文摘
The lipopeptidyl nucleoside antibiotics represented by A-90289, caprazamycin, and muraymycin are structurally highlighted by a nucleoside core that contains a nonproteinogenic 尾-hydroxy-伪-amino acid named 5鈥?C-glycyluridine (GlyU). Bioinformatic analysis of the biosynthetic gene clusters revealed a shared open reading frame encoding a protein with sequence similarity to serine hydroxymethyltransferases, resulting in the proposal that this shared enzyme catalyzes an aldol-type condensation with glycine and uridine-5鈥?aldehyde to furnish GlyU. Using LipK involved in A-90289 biosynthesis as a model, we now functionally assign and characterize the enzyme responsible for the C鈥揅 bond-forming event during GlyU biosynthesis as an l-threonine:uridine-5鈥?aldehyde transaldolase. Biochemical analysis revealed this transformation is dependent upon pyridoxal-5鈥?phosphate, the enzyme has no activity with alternative amino acids, such as glycine or serine, as aldol donors, and acetaldehyde is a coproduct. Structural characterization of the enzyme product is consistent with stereochemical assignment as the threo diastereomer (5鈥?i>S,6鈥?i>S)-GlyU. Thus this enzyme orchestrates C鈥揅 bond breaking and formation with concomitant installation of two stereocenters to make a new l-伪-amino acid with a nucleoside side chain.