PubMed · 11195518
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2001-01-01. Low-affinity complexes.. https://pubmed.ncbi.nlm.nih.gov/11195518/
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Bacterial luciferase is a heterodimeric (alphabeta) enzyme which catalyzes a light-producing reaction in Vibrio harveyi. In addition to the alphabeta enzyme, the beta subunit can self-associate to form a stable but inactive homodimer [Sinclair, J. F., Ziegler, M. M., and Baldwin, T. O. (1994) Nat. Struct. Biol. 1, 320-326]. The studies reported here were undertaken to explore the role of the subunit interface in the conformational stability of the enzyme. To this end, we constructed four mutant heterodimers in which residues at the subunit interface were changed in an effort to alter the volume of an apparent solvent accessible channel at the interface or to alter H-bonding groups. Equilibrium unfolding data for the heterodimer have been interpreted in terms of a three-state mechanism [Clark, C. A., Sinclair, J. F., and Baldwin, T. O. (1993) J. Biol. Chem. 268, 10773-10779]. However, we found that unfolding for the wild-type and mutant luciferases is better described by a four-state model. This change in the proposed mechanism of unfolding is based on observation of residual structure in the subunits following dissociation of the heterodimeric intermediate. All of the mutants display modest reductions in activity but, surprisingly, no change in the DeltaG2H2O value for subunit dissociation and no measurable change in the equilibrium dissociation constant relative to that of the wild-type heterodimer. However, the DeltaG1H2O value for the formation of the dimeric intermediate that precedes subunit dissociation is reduced for three of the mutants, indicating that mutations at the interface can alter the stability of a region of the alpha subunit that is distant from the interface. We conclude that the interface region communicates with the distal domains of this subunit, probably through the active center region of the enzyme.
The cooperativity in terms of enthalpy contribution for beta-sheet formation of polyglycine models in a vacuum has been studied theoretically by using a repeating unit approach. No cooperativity is found in the parallel direction for both the parallel and antiparallel beta-sheets. Cooperativity in the perpendicular direction is dependent upon the residue number (m) in each beta-strand. While there is large cooperativity in the acetamide hydrogen-bond chain (m = 0), the cooperativity is not large in beta-sheet networks (m > 0). SCIPCM solvent model calculations also significantly reduce the cooperativity in hydrogen-bond chains. It is concluded that cooperativity is mainly due to long-range electrostatic interactions and not due to the resonance effect.
N-(2-Chlorobenzyl)-1,2,3,4-tetrahydroisoquinoline-1,3-dione, C(16)H(12)ClNO(2), crystallizes in P2(1)/n with three crystallographically independent molecules in the asymmetric unit, which differ slightly in conformation, N-(2-bromo-4-methylphenyl)-1,2,3,4-tetrahydroisoquinoline-1,3-dione, C(16)H(12)BrNO(2), crystallizes in P2(1)/n with one molecule in the asymmetric unit and N-(2,3-dichlorophenyl)-1,2,3,4-tetrahydroisoquinoline-1,3-dione, C(15)H(9)Cl(2)NO(2), crystallizes in P2(1)/c with one molecule in the asymmetric unit. In all three structures, the heterocyclic rings adopt approximately planar conformations. The pyridine rings are orthogonal to the substituted phenyl rings. In all three structures, the crystal packing is stabilized by intermolecular C-H...O hydrogen bonds.