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.