Investigation of quercetin binding sites on chloroplast coupling factor 1.

The quercetin binding sites on spinach chloroplast coupling factor 1 (CF1) have been investigated using direct and competitive binding, stopped-flow, temperature-jump, and fluorescence resonance energy transfer measurements. It was found that 8-anilino-1-naphthalensulfonic acid (ANS) competes with quercetin binding at two sites on the solubilized enzyme which are distinct from the two tight nucleotide binding sites and the 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) reactive site. The bimolecular association of quercetin with CF1 is too fast to measure directly and is followed by two slower conformational changes. The distances from the tight nucleotide sites to the quercetin-ANS sites were estimated as 40-48 A by fluorescence resonance energy transfer using 1,N6-ethenoadenosine diphosphate and 1,N6-ethenoadenylyl imidodiphosphate as donors and quercetin as the acceptor. The distance from the quercetin-ANS site to the NBD-C1 reactive site was found to be about 30 A using ANS as a donor and NBD-C1 reacted with a tyrosine group on CF1 as the energy acceptor. A model is proposed for the relative location of these sites on CF1.