We describe the NMR structure of DsbB, a polytopic helical membrane protein. DsbB, a bacterial cytoplasmic membrane protein, plays a key role in disulfide bond formation. It reoxidizes DsbA, the periplasmic protein disulfide oxidant, using the oxidizing power of membrane-embedded quinones. We determined the structure of an interloop disulfide bond form of DsbB, an intermediate in catalysis. Analysis of the structure and interactions with substrates DsbA and quinone reveals functionally relevant changes induced by these substrates. Analysis of the structure, dynamics measurements, and NMR chemical shifts around the interloop disulfide bond suggest how electron movement from DsbA to quinone through DsbB is regulated and facilitated. Our results demonstrate the extraordinary utility of NMR for functional characterization of polytopic integral membrane proteins and provide insights into the mechanism of DsbB catalysis.