Macromolecular crystallography allows for the determination of structures to an atomic resolution and can provide important insight into the function and mechanism of enzymes. Including enzymes involved in interesting reactions and complex formations. In Wolinella succinogenes a sulfur carrier protein pathway was discovered to serve as the sulfur source for methionine biosynthesis. During this process, the sulfur carrier protein HcyS must form complexes with a variety of proteins to accomplish the C-terminal thiocarboxylate formation and the biosynthesis of methionine. This ability to form the different complex interactions makes the pathway fascinating in its structural characteristics. Additionally, the methionine pathway has a novel sulfur source when compared to other sulfur carrier protein pathways. The sulfur for the thiocarboxylate is supplied by ferredoxin sulfite reductase (FSR). The structure of FSR has been solved and reveals a more complex function than simply reducing sulfite to sulfide. The presence of a cysteine in the active site suggests the FSR may also serve as a sulfurtransferase to control the formation of the sulfur carrier protein thiocarboxylate. The structure of FSR provides further insight into the formation of complexes by HcyS.