We have studied the secondary structure of mitochondrial F1 using infrared spectroscopy. Our results show that in the absence of added nucleotides this complex contains similar percentages of α-helices, β-structures and reverse turns (30%, 28% and 31%, respectively). The influence of ADP and ATP on the different types of secondary structure was determined; when all the nucleotide-binding sites were occupied, small but reproducible changes were observed, corresponding to a decrease in β-structure and an increase in α-helix and reverse turns. The effect of nucleotide binding on the thermal stability of F1 was also studied; the thermal denaturation temperature, 55°C, was increased by 11°C and 7°C by ATP and ADP, respectively. These results indicate that nucleotide binding affects the secondary structure of F1, stabilizing the complex.
