Structural adaptations of heptosyltransferase I enzymes in proteobacteria: Insights into evolutionary resilience and functional dynamics

Since lipopolysaccharide (LPS) molecules are highly variable in their structure while also being essential for bacterial cell survival, we hypothesized that enzymes involved in its biosynthesis may exhibit differences that could be used to predict the likelihood of survival in different ecological niches. We examined the sequence variability of orthologues of heptosyltransferase I (HepI), which is found in all LPS containing Gram-negative bacteria. We identified two different sequence motifs (SS-HepI and SA-HepI) within the N-terminal domain of HepI which correlated with differences in the Lipid A portion of the LPS. Further, we compared the protein structure of HepI from Escherichia coli with a structural model we generated that incorporated the alternate sequence motif. Molecular dynamics simulations of these two proteins showed that proteins with the E. coli-like SS-HepI maintained a larger enzyme active site, while the SA-HepI variant undergoes rearrangements that lead to an approximately 75 ų smaller N-terminal active site pocket. These pocket sizes correspond to the size of the known substrates for these enzymes, where tetra-saccharide and tri-saccharide substrates are used, respectively. This work revealed sequence-structure-function relationships that can be used to determine if a species incorporates a Heptose residue onto Lipid A chains containing one or two 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) residues. Since research suggests that the number of Kdo residues in LPS impacts the overall immune response to the LPS endotoxin, this work could aid in our understanding of the pathogenic effects of human-bacterial interactions. Understanding the sequence-structural adaptations of HepI enzymes across Proteobacteria sheds light on their evolutionary resilience and functional versatility.