A motif for quinone binding sites in respiratory and photosynthetic systems

Many of the membrane-bound protein complexes of respiratory and photosynthe tic systems are reactive with quinones. To date, no clear structural relati onship between sites that bind quinone has been defined, apart from that in the homologous family of "type II" photosynthetic reaction centres. We sho w here that a structural element containing a weak sequence motif is common to the Q(A) and Q(B) sites of bacterial reaction centres and the Q(i) site of the mitochondrial bc(1) complex. Analyses of sequence databases indicat e that this element may also be present in the PsaA/B subunits of photosyst em I, in the ND4 and ND5 subunits of complex I and, possibly, in the mitoch ondrial alternative quinol oxidase. This represents a first step in the str uctural classification of quinone binding sites. (C) 2000 Academic Press.