UV optical absorption by protein radicals in cytochrome c oxidase.

The UV properties of key oxygen intermediates of cytochrome c oxidase have been investigated by transient absorption spectroscopy. The temporal behavior of P(m) species upon aerobic incubation with CO or in the reaction with H(2)O(2) is closely concurred by a new optical shift at 290/260 nm. In the acid-induced conversion of P(m) to F(*), it is replaced by another shift at 323/288 nm. The wavelength and intensity of the UV signal observed in F(*) match closely the properties of model Trp? in agreement with results of ENDOR studies on this species. The UV spectrum of Tyr* gives the closest match with the 290/260 nm signal observed in P(m). On the basis of analysis of possible UV chromophores in CcO and similarity to Tyr*, the 290/260 nm signal is proposed to originate from the H(240)-Y(244)* site. Possible effects of local environment on UV properties of this site are discussed.