ASPARTIC PROTEINASES - FOURIER-TRANSFORM INFRARED SPECTROSCOPIC STUDIES OF A MODEL OF THE ACTIVE SIDE

We synthesized and studied by Fourier transform infrared spectroscopy nine monosalts of diamides as models for the active side of aspartic p roteinases. One compound, the monosalt of meta-aminobenzoic acid diami de of fumaric acid (m-FUM), shows the same biological activity as peps in with regard to the splitting of peptide bonds of the Pro-Thr-Glu-Ph e-Phe(4-NO2)-Arg-Leu heptapeptide. The monosalt of m-FUM forms with ox indole a complex in which the carboxylic acid group of the monosalt of m-FUM is strongly hydrogen bonded with the O atom of the peptide bond of oxindole. When one water molecule is added to this complex, the st rong field of the caboxylate group destabilizes an O-H bond of the wat er molecule. The distorted water molecule attacks the carbon atom of t he peptide group, and the water proton transfers to the peptide N atom . Simultaneously, the C-N bond of the amide group is broken. Hence it is demonstrated that the catalytic mechanism of aspartic acid proteina ses is a base catalysis. The results show that for this catalytic mech anism there are sufficient carboxylic and carboxylate groups, as well as a water molecule in the correct arrangement. It was also demonstrat ed with other monosalts of dicarboxylic acids that well-defined steric conditions of the carboxylic acid and the carboxylate group must be f ulfilled to show hydrolytic activity with regard to oxindole molecules .