Biophysical and Biochemical Properties of Flavocytochrome

The reduction potentials for the heme and FMN prosthetic groups of flavocytochromes 62 from S. cerevisiae and H. anomala are listed in Table I. Values for various modified forms of the enzyme, such as the flavin-free (deflavo) derivative, and the isolated cytochrome domain (the cytochrome 62 core) are also reported in Table I (64-69). The reduction potentials for the heme group are as expected for a 65-type cytochrome (70), with little difference in the values for different forms of protein, e.g., the deflavo-derivative of the holoenzyme and the isolated cytochrome 62. The reduction potentials for the FMN group are not too different from those of the heme (about 50 mV difference), consistent with reversible electron transfer between the two prosthetic groups (10). 

In the H. anomala enzyme the monoelectronic reduction potentials for the FMN (F) group are altered in the presence of pyruvate (58, 59, 71) to such an extent that the value for Fox/Fsq lies above that of the heme (Table I). This effectively means that flavosemiquinone heme 

Reduction Potentials for Prosthetic Groups in Various Forms of Flavocytochrome h-f 

Abbreviations Sx, the cleaved form of flavocytochrome 62 from S. cerevisiae deflavo-Sx, S , in which the FMN prosthetic group has been removed Sx-62-core, the isolated cytochrome domain (or core) of Sx Hj, the intact flavocytochrome 62 from H. anomala deflavo-Hi, Hi in which FMN has been removed and Hi-62-core, the isolated heme domain (or core) of Hi. 

Electronic absorption spectra of flavocytochrome 62 were first reported in 1942 (72). Since then, visible absorption has been an important tool for measuring enzyme concentration. The electronic absorption spectra for S. cerevisiae flavocytochrome 62 in both oxidized and reduced states are shown in Fig. 7. The sharp peaks at 557, 528, and 423 nm (reduced) are characteristic of a 6-type cytochrome. The ratio of the absorbances at 269 and 423 nm (reduced) provides a useful indication of enzyme purity (20). The absorption coefficients generally accepted as most accurate are listed in Table II. There are no significant 

---