A multifunctional sialyltransferase has been cloned from Pasteurella multocida strain P-1059 and expressed in E. coli as a truncated C-terminal His6-tagged recombinant protein (tPm0188Ph). Biochemical studies indicate that the obtained protein is (1) an alpha2,3-sialyltransferase (main function), (2) an alpha2,6-sialyltransferase, (3) an alpha2,3-sialidase, and (4) an alpha2,3-trans-sialidase. The recombinant tPm0188Ph is a powerful tool in the synthesis of structurally diverse sialoside libraries due to its relaxed substrate specificity, high solubility, high expression level, and multifunctionality.

Sialic acids are a subset of nonulosonic acids, which are nine-carbon alpha-keto aldonic acids. Natural existing sialic acid-containing structures are presented in different sialic acid forms, various sialyl linkages, and on diverse underlying glycans. They play important roles in biological, pathological, and immunological processes. Sialobiology has been a challenging and yet attractive research area. Recent advances in chemical and chemoenzymatic synthesis as well as large-scale E. coli cell-based production have provided a large library of sialoside standards and derivatives in amounts sufficient for structure-activity relationship studies. Sialoglycan microarrays provide an efficient platform for quick identification of preferred ligands for sialic acid-binding proteins. Future research on sialic acid will continue to be at the interface of chemistry and biology. Research efforts will not only lead to a better understanding of the biological and pathological importance of sialic acids and their diversity, but could also lead to the development of therapeutics. Keywords biology; carbohydrate; chemistry; CMP-sialic acid synthetase; glycan; sialic acid; sialic acid aldolase; sialidase; sialoside; sialyltransferase Diversity of Sialic Acids in Nature Sialic acids are a Subset of Nonulosonic AcidsNot long ago, it was thought that sialic acids (Sias) were unique inventions of the deuterostome lineage of animals, which emerged around the time of the Cambrian expansion ~530 million years ago, with certain pathogenic bacteria having then "acquired" them from hosts by gene transfer (1-2). However, the relevant genes of bacterial pathogens were then found to be only distantly homologous to corresponding host genes (3). Meanwhile, work from multiple investigators over the last few decades has shown that the unusual 9-carbon backbone of Sias is in fact shared by a larger family of nonulosonic acids (NulOs), which are much more widely distributed in nature (4-6). Furthermore, the key steps in the biosynthesis of nonulosonic acids share remarkable similarities, and the genes involved are homologous. These aspects have recently been discussed extensively in a phylogenomic evaluation of nonulosonic acids (7). Although the other forms of nonulosonic acids, such as legionaminic acid and pseudaminic acid, have sometimes been called "bacterial sialic acids" (4), we prefer to reserve the term Sia for the 9-carbon sugars found both in the deuterostome lineage of animals and in certain bacteria that are based on a neuraminic acid (Neu) or a 2-keto-3-deoxy-nonulosonic acid (Kdn) backbone (7) (Figure 1). Thus, this review will focus only on these "traditional" Sias, emphasizing recent challenges and advances at the interface of chemistry and biology.

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC), popularly known as the "click reaction", serves as the most potent and highly dependable tool for facile construction of simple to complex architectures at the molecular level. Click-knitted threads of two exclusively different molecular entities have created some really interesting structures for more than 15 years with a broad spectrum of applicability, including in the fascinating fields of synthetic chemistry, medicinal science, biochemistry, pharmacology, material science, and catalysis. The unique properties of the carbohydrate moiety and the advantages of highly chemo- and regioselective click chemistry, such as mild reaction conditions, efficient performance with a wide range of solvents, and compatibility with different functionalities, together produce miraculous neoglycoconjugates and neoglycopolymers with various synthetic, biological, and pharmaceutical applications. In this review we highlight the successful advancement of Cu(I)-catalyzed click chemistry in glycoscience and its applications as well as future scope in different streams of applied sciences.

Human heterophile antibodies that agglutinate animal erythrocytes are known to detect the non-human sialic acid N-glycolylneuraminic acid (Neu5Gc). This monosaccharide cannot by itself fill the binding site (paratope) of an antibody and can also be modified and presented in various linkages, on diverse underlying glycans. Thus, we hypothesized that the human anti-Neu5Gc antibody response is diverse and polyclonal. Here we use a novel set of natural and chemoenzymatically-synthesized glycans to show that normal humans have an abundant and diverse spectrum of such anti-Neu5Gc antibodies, directed against a variety of Neu5Gc-containing epitopes. High sensitivity and specificity assays were achieved by using N-acetylneuraminic acid (Neu5Ac)-containing probes (differing from Neu5Gc by one less oxygen atom) as optimal background controls. The commonest anti-Neu5Gc antibodies are of the IgG class. Moreover, the range of reactivity and Ig classes of antibodies varies greatly amongst normal humans, with some individuals having remarkably large amounts, even surpassing levels of some well-known natural blood group and xenoreactive antibodies. We purified these anti-Neu5Gc antibodies from individual human sera using a newly developed affinity method and show that they bind to wild-type but not Neu5Gc-deficient mouse tissues. Moreover, they bind back to human carcinomas that have accumulated Neu5Gc in vivo. As dietary Neu5Gc is primarily found in red meat and milk products, we suggest that this ongoing antigen-antibody reaction may generate chronic inflammation, possibly contributing to the high frequency of diet-related carcinomas and other diseases in humans.

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