A domain needed for the catalytic efficiency of an enzyme model of simple processivity and domain–domain interactions has been characterized by NMR. This domain 4 from phosphomannomutase/phosphoglucomutase (PMM/PGM) closes upon glucose phosphate and mannose phosphate ligands in the active site, and can modestly reconstitute activity of enzyme truncated to domains 1–3. This enzyme supports biosynthesis of the saccharide-derived virulence factors (rhamnolipids, lipo-polysaccharides, and alginate) of the opportunistic bacterial pathogen Pseudomonas aeruginosa. 1H, 13C, and 15N NMR chemical shift assignments of domain 4 of PMM/PGM suggest preservation and independence of its structure when separated from domains 1–3. The face of domain 4 that packs with domain 3 is perturbed in NMR spectra without disrupting this fold. The perturbed residues overlap both the most highly coevolved positions in the interface and residues lining a cavity at the domain interface.