Flavocytochrome electron transfer pathway

Flavin mononucleotide, 3absorption coefficients, 36 270 active site, 36 265-267 catalysis and electron transfer, 36 275-287 carbanion mechanism, 36 277-282 electron acceptors, 36 285-287 electron transfer pathway, 36 275-276, 282-285... 

In presence of TNS, reduction of cytochrome c occurs in the range of minutes instead of seconds, as it is the case for the reduction with flavocytochrome b2 or isolated cytochrome b2 core. Thus, the relative distance and orientation of the FMN, TNS and heme planes induces an electron transfer pathway different from that known for the cytochrome b2 - cytochrome c electron transfer. This clearly shows the importance of cytochrome b2 core in the electron transfer to cytochrome c. 

The physiological pathway of electron transfer in flavocytochrome is from bound lactate to FMN, then FMN to 52-heme, and finally 52-heme to cytochrome c (Fig. 9) (2,11, 80,102). The first step, oxidation of L-lactate to pyruvate with concomitant electron transfer to FMN, is the slowest step in the enzyme turnover (103). With the enzyme from S. cerevisiae, a steady-state kinetic isotope effect (with ferricyanide as electron acceptor) of around 5 was obtained for the oxidation of dl-lactate deuterated at the C position, consistent with the major ratedetermining step being cleavage of the C -H bond (103). Flavocytochrome 52 reduction by [2- H]lactate measured by stopped-flow spectrophotometry resulted in isotope effects of 8 and 6 for flavin and heme reduction, respectively, indicating that C -H bond cleavage is not totally rate limiting (104). 

Fig 9. The physiological pathway of electron transfer in flavocytochrome 62. Fox, Oxidized FMN F, flavosemiquinone F,ed, reduced FMN H x, oxidized heme Hrej, reduced heme Cyt c, cytochrome c. 

This leads us to the following conclusion the flavodehydrogenase domain in the native flavocytochrome hi exerts very little influence on the efficiency of the electron transfer between cytochrome b2 and cytochrome c within the complex. This result implies that similar transition states must be achieved in the reactions of cytochrome c with both free and bound cytochrome b2 Consequently, the electron transfer occurs via the same mechanism involving the same pathway through the intra-molecular reaction complex. Thus, in both flavocytochrome b2 - cyt.c and cyt.b2 core - cyt.c complexes, the relative distance and orientation of the heme b2 and heme c planes are optimal to achieve the electron-transfer process. 

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