Protein-protein interaction within multi-protein complexes from Paracoccus versutus and Paracoccus denitrificans:characterization of the recognition surfaces by high resolution NMR and X-ray crystallography

Methylamine can be used as the sole carbon source of certain methylotrophic bacteria, as Paracoccus versutus and Paracoccus denitrificans. Methylamine dehydrogenase (MADH) catalyses the conversion of methylamine into formaldehyde and donates electrons to the electron transfer protein amicyanin. The crystal structure of the complex of MADH and amicyanin from P. versutus was determined and the rate of electron transfer from the tryptophan tryptophylquinone cofactor of MADH to the copper ion of amicyanin in solution was determined. In general the kinetics are similar to those previously observed for the P. denitrificans system. The complex in solution was studied using nuclear magnetic resonance, observing the signals of perdeuterated, 15N-enriched amicyanin bound to MADH, for both the two systems. Chemical shift perturbation analysis show that amicyanin assumes a well-defined positions in the complexes in solution. The most affected residues are in the interfaces observed in the crystal structures, while smaller perturbations extend to larger areas and can be correlated to secondary effects. For the P. denitrificans system also the interaction between amicyanin and its supposed redox partner, cytochrome c-551i, was studied in solution by high-resolution NMR. The results clearly indicate the formation of a binary complex with amicyanin, while no interactions were observed when amicyanin is in complex with MADH.