Crystal structure of Bacillus stearothermophilus alpha-amylase: possible factors determining the thermostability.
The crystal structure of a thermostable alpha-amylase from Bacillus stearothermophilus (BSTA) has been determined at 2.0 A resolution. The main-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 BSTA, Ile181-Gly182, pushes away the spatially contacting region including Asp207, which corresponds to Ca(2+)-coordinating Asp204 in BLA. As a result, Asp207 cannot coordinate the Ca(2+). (ii) BSTA contains nine fewer hydrogen bonds than BLA, which costs about 12 kcal/mol. This tendency is prominent in the (beta/alpha)(8)-barrel, where 10 fewer hydrogen bonds were observed in BSTA. BLA forms a denser hydrogen bond network in the inter-helical region, which may stabilize alpha-helices in the barrel. (iii) A few small voids observed in the alpha-helical region of the (beta/alpha)(8)-barrel in BSTA decrease inter-helical compactness and hydrophobic interactions. (iv) The solvent-accessible surface area of charged residues in BLA is about two times larger than that in BSTA.
Institute of Applied Biochemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan. mizuno@abr.affrc.go.jp
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D000595:Amino Acid Sequence; D001665:Binding Sites; D003460:Crystallization; D004795:Enzyme Stability; D001411:Geobacillus stearothermophilus / Q000201:enzymology*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D016415:Sequence Alignment; D055672:Static Electricity; D013329:Structure-Activity Relationship; D013696:Temperature; D014961:X-Ray Diffraction; D000516:alpha-Amylases / Q000737:chemistry* / Q000302:isolation & purification
D000516:alpha-Amylases