique metal cluster resides at the bottom of a 15 A-deep active site cleft. Analysis of the structure suggests that arginine hydrolysis is achieved by a metal-activated hydroxide ion that symmetrically bridges the two Mn$\sp{2+}$ ions