Evidence by NMR for mobility of the cytochrome domain within flavocytochrome b2

According to a model proposed by Gervais, M, Groudinsky, O., Risler, Y. and Labeyrie, F. ((1977) Biochem. Biophys. Res. Commun. 77, 1543-1551) flavocytochrome b2 is composed of a central flavodehydrogenase entity of 4 X 45 kDa to which are attached four cytochrome b2 globules of approx. 11 kDa that are released after proteolysis of the connective loops. A possible inherent mobility of the latter with functional significance was suspected. Proton NMR spectra at 400 MHz of the isolated and of the flavodehydrogenase-bound ferricytochrome b2 units have been compared. In the ranges downfield of +12 ppm and upfield from -4 ppm, where hyperfine-shifted heme proton resonances reside, the chemical shifts are identical for the two forms, but the linewidths are markedly broader for flavocytochrome b2. The linewidths of three heme resonances, a methyl at +19 ppm, two single protons at -6 and -8 ppm (most probably from one vinyl) and an unassigned line at -2.4 ppm, all increase by a factor of about 4. Since, in the present case, linewidths are controlled mainly by proton/proton dipolar relaxations which are caused by molecular tumbling, a change in linewidths of about 15 would be expected if the cytochrome b2 globule had no free motion relative to the flavodehydrogenase domain. The present results thus support the previous hypothesis that such a relative mobility, of unknown correlation time and amplitude, actually exists.