INTRAMOLECULAR ELECTRON-TRANSFER RATE BETWEEN ACTIVE-SITE COPPER AND TOPA QUINONE IN PEA SEEDLING AMINE OXIDASE

The equilibrium between the two substrate-reduced forms of pea seedling amine oxidase, one containing Cu(II) and reduced 3-(2,4,5-trihydroxyphenyl)-L-alanine (topa) cofactor and one containing Cu(I) and topa semiquinone, was investigated by visible spectroscopy as a function of temperature. To determine the rate of interconversion between the two species, temperature jump relaxation studies were performed on the substrate-reduced enzyme near room temperature. The yellow radical species was found to approach its equilibrium concentration with a maximum rate constant of 43,000 +/- 3,000 s-1. This rapid equilibration is attributed to intramolecular electron transfer between copper and topa. The data indicate that the Cu(I)/topa(SQ) species is a kinetically competent intermediate in the reaction of amine oxidases with substrates. Furthermore, the extremely rapid electron transfer rate (k(ET) congruent-to 20,000 s-1) suggests that the topa cofactor is in close proximity to the copper atom.