The stereospecificity of bacterial external flavoprotein monooxygenases for nicotinamide adenine dinucleotide

Abstract

It is known that orcinol hydroxylase shows A-stereospecificity for nicotinamide adenine dinucleotide when the enzyme reaction involves the true substrate, orcinol, but when the reaction is carried out in the presence of pseudosubstrates, this enzyme shows an isotope dependent mixed-type Stereospecificity, the degree of which depends on the uncoupling activity of the employed pseudosubstrate [Ribbons, D. W., Ohta, Y. and Higgins, I. J. (1972) in Miami Winter Symposium, The Molecular Basis of Electron Transport (Schultz, J., and Cameron, B. R., eds.), Vol. 4, pp. 251–274, Academic Press, New York]. In this report, the nicotinamide nucleotide stereospecificity of other external flavoprotein monooxygenases from bacterial sources is presented. All of the monooxygenases examined, namely, melilotate hydroxylase, m-hydroxybenzoate-6-hydroxylase, resorcinol hydroxylase, and salicylate hydroxylase, invariably show A-stereospecificity, as in the case of orcinol hydroxylase with true substrate. Unlike orcinol hydroxylase, however, the stereospecificity of salicylate hydroxylase remains A-stereospecific even with pure pseudosubstrate (i.e., benzoate) regardless of the position of the deuterium at the dihydronicotinamide 4-position. On the basis of the information obtained in this investigation, a general rule pertaining to the stereospecificity of nicotinamide nucleotide enzymes is proposed that all external flavoprotein monooxygenases have A-stereospecificity with respect to nicotinamide adenine dinucleotide.