Fig. 2. Schematic of the active site of native aminopeptidase from Aeromonas proteolytica (AAP) For Asp179, there is a 0.04-Å difference in bond length between the two carbon–oxygen bonds of the side chain carboxylate, with the distance to the inner-sphere oxygen being slightly shorter than the dangling oxygen distance. This difference is about twice the ESD of the distance measurement at this resolution. Although a comparison of these two bond distances with each other may be unreliable because the difference between them is near the limits of their error estimates, a comparison can be made between the carbon–oxygen distances of Asp179 and those of Asp117 and Glu152. The carbon–oxygen distance to the inner-sphere oxygen of Asp179 is significantly shorter than the carbon–oxygen distances for the metal ligated oxygen atoms of Glu152 and Asp117 (Δd=0.05–0.07 Å) and is consistent with that for C=O distances measured in small molecules, implying that this bond has more double-bond character. The carbon–oxygen distance for the dangling oxygen compares well
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