Iron-sulfur centers TMADH

The electronic coupling has been calculated for each of the flavin electron transfer complexes described in this chapter and is described below, with the exceptions of CPR, TMADH and PDR. For these three proteins, the two redox cofactors are in direct van der Waals contact, either between the C-7 and C-8 methyl groups of two flavins (CPR), or between the flavin C-8 methyl and a cysteinyl sulfur ligand to the iron-sulfur center (TMADH and PDR). In these cases the coupling between the redox centers should be maximal and the electron transfer rates should depend only on the driving foree and reorganization energy for the electron transfer proeesses. 

Like PDR, trimethylamine dehydrogenase (TMADH) from Methylo-philus methylotrophus W3A1 provides an example of a system in which an iron-sulfur center, in this case a [4Fe-4S] cluster, interacts... 

Although at pH 8 the electron distribution favours the formation of flavin semiquinone and reduced iron-sulfur center, the magnetic moments of the two redox centers do not interact. At pH 10, however, 2-electron-reduced TMADH exhibits the EPR spectrum diagnostic of the spin-mteracting state. In a more detailed analysis using the pH-jump technique, the interconversion of three states of TMADH [state 1, dihy-droflavin-oxidised 4Fe-4S center (formed at pH 6) state 2, flavin semi-quinone-reduced 4Fe-4S center (formed at pH 8) state 3, spin interacting state (formed at pH 10)] were studied in both H2O and D2O (Rohlfs et al., 1995). The kinetics were found to be consistent with a reaction mechanism that involves sequential protonation/deprotonation and electron transfer events (Figure 6). Normal solvent kinetic isotope effects were observed and proton inventory analysis revealed that at least one proton is involved in the reaction between pH 6 and 8 and at least two protons are involved between pH 8 and 10. At least three protonation/... 

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