Structural analysis of the pseudaminic acid synthase PseI from Campylobacter jejuni

Campylobacter jejuni PseI is a pseudaminic acid synthase that condenses the 2,4-diacetamido-2,4,6-trideoxy-l-altrose sugar (6-deoxy AltdiNAc) and phosphoenolpyruvate to generate pseudaminic acid, a sialic acid-like 9-carbon backbone α-keto sugar. Pseudaminic acid is conjugated to cell surface proteins and lipids and plays a key role in the mobility and virulence of C. jejuni and other pathogenic bacteria. To provide insights into the catalytic mechanism of PseI, we performed a structural study on PseI. PseI forms a two-domain structure and assembles into a domain-swapped homodimer. The PseI dimer has two cavities, each of which accommodates a metal ion using conserved histidine residues. A comparative analysis of structures and sequences suggests that the cavity of PseI functions as an active site that binds the 6-deoxy AltdiNAc and phosphoenolpyruvate substrates and mediates their condensation. Furthermore, we propose the substrate binding-induced structural rearrangement of PseI and predict 6-deoxy AltdiNAc recognition residues that are specific to PseI.