| Summary: | 碩士 === 國立中興大學 === 生物化學研究所 === 97 === Pyrroloquinoline quinone (4,5-dihydro-4,5-dioxo-1H-pyrrolo-[2,3-f]quinoline- 2,7,9-tricarboxylic acid, PQQ) is a novel heat-stable, water soluble redox cofactor present in a variety of bacterial alcohol and aldose dehydrogenases. It belongs to the third family of ortho-quinone cofactor following the well-known pyridine nucleotide- (such as NAD) and flavin-dependent (such as FAD) cofactors. Discovery of PQQ has elicited considerable interests due to its role as a growth-promoting factor both in animals and plants. Although the structures and functions of many enzymes using PQQ as a cofactor has been elucidated, relatively little is known about the pathway leading to the biosynthesis of this intriguing cofactor. Recently six genes belonging to the pqqABCDEF operon were found to be essential for the biosynthesis of PQQ, and the biosynthesis of PQQ were found to involve many unique synthesis steps. Structural studies of these genes are necessary to elucidate the synthesis mechanism. However, until now, only the structures of PqqB and PqqC have been determined.We now report the tertiary structure of PqqD from a plant pathogen Xanthomonas campestris determined to a resolution of 1.66 Å using X-ay crystallography. The crystal structure of XcPqqD was found to adopt a dimer of novel saddle-like fold that reveal considerable differences to the structure predicted de novo and carried out in the absence of suitable template. In addition, a phosphate ligand was found to be situated very well in the highly positively-charged central channel. Docking study of PqqD with PQQ also located PQQ at the similar phosphate binding site, indicating that PqqD can possibly bind PQQ and serve as a PQQ carrier.
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