Cation binding to a Bacillus (1,3-1,4)-beta-glucanase. Geometry, affinity and effect on protein stability.
The hybrid Bacillus (1,3-1,4)-beta-glucanase H(A16-M), consisting of 16 N-terminal amino acids derived from the mature form of the B. amyloliquefaciens enzyme and of 198 C-proximal amino acids from the B. macerans enzyme, binds a calcium ion at a site at its molecular surface remote from the active center [T. Keitel, O. Simon, R. Borriss & U. Heinemann (1993) Proc. Natl Acad. Sci. USA 90, 5287-5291]. X-ray diffraction analysis at 0.22-nm resolution of crystals grown in the absence of calcium and in the presence of EDTA shows this site to be occupied by a sodium ion. Whereas the calcium ion has six oxygen atoms in its coordination sphere, two of which are from water molecules, sodium is fivefold coordinated with a fifth ligand belonging to a symmetry-related protein molecule in the crystal lattice. The affinity of H(A16-M) for calcium over sodium has been determined calorimetrically. Calcium binding stabilizes the native three-dimensional structure of the protein as shown by guanidinium chloride unfolding and thermal inactivation experiments. The enhanced enzymic activity of Bacillus beta-glucanases at elevated temperatures in the presence of calcium ions is attributed to a general stabilizing effect by the cation.
Keitel
T
T
Institut für Kristallographie, Freie Universität Berlin, Germany.
Meldgaard
M
M
Heinemann
U
U
D001407:Bacillus / Q000201:enzymology*; D002412:Cations; D002478:Cells, Cultured; D018360:Crystallography, X-Ray; D004795:Enzyme Stability; D006026:Glycoside Hydrolases / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D013816:Thermodynamics
D002412:Cations; D006026:Glycoside Hydrolases; C028020:licheninase