Figure 2.
Fig. 2. Mechanism of human arginase I. New features
suggested by the 1.29-Å resolution structure of the
complex with ABH are the inner-sphere coordination of 2-NH[2] to
in
the tetrahedral intermediate, and the protonation of the amino
leaving group of L-ornithine by the conformationally flexible
imidazolium group of general acid H141. D128 may also donate a
proton to L-ornithine before product release. In the final step
of catalysis, the H141 imidazole may serve as a general base to
abstract a proton from the metal-bridging water molecule
(possibly through an intervening solvent molecule). For clarity,
only the side chain guanidinium group of substrate L-arginine
and the side chain amino group of product L-ornithine are
indicated.
2-NH[2] to
in
the tetrahedral intermediate, and the protonation of the amino
leaving group of L-ornithine by the conformationally flexible
imidazolium group of general acid H141. D128 may also donate a
proton to L-ornithine before product release. In the final step
of catalysis, the H141 imidazole may serve as a general base to
abstract a proton from the metal-bridging water molecule
(possibly through an intervening solvent molecule). For clarity,
only the side chain guanidinium group of substrate L-arginine
and the side chain amino group of product L-ornithine are
indicated.