<p>The molybdenum cofactor (Moco) is an essential enzyme cofactor found in all kingdoms of life. Moco plays central roles in many vital biological processes, and must be biosynthesized de novo. During its biosynthesis, the characteristic pyranopterin ring of Moco is constructed by a complex rearrangement of guanosine 5'-­triphosphate (GTP) into cyclic pyranopterin (cPMP) through the action of two enzymes, MoaA and MoaC. However, the mechanisms and the functions of the two enzymes are under significant debate. To elucidate their physiological roles, I took a multidisciplinary approach to functionally characterize MoaA and MoaC in vivo and in vitro. In this dissertation, I report the first isolation and characterization of the physiological MoaC substrate, 3',8-­ cyclo-­7,8-­dihydro-­guanosine 5'-triphosphate (3',8-cH2GTP). I also report the first X-­ray crystal structures of MoaC in complex with this highly air sensitive substrate, and its product cPMP. These studies, combined with in vitro experiments using substrate analogs, catalytically impaired mutants, and synthetic peptides, have enabled me to delineate the functions of the Moco biosynthetic enzymes, MoaA and MoaC, and proposed mechanistic models for their roles in the formation of cPMP.</p> / Dissertation