Molecular dynamics simulations shed light into the donor substrate specificity of vertebrate poly-alpha-2,8-sialyltransferases ST8Sia IV.

BACKGROUND: Sialic acids are essential monosaccharides influencing several biological processes and disease states. The sialyltransferases catalyze the transfer of Sia residues to glycoconjugates playing critical roles in cellular recognition and signaling. Despite their importance, the molecular mechanisms underlying their substrate specificity, especially between different organisms, remain poorly understood. Recently, the human ST8Sia IV, a key enzyme in the synthesis of polysialic acids, was found to accept only CMP-Neu5Ac as a sugar-donor, whereas the whitefish Coregonus maraena enzyme showed a wider donor substrate specificity, accepting CMP-Neu5Ac, CMP-Neu5Gc, and CMP-Kdn. However, what causes these differences in donor substrate specificity is unknown. METHODS: Computational approaches were used to investigate the structural and biochemical determinants of the donor substrate specificity in ST8Sia IV. Accurate structural models of the human and fish ST8Sia IV catalytic domains and their complexes with three sialic acid donors (CMP-Neu5Ac, CMP-Neu5Gc, and CMP-Kdn) were generated. Subsequently, molecular dynamics simulations were conducted to analyze the stability and interactions within these complexes and identify differences in complex stability and substrate binding sites between the two ST8Sia IV. RESULTS: Our MD simulations revealed that the human enzyme effectively stabilizes CMP-Neu5Ac, whereas CMP-Neu5Gc and CMP-Kdn are unstable and explore different conformations. In contrast, the fish ST8Sia IV stabilizes all three donor substrates. Based on these data, we identified the key interacting residues for the different Sias parts of the substrate donors. GENERAL SIGNIFICANCE: This work advances our knowledge of the enzymatic mechanisms governing sialic acid transfer, shedding light on the evolutionary adaptations of sialyltransferases.

Salmonid polysialyltransferases to generate a variety of sialic acid polymers.

The human polysialyltransferases ST8Sia II and ST8Sia IV catalyze the transfer of several Neu5Ac residues onto glycoproteins forming homopolymers with essential roles during different physiological processes. In salmonids, heterogeneous set of sialic acids polymers have been described in ovary and on eggs cell surface and three genes st8sia4, st8sia2-r1 and st8sia2-r2 were identified that could be implicated in these heteropolymers. The three polysialyltransferases from the salmonid Coregonus maraena were cloned, recombinantly expressed in HEK293 cells and the ST8Sia IV was biochemically characterized. The MicroPlate Sialyltransferase Assay and the non-natural donor substrate CMP-SiaNAl were used to demonstrate enzyme activity and optimize polysialylation reactions. Polysialylation was also carried out with natural donor substrates CMP-Neu5Ac, CMP-Neu5Gc and CMP-Kdn in cell-free and cell-based assays and structural analyses of polysialylated products using the anti-polySia monoclonal antibody 735 and endoneuraminidase N and HPLC approaches. Our data highlighted distinct specificities of human and salmonid polysialyltransferases with notable differences in donor substrates use and the capacity of fish enzymes to generate heteropolymers. This study further suggested an evolution of the biological functions of polySia. C. maraena ST8Sia IV of particular interest to modify glycoproteins with a variety of polySia chains.

A phylogenetic view and functional annotation of the animal ß1,3-glycosyltransferases of the GT31 CAZy family.

The formation of ß1,3-linkages on animal glycoconjugates is catalyzed by a subset of ß1,3-glycosyltransferases grouped in the Carbohydrate-Active enZYmes family glycosyltransferase-31 (GT31). This family represents an extremely diverse set of ß1,3-N-acetylglucosaminyltransferases [B3GNTs and Fringe ß1,3-N-acetylglucosaminyltransferases], ß1,3-N-acetylgalactosaminyltransferases (B3GALNTs), ß1,3-galactosyltransferases [B3GALTs and core 1 ß1,3-galactosyltransferases (C1GALTs)], ß1,3-glucosyltransferase (B3GLCT) and ß1,3-glucuronyl acid transferases (B3GLCATs or CHs). The mammalian enzymes were particularly well studied and shown to use a large variety of sugar donors and acceptor substrates leading to the formation of ß1,3-linkages in various glycosylation pathways. In contrast, there are only a few studies related to other metazoan and lower vertebrates GT31 enzymes and the evolutionary relationships of these divergent sequences remain obscure. In this study, we used bioinformatics approaches to identify more than 920 of putative GT31 sequences in Metazoa, Fungi and Choanoflagellata revealing their deep ancestry. Sequence-based analysis shed light on conserved motifs and structural features that are signatures of all the GT31. We leverage pieces of evidence from gene structure, phylogenetic and sequence-based analyses to identify two major subgroups of GT31 named Fringe-related and B3GALT-related and demonstrate the existence of 10 orthologue groups in the Urmetazoa, the hypothetical last common ancestor of all animals. Finally, synteny and paralogy analysis unveiled the existence of 30 subfamilies in vertebrates, among which 5 are new and were named C1GALT2, C1GALT3, B3GALT8, B3GNT10 and B3GNT11. Altogether, these various approaches enabled us to propose the first comprehensive analysis of the metazoan GT31 disentangling their evolutionary relationships.

A practical approach to reconstruct evolutionary history of animal sialyltransferases and gain insights into the sequence-function relationships of Golgi-glycosyltransferases.

In higher vertebrates, sialyltransferases catalyze the transfer of sialic acid residues, either Neu5Ac or Neu5Gc or KDN from an activated sugar donor, which is mainly CMP-Neu5Ac in human tissues, to the hydroxyl group of another saccharide acceptor. In the human genome, 20 unique genes have been described that encode enzymes with remarkable specificity with regards to their acceptor substrates and the glycosidic linkage formed. A systematic search of sialyltransferase-related sequences in genome and EST databases and the use of bioinformatic tools enabled us to investigate the evolutionary history of animal sialyltransferases and propose original models of divergent evolution of animal sialyltransferases. In this chapter, we extend our phylogenetic studies to the comparative analysis of the environment of sialyltransferase gene loci (synteny and paralogy studies), the variations of tissue expression of these genes and the analysis of amino-acid position evolution after gene duplications, in order to assess their sequence-function relationships and the molecular basis underlying their functional divergence.