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Figure 1.
Figure 1. L-Arginine catabolism. a, Structure-based mechanism
of arginase^16, in which metal-activated hydroxide ion attacks
the substrate guanidinium group to form a tetrahedral
intermediate (for clarity, only the side chain atoms of
substrate L-arginine are shown). Proton transfer mediated by Asp
128 facilitates collapse of this intermediate to form products
L-ornithine and urea. Following product dissociation, a
nucleophilic metal-bridging hydroxide ion is regenerated from a
metal-bridging water by proton transfer to bulk solvent. His 141
may function as a proton shuttle as indicated. b, Reciprocal
coordination of arginase and nitric oxide pathways; note that N^
 -hydroxy-L-arginine
is an intermediate in the NO synthase reaction. c, The arginase
inhibitor 2(S)-amino-6-boronohexanoic acid (ABH) is an isostere
of L-arginine.
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