Crystal structure of human arginase I at 1.29-Å resolution and exploration of inhibition in the immune response

Human arginase I is a potential target for therapeutic intervention in diseases linked to compromised L-arginine homeostasis. Here, we report high-affinity binding of the reaction coordinate analogue inhibitors 2(S)-amino-6-boronohexanoic acid (ABH, K-d = 5 nM) and S-(2-boronoethyl)-L-cysteine (BEC, K-d = 270 nM) to human arginase 1, and we report x-ray crystal structures of the respective enzyme-inhibitor complexes at 1.29- and 1.94-angstrom resolution determined from crystals twinned by hemilhedry. The ultrahigh-resolution structure of the human arginase I-ABH complex yields an unprecedented view of the binuclear manganese cluster and illuminates the structural basis for nanomolar affinity: bidentate inner-sphere boronate-manganese coordination interactions and fully saturated hydrogen bond networks with inhibitor a-amino and a-carboxylate groups. These interactions are therefore implicated in the stabilization of the transition state for L-arginine hydrolysis. Electron density maps also reveal that active-site residue H141 is protonated as the imidazolium cation. The location of H141 is such that it could function as a general acid to protonate the leaving amino group Of L-ornithine during catalysis, and this is a revised mechanistic proposal for arginase. This work serves as a foundation for studying the structural and chemical biology of arginase I in the immune response, and we demonstrate the inhibition of arginase activity by ABH in human and murine myeloid cells.