Studies on Two Convergently Evolved Cysteate Synthases in Sulfonolipid Biosynthesis

Capnine-like sulfonolipids are sulfonate-containing analogs
of
sphingolipids found in many Bacteroidetes bacteria, where they govern
essential functions such as gliding motility, outer membrane polysaccharide
assembly, and antibiotic susceptibility. In gut-associated anaerobic
Bacteroidetes, these sulfonolipids also modulate host–microbe
interactions. In aerobic bacteria, the capnine precursor cysteate
is produced by a pyridoxal phosphate (PLP)-dependent cysteate synthase
(CapA1), a close homologue of cystathionine β-synthase (CBS).
By contrast, the mechanism of cysteate production in anaerobic Bacteroidetes
bacteria has not been biochemically studied. Herein, we report the
characterizations of archaeal cysteate synthase homologue from the
anaerobic bacteria Alistipes finegoldii (AfCapA2). Biochemical assays confirm its ability
to catalyze the conversion of O-phosphoserine (OPS)
to cysteate. Crystal structures of AfCapA2 in complex
with PLP and OPS-PLP identify essential catalytic residues and reveal
a structural similarity to threonine synthase, unlike CapA1, which
is more similar to CBS. Comparative analysis of CapA1 and this nonorthologous
CapA2, including structural differences, catalytic versatility, and
phylogenetic distribution across Bacteroidetes, suggests convergent
evolution of cysteate synthase activity. Our work clarifies the details
of sulfonolipid synthesis in anaerobic bacteria and the biochemical
origins of this structurally distinctive lipid in the gut microbiome.