Crystal structure of Bacillus stearothermaphilus alpha-amylase: Possible factors determining the thermostability

The crystal structure of a thermostable alpha -amylase from Bacillus stearo thermophilus (BSTA) has been determined at 2.0 Angstrom resolution. The mai n-chain fold is almost identical to that of the known crystal structure of Bacillus Licheniformis alpha -amylase (BLA), BLA is known to be more stable than BSTA. A structural comparison between the crystal structures of BSTA and BLA showed significant differences that may account for the difference in their thermostabilities, as follows. (i) The two-residue insertion in BS TA, Ile181-Gly182, pushes away the spatially contacting region including As p207, which corresponds to Ca2+-coordinating Asp204 in BLA. As a result, As p207 cannot coordinate the Ca2+, (ii) BSTA contains nine fewer hydrogen bon ds than BLA, which costs about 12 kcal/mol. This tendency is prominent in t he (beta/alpha)(8)-barrel, where 10 fewer hydrogen bonds were observed in B STA. BLA forms a denser hydrogen bond network in the interhelical region, w hich may stabilize alpha -helices in the barrel. (iii) A few small voids ob served in the alpha -helical region of the (beta/alpha)(8)-barrel in BSTA d ecrease inter-helical compactness and hydrophobic interactions. (iv) The so lvent-accessible surface area of charged residues in BLA is about two times larger than that in BSTA.