In this thesis, protein-protein-interactions in the formate metabolism of Escherichia coli were identified and structurally characterized by mass spectrometry (MS)-based methods. One aim was to shed light on the interactions of Fdh-N and Fdh-O with proteins of the cell wall. For four identified Fdh-binding partners, namely the chaperone Skp, the L, D-transpeptidase ErfK, the oligopeptide binding protein OppA, and the chaperone TolB, interface regions within the Fdh/protein complexes were mapped. Detailed structural and functional insights into the mechanisms that support the post-maturation of the multi-subunit enzymes Fdh-N and Fdh-O in the periplasm of E. coli were obtained. Another part of this work deals with the structural characterization of the FocA/PflB complex and its regulatory effect on formate transport.