Ca. Smith et al., Calcium-mediated thermostability in the subtilisin superfamily: The crystal structure of Bacillus Ak.1 protease at 1.8 angstrom resolution, J MOL BIOL, 294(4), 1999, pp. 1027-1040
Proteins of the subtilisin superfamily (subtilases) are widely distributed
through many Living species, where they perform a variety of processing fun
ctions. They are also used extensively in industry. In many of these enzyme
s, bound calcium ions play a key role in protecting against autolysis and t
hermal denaturation. We have determined the crystal structure of a highly t
hermostable protease from Bacillus sp. Ak.1 that is strongly stabilized by
calcium. The crystal structure, determined at 1.8 Angstrom resolution (R=0.
182, R-free = 0.247), reveals the presence of four bound cations, three Ca2
+ and one Naf. Two of the Ca2+ binding sites, Ca-l and Ca-2, correspond to
sites also found in thermitase and the mesophilic subtilisins. The third ca
lcium ion, however, is at a novel site that is created by two key amino aci
d substitutions near Ca-l, and has not been observed in any other subtilase
. This site, acting cooperatively with Ca-l, appears to give substantially
enhanced thermostability, compared with thermitase. Comparisons with the me
sophilic subtilisins also point to the importance of aromatic clusters, red
uced hydrophobic surface and constrained N and C termini in enhancing the t
hermostability of thermitase and Ak.1 protease. The Ak.1 protease also cont
ains an unusual Cys-X-Cys disulfide bridge that modifies the active site cl
eft geometry. (C) 1999 Academic Press.