Amalgamation of Nucleosides and Amino Acids in Antibiotic Biosynthesis

The rapid increase in antibiotic resistance demands the identification of novel antibiotics with novel targets. One potential antibacterial target is the biosynthesis of peptidoglycan cell wall, which is both ubiquitous and necessary for bacterial survival. Both the caprazamycin-related compounds A-90289 and muraminomicin, as well as the capuramycin-related compounds A-503083 and A-102395 are potent inhibitors of the translocase I enzyme, one of the key enzymes required for cell wall biosynthesis. The caprazamycin-related compounds contain a core nonproteinogen b-hydroxy-a-amino acid referred to as 5’-C-glycyluridine (GlyU). Residing within the biosynthetic gene clusters of the aforementioned compounds is a shared open reading frame which encodes a putative serine hydroxymethyltransferase (SHMT). The revelation of this shared open reading frame resulted in the proposal that this putative SHMT catalyzes an aldol-type condensation reaction utilizing glycine and uridine-5’-aldehyde, resulting in the GlyU core. The enzyme LipK involved in A-90289 biosynthesis was used as a model to functionally assign this putative SHMT to reveal its functions as an l-threonine: uridine-5’-aldehyde transaldolases. Biochemical analysis indicates enzymatic activity is dependent upon pyridoxal-5’-phosphate, is non-reactive with alternative amino acids, and produces acetaldehyde as a co-product. Structural characterization of the enzymatic product is consistent with (5’S,6’S)-GlyU indicating that this enzyme orchestrates a C-C bond breaking and formation resulting in two new stereocenters to make a new l-a-amino acid. The same activity was demonstrated for the LipK homologues involved in the biosynthesis of muraminomicin, A-503083, and A-102395. This l-threonine: uridine-5’-aldehyde transaldolase was used with alternative aldehyde substrates to prepare unusual l-a-amino acids, suggesting the potential for exploiting this enzyme to make new compounds