A study of the mitochondrial F1-ATPase tryptophan phosphorescence at 273 K

The bovine heart mitochondrial F1-ATPase complex exhibits an intrinsic tryptophan phosphorescence that can be used to monitor structural changes of the ?-subunit. The phosphorescence decay rate of F1 containing the tightly bound nucleotides increases upon addition of adenine nucleoside triphosphate in the presence of magnesium. The average phosphorescence lifetime of this enzyme preparation decreases from 10.2 to 7.8 ms upon Mg-ATP addition. Since increasing phosphorescence decay rate is related to increasing flexibility of proteins, Mg-ATP added to the F1-ATPase complex can enhance the flexibility of the protein structure surrounding the chromophore. Experiments carried out on F1 prepared with the three noncatalytic sites filled and the three catalytic sites vacant show a significant increase of the phosphorescence lifetime from 6.4 ms to 7.6 ms upon Mg-ATP addition. These results suggest that the mitochondrial F1-ATPase ?-subunit conformation senses differently the nucleoside triphosphate binding to catalytic or noncatalytic sites.