Whilst the mechanism employed by inverting sialyltransferases has been well studied, the retaining mechanism utilised by KpsS1 is an intriguing open question that is difficult to determine without in depth structural characterisation of the active site. However, the overall high per-residue confidence (pLDDT) scores in KpsS1's AlphaFold model make it a suitable foundation from which to base preliminary mechanistic hypotheses. This model predicts KpsS1 adopts a GT-B fold comprised of two β/α/β Rossmann domains linked by a flexible αhelix rich region (Figure ). A DALI search against the PDB using this model returned two retaining β-Kdo transferases, KpsC from E. coli (GT107 family), and WbbB from Raoultella terrigena (GT99 family). Structural studies of both KpsC and WbbB suggest that an internal return SNi mechanism, often proposed for retaining GTs, is unlikely for these enzymes due to narrow active sites. Moreover the presence of an indispensable Asp residue in a conserved QXXXD motif in the active site of both enzymes, raised the possibility of a double displacement mechanism, a hypothesis recently reinforced by trapping of mutant Kdo-WbbB/KpsC (QXXXD) covalent enzyme intermediates. Although analysis of the KpsS1 sequence revealed an equivalent QNGLD motif, it also confirmed the presence of an extended H(F/Y)HPE motif, similar to an invariant HP motif in the retaining b-Kdo-transferases, a CMP-binding motif also highly conserved in inverting sialyltransferases. This H(F/Y)HPE motif, alongside a unique PDYM motif also not present in the GT99 and GT107 b-Kdo-transferases, were both found to be highly conserved in putative KpsS1 PseTs returned in a BLASTp search (Supplementary Figure ), including from other A. baumannii strains (Supplementary Figure ). These include KpsS1KL23 which shares 84% sequence identity to KpsS1, KpsS2KL33 which is predicted to form the same a(2,6)linkage to a b-D-Glcp as KpsS1, and KpsS2KL42 from the A. baumannii LUH5550 K42 gene cluster, which likely utilises CMP-Pse5Ac7Hb as a donor, a Pse sugar modified with (R)-3hydroxybutanoyl group at N7. Conversely, the CPS of A. baumannii RBH4 K6 contains an equatorial b(2,6)-Pse5Ac7Ac linkage, and notably kpsS1 is replaced in this KL6 capsule locus by a sequence with similarity to characterised inverting sialyltransferases. A webFlaGs search, which identifies putative orthologues to inputted sequences based on shared sequence identity and genomic context, also returned a number of putative PseT sequences from more divergent bacterial species (Supplementary Figure ) in which both the H(F/Y)HPE and PDYM motifs were highly conserved (Supplementary Figure ). Notably, the KpsS1 AlphaFold model lacks confidence in a short, loop embedded a3 helix (Figure ), indicative of a flexible region, which covers a large cavity in the enzyme where these conserved motifs are clustered (Figure ), potentially highlighting their positioning in a conformationally flexible active site.