Mechanism of a Standalone ß-Lactone Synthetase: New Continuous Assay for a Widespread ANL Superfamily Enzyme.

Enzyme-catalyzed ß-lactone formation from ß-hydroxy acids is a crucial step in bacterial biosynthesis of ß-lactone natural products and membrane hydrocarbons. We developed a novel, continuous assay for ß-lactone synthetase activity using synthetic ß-hydroxy acid substrates with alkene or alkyne moieties. ß-Lactone formation is followed by rapid decarboxylation to form a conjugated triene chromophore for real-time evaluation by UV/Vis spectroscopy. The assay was used to determine steady-state kinetics of a long-chain ß-lactone synthetase, OleC, from the plant pathogen Xanthomonas campestris. Site-directed mutagenesis was used to test the involvement of conserved active site residues in Mg2+ and ATP binding. A previous report suggested OleC adenylated the substrate hydroxy group. Here we present several lines of evidence, including hydroxylamine trapping of the AMP intermediate, to demonstrate the substrate carboxyl group is adenylated prior to making the ß-lactone final product. A panel of nine substrate analogues were used to investigate the substrate specificity of X. campestris OleC by HPLC and GC-MS. Stereoisomers of 2-hexyl-3hydroxyoctanoic acid were synthesized and OleC preferred the (2R,3S) diastereomer consistent with the stereo-preference of upstream and downstream pathway enzymes. This biochemical knowledge was used to guide phylogenetic analysis of the ß-lactone synthetases to map their functional diversity within the acyl-CoA synthetase, NRPS adenylation domain, and luciferase superfamily.

Bacterial amylases enable glycogen degradation by the vaginal microbiome.

The human vaginal microbiota is frequently dominated by lactobacilli and transition to a more diverse community of anaerobic microbes is associated with health risks. Glycogen released by lysed epithelial cells is believed to be an important nutrient source in the vagina. However, the mechanism by which vaginal bacteria metabolize glycogen is unclear, with evidence implicating both bacterial and human enzymes. Here we biochemically characterize six glycogen-degrading enzymes (GDEs), all of which are pullanases (PulA homologues), from vaginal bacteria that support the growth of amylase-deficient Lactobacillus crispatus on glycogen. We reveal variations in their pH tolerance, substrate preferences, breakdown products and susceptibility to inhibition. Analysis of vaginal microbiome datasets shows that these enzymes are expressed in all community state types. Finally, we confirm the presence and activity of bacterial and human GDEs in cervicovaginal fluid. This work establishes that bacterial GDEs can participate in the breakdown of glycogen, providing insight into metabolism that may shape the vaginal microbiota.