Studies on Two Convergently Evolved Cysteate Synthases in Sulfonolipid Biosynthesis

Area of Science:

• Microbiology
• Biochemistry
• Structural Biology

Background:

• Capnine-like sulfonolipids are vital for Bacteroidetes bacteria, impacting motility, outer membrane assembly, and host interactions.
• Cysteate, a precursor, is synthesized by CapA1 in aerobic bacteria, but the mechanism in anaerobic bacteria remains unknown.
• Understanding cysteate synthesis is crucial for deciphering sulfonolipid roles in the gut microbiome.

Purpose of the Study:

• To biochemically characterize the cysteate synthase from anaerobic bacteria *Alistipes finegoldii* (AfCapA2).
• To elucidate the structural and evolutionary relationship of AfCapA2 to other cysteate synthases.
• To clarify the mechanism of sulfonolipid synthesis in anaerobic gut bacteria.

Main Methods:

• Biochemical assays to test AfCapA2's enzymatic activity.
• X-ray crystallography to determine the structure of AfCapA2 with its substrate and cofactor.
• Comparative sequence and structural analysis with known cysteate synthases (CapA1) and related enzymes.

Main Results:

• AfCapA2 catalyzes the conversion of O-phosphoserine (OPS) to cysteate.
• Crystal structures reveal AfCapA2 is structurally similar to threonine synthase, unlike the CBS-like CapA1.
• Phylogenetic and structural comparisons suggest convergent evolution of cysteate synthase activity in Bacteroidetes.

Conclusions:

• The characterization of AfCapA2 provides the first biochemical insight into cysteate production in anaerobic bacteria.
• Structural and mechanistic differences between AfCapA2 and CapA1 highlight distinct evolutionary pathways for cysteate synthesis.
• This work deepens our understanding of sulfonolipid biosynthesis and its implications for gut microbiome function.