Abstract

Glucosamine-6-phosphate (GlcN-6-P) synthase is responsible for catalysis of the first and practically irreversible step in hexosamine metabolism. As UDP-GlcNAc, the final product of the hexosamine path, constitutes one of the essential substrates for assembly of bacterial and fungal cell walls, the enzyme is an interesting target for antimicrobial therapy.The structure of E. coli enzyme, known since 2001, forms a dimer of two identical subunits and each subunit is composed of two domains: N-terminal glutamine amide transfer domain (GAH) and C-terminal isomerase domain (ISOM). As for the C. albicans enzyme, for the moment only the ISOM domain crystal structure has been known and the predictions of the glutaminase domain have been made with homology modeling.The sequence alignment of the prokaryotic enzyme (GlmS) and its eukaryotic counterpart (Gfa) shows 41% of identity. All essential residues of the active sites are highly conserved. Despite these similarities, there are significant discrepancies in sequence, related to differences in structure and regulatory mechanisms. Firstly, in the eukaryotic structures, there is an insertion of 30-80 amino acids of unknown function at the C-end of the GAH domain. Moreover, as Gfa forms a tetramer, the areas responsible for tetramerisation are only conserved in the eukaryotic sequences as well as residues responsible for binding of UDP-GlcNAc, the unique inhibitor of the eukaryotic GlcN-6-P synthase. Both areas are located within the ISOM domain.The mechanism of Gfa inhibition by UDP-GlcNAc has not been clearly established yet. However, the latest research carried with computational methods reveal the key role of the 18 C-terminal residues. Rigidification of this region upon UDP-GlcNAc binding could trouble the communication and signal transferring between domains, resulting in the enzyme inhibition.The eukaryotic enzyme can also be regulated by phosphorylation/dephosphorylation mediated by protein kinase A and protein phosphatase. Several residues, namely: Ser208, Ser235, Ser205 and Ser243 have been identified as potential phosphorylation sites of different eukaryotic GlcN-6-P synthase.Profound study of mechanism of the enzyme activity regulation is highly desirable but impossible as long as the crystal structure of the entire enzyme remains unknown.

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