Three-dimensional structure of N5-carboxyaminoimidazole ribonucleotide synthetase: a member of the ATP grasp protein superfamily.
Escherichia coli PurK, a dimeric N5-carboxyaminoimidazole ribonucleotide (N5-CAIR) synthetase, catalyzes the conversion of 5-aminoimidazole ribonucleotide (AIR), ATP, and bicarbonate to N5-CAIR, ADP, and Pi. Crystallization of both a sulfate-liganded and the MgADP-liganded E. coli PurK has resulted in structures at 2.1 and 2.5 A resolution, respectively. PurK belongs to the ATP grasp superfamily of C-N ligase enzymes. Each subunit of PurK is composed of three domains (A, B, and C). The B domain contains a flexible, glycine-rich loop (B loop, T123-G130) that is disordered in the sulfate-PurK structure and becomes ordered in the MgADP-PurK structure. MgADP is wedged between the B and C domains, as with all members of the ATP grasp superfamily. Other enzymes in this superfamily contain a conserved Omega loop proposed to interact with the B loop, define the specificity of their nonnucleotide substrate, and protect the acyl phosphate intermediate formed from this substrate. PurK contains a minimal Omega loop without conserved residues. In the reaction catalyzed by PurK, carboxyphosphate is the putative acyl phosphate intermediate. The sulfate of the sulfate ion-liganded PurK interacts electrostatically with Arg 242 and the backbone amide group of Asn 245, components of the J loop of the C domain. This sulfate may reveal the location of the carboxyphosphate binding site. Conserved residues within the C-terminus of the C domain define a pocket that is proposed to bind AIR in collaboration with an N-terminal strand loop helix motif in the A domain (P loop, G8-L1). The P loop is proposed to bind the phosphate of AIR on the basis of similar binding sites observed in PurN and PurE and proposed in PurD and PurT, four other enzymes in the purine pathway.
Thoden
J B
JB
Department of Biochemistry, University of Wisconsin-Madison 53705, USA.
Kappock
T J
TJ
Stubbe
J
J
Holden
H M
HM
D000251:Adenosine Triphosphatases / Q000737:chemistry* / Q000378:metabolism; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D002262:Carboxy-Lyases; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D019281:Dimerization; D004926:Escherichia coli / Q000201:enzymology; D029968:Escherichia coli Proteins; D006056:Golgi Apparatus / Q000737:chemistry* / Q000378:metabolism; D008024:Ligands; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009943:Organophosphorus Compounds / Q000737:chemistry / Q000378:metabolism; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D010710:Phosphates; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D012265:Ribonucleotides / Q000737:chemistry* / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; D008024:Ligands; D008565:Membrane Proteins; D009943:Organophosphorus Compounds; D010446:Peptide Fragments; D010710:Phosphates; D012265:Ribonucleotides; C011416:carboxyaminoimidazole ribotide; C015732:carboxy phosphate; D000251:Adenosine Triphosphatases; D002262:Carboxy-Lyases; C075015:purK protein, E coli