Putative isomerase involved in the N-glycosylation pathway. Required for the appearance of the methyl ester of hexuronic acid found at position four of the pentasaccharide N-linked to the S-layer glycoprotein. Either involved in preparing the third sugar for attachment of the fourth pentasaccharide subunit or processing the fourth sugar prior to its addition to the lipid-linked trisaccharide. Cell surface structure biogenesis; S-layer biogenesis. Defects in N-glycosylation pathway, characterized by impaired addition of the fourth sugar of the pentasaccharide: both dolichol phosphate, the lipid carrier used in H.volcanii N-glycosylation, and modified S-layer glycoprotein Asn residues only present the first three subunits of the pentasaccharide.