The peptidases in clan MH are known as cocatalytic zinc peptidases that have two zinc ions in the active site, but their metal preference has not been rigorously investigated. In this study, the molecular basis for metal preference is provided from the structural and biochemical analyses. Kinetic studies of Pseudomonas aeruginosa aspartyl aminopeptidase (PaAP) which belongs to peptidase family M18 in clan MH revealed that its peptidase activity is dependent on Co2+ rather than Zn2+: the K-cat (s(-1)) values of PaAP were 0.006, 5.10 and 0.43 in no-metal, Co2+, and Zn2+ conditions, respectively. Consistently, addition of low concentrations of Co2+ to PaAP previously saturated with Zn2+ greatly enhanced the enzymatic activity, suggesting that Co2+ may be the physiologically relevant cocatalytic metal ion of PaAP. The crystal structures of PaAP complexes with Co2+ or Zn2+ commonly showed two metal ions in the active site coordinated with three conserved residues and a bicarbonate ion in a tetragonal geometry. However, Co2+- and Zn2+-bound Structures showed no noticeable alterations relevant to differential effects of metal species, except the relative orientation of Glu-265, a general base in the active site. The characterization of mutant PaAP revealed that the first metal binding site is primarily responsible for metal preference. Similar to PaAP, Streptococcus pneumonia glutamyl aminopeptidase (SpGP), belonging to aminopeptidase family M42 in clan MH, also showed requirement for Co2+ for maximum activity. These results proposed that clan MH peptidases might be a cocatalytic cobalt peptidase rather than a zinc-dependent peptidase. (c) 2014 Elsevier Inc. All rights reserved.