Activation of the corrinoid [Fe-S] protein (CoFeSP), involved in reductive CO(2) conversion, requires the reduction of the Co(II) center by the [Fe-S] protein RACo, which according to the reduction potentials of the two proteins would correspond to an uphill electron transfer. In our resonance Raman spectroscopic work, we demonstrate that, as a conformational gate for the corrinoid reduction, complex formation of Co(II)FeSP and RACo specifically alters the structure of the corrinoid cofactor by modifying the interactions of the Co(II) center with the axial ligand. On the basis of various deletion mutants, the potential interaction domains on the partner proteins can be predicted.