title: Profactor IX propeptide and glutamate substrate binding sites on the vitamin K-dependent carboxylase identified by site-directed mutagenesis.
abstract: The vitamin K-dependent carboxylase, a constituent of the endoplasmic reticulum membrane, catalyzes the conversion of reduced vitamin K to vitamin K epoxide and the concomitant conversion of glutamic acid to gamma-carboxyglutamic acid. To study structure-function relationships in the enzyme, seventeen clusters of charged residues of the bovine gamma-glutamyl carboxylase were substituted with alanines using site-specific mutagenesis. Wild-type and mutant carboxylase species were expressed in Chinese hamster ovary cells with an immunodetectable octapeptide inserted at their amino-terminal ends. Out of 17 mutant carboxylase species that contain a total of 41 charged residue to alanine substitutions, K217A/K218A (CBX217/218), R234A/H235A (CBX234/235), R359A/H360A/K361A (CBX359/360/361), R406A/H408A (CBX406/408), and R513A/K515A (CBX513/515) had impaired carboxylase activity compared with the wild-type enzyme. The vitamin K epoxidase activities of these mutants were reduced in parallel with the carboxylase activities. CBX217/218 appears to be inactive. High propeptide concentrations were required for stimulation of carboxylation of FLEEL by CBX234/235, CBX406/408, and CBX513/515, suggesting defects in the propeptide binding site. CBX359/360/361 showed normal affinity for the propeptide, FLEEL, proPT28, and vitamin K hydroquinone but exhibited a low catalytic rate for carboxylation. These results suggest that residue 217, residue 218, or both are either critical for catalysis or for maintaining the structure of a catalytically active enzyme. Regions around residues 234, 406, and 513 define in part the propeptide binding site, while the regions around residue 359 are involved in catalysis.
authors.affiliation: New England Medical Center, Department of Medicine, Tufts University School of Medicine, Boston, Massachusetts 02111, USA.
authors.forename: I
authors.initials: I
authors.lastname: Sugiura
authors.forename: B
authors.initials: B
authors.lastname: Furie
authors.forename: C T
authors.initials: CT
authors.lastname: Walsh
authors.forename: B C
authors.initials: BC
authors.lastname: Furie
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D019735:Carbon-Carbon Ligases; D002417:Cattle; D006224:Cricetinae; D005164:Factor IX / Q000378:metabolism*; D018698:Glutamic Acid / Q000378:metabolism*; D007700:Kinetics; D008025:Ligases / Q000235:genetics / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011498:Protein Precursors / Q000378:metabolism*; D011994:Recombinant Proteins; D013329:Structure-Activity Relationship; D010837:Vitamin K 1 / Q000031:analogs & derivatives / Q000494:pharmacology; D064417:Vitamin K Epoxide Reductases
chemical_list: D011498:Protein Precursors; D011994:Recombinant Proteins; C049838:factor IX Cambridge; D018698:Glutamic Acid; C028843:vitamin K1 hydroquinone; D010837:Vitamin K 1; D005164:Factor IX; D006899:Mixed Function Oxygenases; D064417:Vitamin K Epoxide Reductases; D008025:Ligases; D019735:Carbon-Carbon Ligases; C020660:glutamyl carboxylase