title: Specificity determining residues in ammonia- and glutamine-dependent carbamoyl phosphate synthetases.
abstract: Carbamoyl phosphate synthetases (CPSs) utilize either glutamine or ammonia for the ATP-dependent generation of carbamoyl phosphate. In glutamine-utilizing CPSs (e.g. the single Escherichia coli CPS and mammalian CPS II), the hydrolysis of glutamine to yield ammonia is catalyzed at a triad-type glutamine amidotransferase domain. Non-glutamine-utilizing CPSs (e.g. rat and human CPS I), lacking the catalytic cysteine residue, can generate carbamoyl phosphate only in the presence of free ammonia. Frog CPS I (fCPS I), unlike mammalian CPS Is, retains most of the glutamine amidotransferase residues conserved in glutamine-utilizing CPSs, including an intact catalytic triad, and could therefore be expected to use glutamine. Our work with native fCPS I provides the first demonstration of the inability of this enzyme to bind/utilize glutamine. To determine why fCPS I is unable to utilize glutamine, we compared sequences of glutamine-using and non-glutamine-using CPSs to identify residues that are present or conservatively substituted in all glutamine-utilizing CPSs but absent in fCPS I. We constructed the site-directed mutants Q273E, L270K, Q273E/N240S, and Q273E/L270K in E. coli CPS and have determined that simultaneous occurrence of the two substitutions, Gln-->Glu and Leu-->Lys, found in the frog CPS I glutamine amidotransferase domain are sufficient to eliminate glutamine utilization by the E. coli enzyme.
authors.affiliation: Department of Biology, Northeastern University, Boston, MA 02115, USA.
authors.forename: Amna
authors.initials: A
authors.lastname: Saeed-Kothe
authors.forename: Susan G
authors.initials: SG
authors.lastname: Powers-Lee
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000495:Allosteric Site; D000595:Amino Acid Sequence; D000641:Ammonia / Q000737:chemistry; D000818:Animals; D001665:Binding Sites; D015153:Blotting, Western; D002222:Carbamoyl-Phosphate Synthase (Ammonia) / Q000737:chemistry* / Q000378:metabolism*; D002223:Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzing) / Q000737:chemistry* / Q000378:metabolism*; D003545:Cysteine / Q000737:chemistry; D004926:Escherichia coli / Q000201:enzymology / Q000378:metabolism; D018698:Glutamic Acid / Q000737:chemistry; D005973:Glutamine / Q000737:chemistry; D006801:Humans; D007700:Kinetics; D007930:Leucine / Q000737:chemistry; D008239:Lysine / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011898:Ranidae; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D011994:Recombinant Proteins; D005973:Glutamine; D018698:Glutamic Acid; D000641:Ammonia; D000255:Adenosine Triphosphate; D002222:Carbamoyl-Phosphate Synthase (Ammonia); D002223:Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzing); D007930:Leucine; D008239:Lysine; D003545:Cysteine