2NSJ

E. coli PurE H45Q mutant complexed with CAIR

Summary for 2NSJ

• Entry DOI: 10.2210/pdb2nsj/pdb
• Related: 2ATE 2NSH 2NSL
• Descriptor: Phosphoribosylaminoimidazole carboxylase catalytic subunit, 5-AMINO-1-(5-O-PHOSPHONO-BETA-D-RIBOFURANOSYL)-1H-IMIDAZOLE-4-CARBOXYLIC ACID (3 entities in total)
• Functional Keywords: central three-layer alpha-beta-alpha sandwich, kinked c-terminal helix, lyase
• Biological source: Escherichia coli
• Total number of polymer chains: 1
• Total formula weight: 18128.45

Authors

Ealick, S.E.,Morar, M. (deposition date: 2006-11-04, release date: 2007-04-24, Last modification date: 2023-08-30)

Primary citation

Hoskins, A.A.,Morar, M.,Kappock, T.J.,Mathews, I.I.,Zaugg, J.B.,Barder, T.E.,Peng, P.,Okamoto, A.,Ealick, S.E.,Stubbe, J.
N(5)-CAIR Mutase: Role of a CO(2) Binding Site and Substrate Movement in Catalysis.
Biochemistry, 46:2842-2855, 2007

PubMed Abstract: N5-Carboxyaminoimidazole ribonucleotide mutase (N5-CAIR mutase or PurE) from Escherichia coli catalyzes the reversible interconversion of N5-CAIR to carboxyaminoimidazole ribonucleotide (CAIR) with direct CO2 transfer. Site-directed mutagenesis, a pH-rate profile, DFT calculations, and X-ray crystallography together provide new insight into the mechanism of this unusual transformation. These studies suggest that a conserved, protonated histidine (His45) plays an essential role in catalysis. The importance of proton transfers is supported by DFT calculations on CAIR and N5-CAIR analogues in which the ribose 5'-phosphate is replaced with a methyl group. The calculations suggest that the nonaromatic tautomer of CAIR (isoCAIR) is only 3.1 kcal/mol higher in energy than its aromatic counterpart, implicating this species as a potential intermediate in the PurE-catalyzed reaction. A structure of wild-type PurE cocrystallized with 4-nitroaminoimidazole ribonucleotide (NO2-AIR, a CAIR analogue) and structures of H45N and H45Q PurEs soaked with CAIR have been determined and provide the first insight into the binding of an intact PurE substrate. A comparison of 19 available structures of PurE and PurE mutants in apo and nucleotide-bound forms reveals a common, buried carboxylate or CO2 binding site for CAIR and N5-CAIR in a hydrophobic pocket in which the carboxylate or CO2 interacts with backbone amides. This work has led to a mechanistic proposal in which the carboxylate orients the substrate for proton transfer from His45 to N5-CAIR to form an enzyme-bound aminoimidazole ribonucleotide (AIR) and CO2 intermediate. Subsequent movement of the aminoimidazole moiety of AIR reorients it for addition of CO2 at C4 to generate isoCAIR. His45 is now in a position to remove a C4 proton to produce CAIR.

PubMed: 17298082
DOI: 10.1021/bi602436g

Experimental method

X-RAY DIFFRACTION (2.31 Å)