In this study, chemical promiscuity
of a binuclear metallohydrolase Streptomyces griseus aminopeptidase (SgAP) has been investigated using
DFT calculations. SgAP catalyzes two diverse reactions,
peptide and phosphoester hydrolyses,
using its binuclear (Zn–Zn) core. On the basis of the experimental
information, mechanisms of these reactions have been investigated
utilizing leucine p-nitro aniline (Leu-pNA) and bis(4-nitrophenyl) phosphate (BNPP) as the substrates. The
computed barriers of 16.5 and 16.8 kcal/mol for the most plausible
mechanisms proposed by the DFT calculations are in good agreement
with the measured values of 13.9 and 18.3 kcal/mol for the Leu-pNA and BNPP hydrolyses, respectively. The former was found
to occur through the transfer of two protons, while the latter with
only one proton transfer. They are in line with the experimental observations.
The cleavage of the peptide bond was the rate-determining process
for the Leu-pNA hydrolysis. However, the creation
of the nucleophile and its attack on the electrophile phosphorus atom
was the rate-determining step for the BNPP hydrolysis. These calculations
showed that the chemical nature of the substrate and its binding mode
influence the nucleophilicity of the metal bound hydroxyl nucleophile.
Additionally, the nucleophilicity was found to be critical for the
Leu-pNA hydrolysis, whereas double Lewis acid activation
was needed for the BNPP hydrolysis. That could be one of the reasons
why peptide hydrolysis can be catalyzed by both mononuclear and binuclear
metal cofactors containing hydrolases, while phosphoester hydrolysis
is almost exclusively by binuclear metallohydrolases. These results
will be helpful in the development of versatile catalysts for chemically
distinct hydrolytic reactions.