ABSTRACT
Tethered dimers incorporating natural alpha-amino acid end groups were synthesized, including examples in which the previously reported esterase-sensitive ester linker was replaced with more stable amide or ether linkers. These compounds remained effective both as inhibitors of NAD synthetase and as potent antibacterial agents for Gram-positive strains. Studies on nonspecific effects, including detergent properties and promiscuous inhibition, suggested little contribution to observed activities.
Subject(s)
Amide Synthases/antagonists & inhibitors , Amino Acids/chemical synthesis , Anti-Bacterial Agents/chemical synthesis , Gram-Positive Bacteria/drug effects , NAD/metabolism , Amide Synthases/metabolism , Amides/chemistry , Amino Acids/chemistry , Amino Acids/pharmacology , Anti-Bacterial Agents/chemistry , Anti-Bacterial Agents/pharmacology , Bacillus subtilis/enzymology , Detergents/chemistry , Dimerization , Ethers/chemistry , Microbial Sensitivity Tests , Octoxynol/chemistry , Structure-Activity RelationshipABSTRACT
Apolipoprotein (apo) A-I mutants were constructed for FRET studies to distinguish between two possible lipid-free conformers, a globular helix bundle and an elongated helical hairpin. Mutants containing a single Trp at position 50 were prepared by replacing Trps at positions 8, 72, and 108 with Phe (W@50). Two mutants were constructed from W@50 by incorporating Cys at Arg83 (W@50R83C) or Arg173 (W@50R173C) for attachment of the fluorescent probe AEDANS. Secondary structure of the mutants is very similar to wild type (wt) apo A-I, and fluorescence emission indicates that W50 is protected from solvent. Thermal stabilities of the AEDANS-labeled mutants are also similar to wt. These results indicate that no discernible changes occur in structure or stability as a result of mutations or labeling. The FRET data from W@50 to AEDANS are well-represented by a single distance distribution function with a distance of approximately 22 A for W@50R83C and approximately 19 A for W@50R173C. These distances are consistent with theoretical values calculated from a helical bundle model but not from a helical hairpin. A probability distance distribution function yields significantly small half-width values of 5.6 and 3.7 A, respectively, suggesting low conformational dynamics in both mutants. Differential scanning calorimetry (DSC) was performed on wt and a C-terminal deletion mutant, Delta(187-243), to obtain information on domain architecture. Contrary to expectations, both proteins unfold cooperatively. The results are consistent with the presence of a single folded domain within residues 1-186. These results support the presence of a discrete globular bundle conformation for lipid-free apo A-I.
Subject(s)
Apolipoprotein A-I/chemistry , Lipids/chemistry , Models, Molecular , Apolipoprotein A-I/genetics , Circular Dichroism , Electrophoresis, Polyacrylamide Gel , Fluorescence Resonance Energy Transfer , Humans , Mutagenesis , NaphthalenesulfonatesABSTRACT
Dimethyl sulfoxide (DMSO) is commonly used as a cosolvent to improve the aqueous solubility of small organic compounds. Its use in a screen to identify novel inhibitors of the enzyme NAD(+) synthetase led to this investigation of its potential effects on the structure and stability of this 60-kD homodimeric enzyme. Although no effects are observed on the enzyme's catalytic activity, as low as 2.5% (v/v) DMSO led to demonstrable changes in the stability of the dimer and its unfolding mechanism. In the absence of DMSO, the dimer behaves hydrodynamically as a single ideal species, as determined by equilibrium analytical ultracentrifugation, and thermally unfolds according to a two-state dissociative mechanism, based on analysis by differential scanning calorimetry (DSC). In the presence of 2.5% (v/v) DMSO, an equilibrium between the dimer and monomer is now detectable with a measured dimer association constant, K(a), equal to 5.6 x 10(6)/M. DSC curve analysis is consistent with this finding. The data are best fit to a three-state sequential unfolding mechanism, most likely representing folded dimer <==> folded monomer <==> unfolded monomer. The unusually large change in the relative stabilities of dimer and monomer, e.g., the association equilibrium shifts from an infinitely large K(a) down to approximately 10(6)/M, in the presence of relatively low cosolvent concentration is surprising in view of the significant buried surface area at the dimer interface, roughly 20% of the surface area of each monomer is buried. A hypothetical structural mechanism to explain this effect is presented.
