The enzyme mechanism of the multicopper oxidase (MCO) SLAC from Streptomyces coelicolor was investigated by structural (XRD), spectroscopic (optical, EPR), and kinetics (stopped-flow) experiments on variants in which residue Tyr108 had been replaced by Phe or Ala through site-directed mutagenesis. Contrary to the more common three-domain MCOs, a tyrosine in the two-domain SLAC is found to participate in the enzyme mechanism by providing an electron during oxygen reduction, giving rise to the temporary appearance of a tyrosyl radical. The relatively low k(cat)/K-M of SLAC and the involvement of Y108 in the enzyme mechanism may reflect an adaptation to a milieu in which there is an imbalance between the available reducing and oxidizing co-substrates. The purported evolutionary relationship between the two-domain MCOs and human ceruloplasmin appears to extend not only to the 3D structure and the mode of binding of the Cu's in the trinuclear center, as noted before, but also to the enzyme mechanism.