Role of a strictly conserved active site tyrosine in cofactor genesis in the copper amine oxidase from Hansenula...

The copper amine oxidases catalyze the O(2)-dependent, two-electron oxidation of amines to aldehydes at an active site that contains Cu(II) and topaquinone (TPQ) cofactor. TPQ arises from the autocatalytic, post-translational oxidation of a tyrosine side chain within the same active site. The contributions of individual active site amino acids to each of these chemical processes are being delineated. Previously, using the amine oxidase from the yeast Hansenula polymorpha (HPAO), mutations of a strictly conserved and structurally pivotal active site tyrosine (Y305) were studied and their effects on the catalytic cycle demonstrated [Hevel, J. M., Mills, S. A., and Klinman, J. P. (1999) Biochemistry 38, 3683-3693]. This study examines mutations at the same position for their effects on cofactor generation. While the Y305A mutation had moderate effects on the kinetics of catalysis (2.5- and 8-fold effects on k(cat) using ethylamine and benzylamine as substrates), the same mutation slows cofactor formation by approximately 45-fold relative to that of the wild-type (WT). Additionally, the Y305A mutant forms at least two species: primarily TPQ at lower pH and a species with a blue-shifted absorbance at high pH (lambda(max) = 400 nm). The 400 nm species does not react with phenylhydrazine or ethylamine and is stable toward pH buffer exchange, long-term storage (>3 weeks), incubation at high temperatures, or incubation with reductants and colorimetric peroxide quenching reagents. A similar species accumulates appreciably even at approximately neutral pH in the Y305F mutant, despite the fact that the rate of TPQ formation is reduced only 3-fold relative to that of WT HPAO. This small impact of Y305F on the rate of biogenesis contracts with a decrease in k(cat) (using ethylamine as the substrate) of 125-fold. The opposing effects of mutations at position 305 in biogenesis versus catalysis indicate that a single residue can be recruited for different roles during these processes.