title: Characterization and implications of Ca2+ binding to pectate lyase C.
abstract: Ca(2+) is essential for in vitro activity of Erwinia chrysanthemi pectate lyase C (PelC). Crystallographic analyses of 11 PelC-Ca(2+) complexes, formed at pH 4.5, 9.5, and 11.2 under varying Ca(2+) concentrations, have been solved and refined at a resolution of 2.2 A. The Ca(2+) site represents a new motif for Ca(2+), consisting primarily of beta-turns and beta-strands. The principal differences between PelC and the PelC-Ca(2+) structures at all pH values are the side-chain conformations of Asp-129 and Glu-166 as well as the occupancies of four water molecules. According to calculations of pK(a) values, the presence of Ca(2+) and associated structural changes lower the pK(a) of Arg-218, the amino acid responsible for proton abstraction during catalysis. The Ca(2+) affinity for PelC is weak, as the K(d) was estimated to be 0.132 (+/-0.004) mm at pH 9.5, 1.09 (+/-0.29) mm at pH 11.2, and 5.84 (+/-0.41) mm at pH 4.5 from x-ray diffraction studies and 0.133 (+/-0.045) mm at pH 9.5 from intrinsic tryptophan fluorescence measurements. Given the pH dependence of Ca(2+) affinity, PelC activity at pH 4.5 has been reexamined. At saturating Ca(2+) concentrations, PelC activity increases 10-fold at pH 4.5 but is less than 1% of maximal activity at pH 9.5. Taken together, the studies suggest that the primary Ca(2+) ion in PelC has multiple functions.
authors.affiliation: Department of Physiology and Biophysics, University of California, Irvine, California 92697-4560, USA.
authors.forename: Steven R
authors.initials: SR
authors.lastname: Herron
authors.forename: Robert D
authors.initials: RD
authors.lastname: Scavetta
authors.forename: Michael
authors.initials: M
authors.lastname: Garrett
authors.forename: Margaret
authors.initials: M
authors.lastname: Legner
authors.forename: Frances
authors.initials: F
authors.lastname: Jurnak
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D018360:Crystallography, X-Ray; D016974:Dickeya chrysanthemi / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007527:Isoenzymes / Q000737:chemistry* / Q000378:metabolism*; D011133:Polysaccharide-Lyases / Q000737:chemistry* / Q000378:metabolism*; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
chemical_list: D007527:Isoenzymes; D011133:Polysaccharide-Lyases; C023911:pectate lyase; D002118:Calcium