MOLECULAR SPECIFICITY OF A MEDIUM-CHAIN ACYL-COA SYNTHETASE FOR SUBSTRATES AND INHIBITORS - CONFORMATIONAL-ANALYSIS

Amino acid conjugation is an important route of detoxification of xeno biotic and endogenous carboxylic acids. The specificity of the purifie d medium chain acyl-CoA synthetase catalyzing the first reaction of am ino acid conjugation was investigated further for substrates and inhib itors. Molecular modeling techniques were applied to derive the molecu lar characteristics of substrates and inhibitors for the medium chain acyl-CoA synthetase. The purified enzyme accepted not only straight me dium chain fatty acids but also aromatic acids. Of the arylacetic acid s, activity was obtained with naphthylacetic acids, whereas introducti on of a methyl group at the alpha-position caused loss of activity. Hi gh activity was also observed with cyclohexanoic acid. Diflunisal, 2-h ydroxydodecanoic acid, and nalidixic acid inhibited the medium chain a cyl-CoA synthetase activity for hexanoic acid, with K-i values of 0.8, 44, and 12.3 mu M, respectively. The inhibitory carboxylic acids were competitive with respect to hexanoic acid. The hydroxyl or ketone (ox o) groups at the beta-position of carboxylic acids were an important d eterminant for inhibitory activity. All substrates and inhibitors cont ained a flat hydrophobic region coplanar to the carboxylate group. In addition, the substrates had negative values for charge on the carbon in the beta-position of carboxylic acids. (C) 1998 Elsevier Science In c.