J. Santolini et al., INTERRELATION BETWEEN HIGH AND LOW-AFFINITY TENTOXIN BINDING-SITES INCHLOROPLAST F1-ATPASE REVEALED BY SYNTHETIC ANALOGS, The Journal of biological chemistry, 273(6), 1998, pp. 3343-3350
Eight synthetic analogues of tentoxin (cyclo-(L-N-Me-Glu(1)-L-Leu(2)-N
-Me Delta(z)Phe(3)-Gly(4))) modified in residues 1, 2, and 3 were chec
ked for their ability to inhibit and reactivate the ATPase activity of
the activated soluble part of chloroplast ATP synthase. The data were
consistent with a model involving two binding sites of different affi
nities for the toxins. The occupancy of the high affinity site (or tig
ht site) gave rise to an inactive complex, whereas filling both sites
(tight + loose) gave rise to a complex of variable activity, dependent
on the toxin analogue. Competition experiments between tentoxin and n
onreactivating analogues allowed discrimination between the absence of
binding and a nonproductive binding to the site of lower affinity (or
loose site). The affinity for the loose site was not affected signifi
cantly by the modifications of the tentoxin molecule, whereas the affi
nity for the tight site was found notably changed. Increasing the size
of side chain 1 or 2 and introducing a net electrical charge both res
ulted in a decrease of affinity for the tight site, but the second cha
nge dominated the first one. The activity of different ternary complex
es enzyme-tentoxin-analogue depended on the nature of the toxin bound
on each site and not only on that bound on the loose site. This demons
trates that the reactivation process results from an interaction, dire
ct or not, between these two binding sites. Possible molecular mechani
sms are discussed.