Secondary structure and the subunits of FqF

Owing to the instability of the isolated subunits of FqF, direct measurement of the secondary structure by circular dichroic spectroscopy or infrared spectroscopy has been limited to the subunits of TF, [30,78], For example, the secondary structures of TFj and its subunits are lost in the presence of 8 M urea, but restored on its removal [30]. The secondary structure has also been estimated from the amino acid sequence of EF, [11,21] by the method of Chou and Fassman [79]. Table 5.2 summarizes the contents of a-heUces and )8-sheets of subunits of TF, [30] and EF, [11]. Similar values were obtained for TF, and EF,. The CO-stretching vibrations of the a and subunits of TF, were 1648 cm and 1640 cm , respectively, indicating that the subunit has a greater amount of antiparallel )8-sheet structure than the subunit [80]. 

The a-helix contents of the 8 [81] and [82] subunits of EF, were also measured directly. The high a-heUx content of the subunit is consistent with the long Stokes radius of the subunit [81], which might be a stalk connecting the afiy complex with Fo- 

The validity of applying the method of Chou and Fassman [79] to the membrane proteins is still controversial, but the estimated values of the a, b and c subunits of EFq are also shown in Table 5.2. The b subunit is also very rich in a-helices, and for the most part hydrophilic residues, except for the 22 residues at the N terminus [11,21]. The distribution of the secondary structure in these Fq subunits has been described in detail [67]. As shown in Fig. 5.6, a Rossmann fold [83], an alternating structure of a-helices and -sheets, is found in the subunit y8 around residues 

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