title: 4-Oxalocrotonate tautomerase, a 41-kDa homohexamer: backbone and side-chain resonance assignments, solution secondary structure, and location of active site residues by heteronuclear NMR spectroscopy.
abstract: 4-Oxalocrotonate tautomerase (4-OT), a homohexamer consisting of 62 residues per subunit, catalyzes the isomerization of unsaturated alpha-keto acids using Pro-1 as a general base (Stivers et al., 1996a, 1996b). We report the backbone and side-chain 1H, 15N, and 13C NMR assignments and the solution secondary structure for 4-OT using 2D and 3D homonuclear and heteronuclear NMR methods. The subunit secondary structure consists of an alpha-helix (residues 13-30), two beta-strands (beta 1, residues 2-8; beta 2, residues 39-45), a beta-hairpin (residues 50-57), two loops (I, residues 9-12; II, 34-38), and two turns (I, residues 30-33; II, 47-50). The remaining residues form coils. The beta 1 strand is parallel to the beta 2 strand of the same subunit on the basis of cross stand NH(i)-NH(j) NOEs in a 2D 15N-edited 1H-NOESY spectrum of hexameric 4-OT containing two 15N-labeled subunits/hexamer. The beta 1 strand is also antiparallel to another beta 1 strand from an adjacent subunit forming a subunit interface. Because only three such pairwise interactions are possible, the hexamer is a trimer of dimers. The diffusion constant, determined by dynamic light scattering, and the rotational correlation time (14.5 ns) estimated from 15N T1/T2 measurements, are consistent with the hexameric molecular weight of 41 kDa. Residue Phe-50 is in the active site on the basis of transferred NOEs to the bound partial substrate 2-oxo-1,6-hexanedioate. Modification of the general base, Pro-1, with the active site-directed irreversible inhibitor, 3-bromopyruvate, significantly alters the amide 15N and NH chemical shifts of residues in the beta-hairpin and in loop II, providing evidence that these regions change conformation when the active site is occupied.
authors.affiliation: Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205-2185, USA.
authors.forename: J T
authors.initials: JT
authors.lastname: Stivers
authors.forename: C
authors.initials: C
authors.lastname: Abeygunawardana
authors.forename: C P
authors.initials: CP
authors.lastname: Whitman
authors.forename: A S
authors.initials: AS
authors.lastname: Mildvan
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D018360:Crystallography, X-Ray; D007535:Isomerases / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012996:Solutions
chemical_list: D012996:Solutions; C021918:4-oxalocrotonate tautomerase; D007535:Isomerases