Active site location in an intracellular subtilisin protease (a serine protease)

Hey!

I've been reading this paper which describes the structure of an intracellular subtilisin protease from Bacillus clausii, which is a type of subtilisin-like serine protease (as opposed to a trypsin/chymotrypsin serine protease). Subtilisin-like serine proteases usually have a conserved catalytic triad of Asp-32, His-64 and Ser-221, however in this paper, the protein seems to have an active site of Asp-49, His-86 and Ser-250. Why is this?

In addition, for the purposes of stabilising this protein for study, the active serine was mutated to Ala-250, could this have anything to do with the location of the active site?

Any help would be greatly appreciated, Thanks!

Vévodová, J., Gamble, M., Kunze, G., Ariza, A., Dodson, E., Jones, D. D. and Wilson, K. S. 2010. Crystal Structure of an Intracellular Subtilisin Reveals Novel Structural Features Unique to this Subtilisin Family. Structure. 18(6), pp. 744-755.

http://www.sciencedirect.com/science/article/pii/S0969212610001401