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Mechanism of pyranopterin ring formation in molybdenum cofactor biosynthesis

The molybdenum cofactor (Moco) is essential for all kingdoms of life, plays central roles in various biological processes, and must be biosynthesized de novo. During Moco biosynthesis, the characteristic pyranopterin ring is constructed by a complex rearrangement of guanosine 5′-triphosphate (GTP) i...

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Published in:Proceedings of the National Academy of Sciences - PNAS 2015-05, Vol.112 (20), p.6347-6352
Main Authors: Hover, Bradley M., Tonthat, Nam K., Schumacher, Maria A., Yokoyama, Kenichi
Format: Article
Language:English
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Summary:The molybdenum cofactor (Moco) is essential for all kingdoms of life, plays central roles in various biological processes, and must be biosynthesized de novo. During Moco biosynthesis, the characteristic pyranopterin ring is constructed by a complex rearrangement of guanosine 5′-triphosphate (GTP) into cyclic pyranopterin (cPMP) through the action of two enzymes, MoaA and MoaC (molybdenum cofactor biosynthesis protein A and C, respectively). Conventionally, MoaA was considered to catalyze the majority of this transformation, with MoaC playing little or no role in the pyranopterin formation. Recently, this view was challenged by the isolation of 3′,8-cyclo-7,8-dihydro-guanosine 5′-triphosphate (3′,8-cH ₂GTP) as the product of in vitro MoaA reactions. To elucidate the mechanism of formation of Moco pyranopterin backbone, we performed biochemical characterization of 3′,8-cH ₂GTP and functional and X-ray crystallographic characterizations of MoaC. These studies revealed that 3′,8-cH ₂GTP is the only product of MoaA that can be converted to cPMP by MoaC. Our structural studies captured the specific binding of 3′,8-cH ₂GTP in the active site of MoaC. These observations provided strong evidence that the physiological function of MoaA is the conversion of GTP to 3′,8-cH ₂GTP (GTP 3′,8-cyclase), and that of MoaC is to catalyze the rearrangement of 3′,8-cH ₂GTP into cPMP (cPMP synthase). Furthermore, our structure-guided studies suggest that MoaC catalysis involves the dynamic motions of enzyme active-site loops as a way to control the timing of interaction between the reaction intermediates and catalytically essential amino acid residues. Thus, these results reveal the previously unidentified mechanism behind Moco biosynthesis and provide mechanistic and structural insights into how enzymes catalyze complex rearrangement reactions. Significance The molybdenum cofactor (Moco) is an enzyme cofactor critical for the survival of almost all organisms from all kingdoms of life, and its biosynthesis is associated with various medical conditions such as inheritable human diseases and bacterial pathogenesis. The characteristic pyranopterin backbone of Moco is formed by the action of two enzymes, MoaA and MoaC (molybdenum cofactor biosynthesis protein A and C, respectively). Conventionally, MoaA was considered responsible for the majority of the transformation. In contrast to this view, the combined studies reported here revealed that it is MoaC that is responsible for the m
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1500697112