title: Mechanisms of activation of phosphoenolpyruvate carboxykinase from Escherichia coli by Ca2+ and of desensitization by trypsin.
abstract: The 1.8-A resolution structure of the ATP-Mg(2+)-Ca(2+)-pyruvate quinary complex of Escherichia coli phosphoenolpyruvate carboxykinase (PCK) is isomorphous to the published complex ATP-Mg(2+)-Mn(2+)-pyruvate-PCK, except for the Ca(2+) and Mn(2+) binding sites. Ca(2+) was formerly implicated as a possible allosteric regulator of PCK, binding at the active site and at a surface activating site (Glu508 and Glu511). This report found that Ca(2+) bound only at the active site, indicating that there is likely no surface allosteric site. (45)Ca(2+) bound to PCK with a K(d) of 85 micro M and n of 0.92. Glu508Gln Glu511Gln mutant PCK had normal activation by Ca(2+). Separate roles of Mg(2+), which binds the nucleotide, and Ca(2+), which bridges the nucleotide and the anionic substrate, are implied, and the catalytic mechanism of PCK is better explained by studies of the Ca(2+)-bound structure. Partial trypsin digestion abolishes Ca(2+) activation (desensitizes PCK). N-terminal sequencing identified sensitive sites, i.e., Arg2 and Arg396. Arg2Ser, Arg396Ser, and Arg2Ser Arg396Ser (double mutant) PCKs altered the kinetics of desensitization. C-terminal residues 397 to 540 were removed by trypsin when wild-type PCK was completely desensitized. Phe409 and Phe413 interact with residues in the Ca(2+) binding site, probably stabilizing the C terminus. Phe409Ala, DeltaPhe409, Phe413Ala, Delta397-521 (deletion of residues 397 to 521), Arg396(TAA) (stop codon), and Asp269Glu (Ca(2+) site) mutations failed to desensitize PCK and, with the exception of Phe409Ala, appeared to have defects in the synthesis or assembly of PCK, suggesting that the structure of the C-terminal domain is important in these processes.
authors.affiliation: Department of Biochemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5E5.
authors.forename: Athena
authors.initials: A
authors.lastname: Sudom
authors.forename: Robert
authors.initials: R
authors.lastname: Walters
authors.forename: Landon
authors.initials: L
authors.lastname: Pastushok
authors.forename: Douglas
authors.initials: D
authors.lastname: Goldie
authors.forename: Lata
authors.initials: L
authors.lastname: Prasad
authors.forename: Louis T J
authors.initials: LT
authors.lastname: Delbaere
authors.forename: Hughes
authors.initials: H
authors.lastname: Goldie
mesh_terms: D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004789:Enzyme Activation; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics; D008274:Magnesium / Q000378:metabolism; D008345:Manganese / Q000378:metabolism; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D019764:Phosphoenolpyruvate Carboxykinase (ATP) / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D010957:Plasmids / Q000235:genetics; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D019289:Pyruvic Acid / Q000378:metabolism; D014357:Trypsin / Q000378:metabolism*
chemical_list: D008345:Manganese; D019289:Pyruvic Acid; D014357:Trypsin; D019764:Phosphoenolpyruvate Carboxykinase (ATP); D008274:Magnesium; D002118:Calcium