Enzymatic characterization of a serralysin-like metalloprotease from the entomopathogen bacterium, Xenorhabdus

We investigated the enzymatic properties of a serralysin-type metalloenzyme, provisionally named as protease B, which is secreted by Xenorhabdus bacterium, and probably is the ortholog of PrA peptidase of Photorhabdus bacterium. Testing the activity on twenty-two oligopeptide substrates we found that protease B requires at least three amino acids N-terminal to the scissile bond for detectable hydrolysis. On such substrate protease B was clearly specific for positively charged residues (Arg and Lys) at the P1 substrate position and was rather permissive in the others. Interestingly however, it preferred Ser at P1 in the oligopeptide substrate which contained amino acids also C-terminal to the scissile bond, and was cleaved with the highest kcat/KM value. The pH profile of activity, similarly to other serralysins, has a wide peak with high values between pH 6.5 and 8.0. The activity was slightly increased by Cu2+ and Co2+ ions, it was not sensitive for serine protease inhibitors, but it was inhibited by 1,10-phenanthroline, features shared by many Zn-metalloproteases. At the same time, EDTA inhibited the activity only partially even either after long incubation or in excess amount, and Zn2+ was inhibitory (both are unusual among serralysins). The 1,10-phenanthroline inhibited activity could be restored with the addition of Mn2+, Cu2+ and Co2+ up to 90–200% of its original value, while Zn2+ was inefficient. We propose that both the Zn inhibition of protease B activity and its resistance to EDTA inhibition might be caused by an Asp in position 191 where most of the serralysins contain Asn.

Research Highlights
► Some properties of Xenorhabdus PrtA protease are rare among serralysins.
► It cleaves next to Lys, Arg in P4-P1 peptides but next to Ser in a P4-P4’ peptide.
► It is inhibited by Zn2+ and is less sensitive to inhibition by EDTA.
► Its activity is enhanced by Cu2+ and Co2+ and rescued by Cu, Co, Mn but not by Zn ion.
► The unusual effect of Zn2+ and EDTA can be explained by an Asn191Asp substitution.