Galectin (Gal) family members are a type of soluble lectin, and they play important roles in immunomodulation. Their redundant roles have been proposed. We previously found that Gal-1 promotes the formation of Ab-secreting plasma cells, but B cells from Gal-1–deficient and control animals produce comparable amounts of Abs. In the current study, we used synthetic sulfomodified N-acetyllactosamine (LacNAc) analogs and short hairpin RNAs for Gal-8 to demonstrate a redundancy in the effects of Gal-1 and Gal-8 on plasma cell formation. Gal-1 and Gal-8 were both expressed during plasma cell differentiation, and both Gals promoted the formation of plasma cells. Gal-1 and Gal-8 bound better to mature B cells than to plasma cells, and the expression of glycosyltransferase enzymes changed during differentiation, with a decrease in mannosyl (α-1,6-)-glycoprotein β-1,6-N-acetyl-glucosaminyltransferase and N-acetylglucosaminyltransferase-1 mRNAs in plasma cells. Synthetic sulfomodified Galβ1-3GlcNAc disaccharides (type 1 LacNAcs) selectively prevented Gal-8 binding, leading to a blockade of Ab production in Gal-1–deficient B cells. Furthermore, synthetic type 1 LacNAcs that were able to block the binding of both Gals greatly reduced the effect of exogenously added recombinant Gal-1 and Gal-8 on promoting Ab production. These results reveal a novel role for Gal-8 in collaboration with Gal-1 in plasma cell formation, and suggest the possibility of using distinct LacNAc ligands to modulate the function of Gals.

Biomimicry valuably allows the understanding of the essential chemical components required to recapitulate biological function, yet direct strategies for evaluating the roles of amino acids in proteins can be limited by access to suitable, subtly-altered unnatural variants. Here we describe a strategy for dissecting the role of histidine residues in enzyme active sites using unprecedented, chemical, post-translational side-chain-β,γ C–N bond formation. Installation of dehydroalanine (as a “tag”) allowed the testing of nitrogen conjugate nucleophiles in “aza-Michael”-1,4-additions (to “modify”). This allowed the creation of a regioisomer of His (iso-His, Hisiso) linked instead through its pros-Nπ atom rather than naturally linked via C4, as well as an aza-altered variant aza-Hisiso. The site-selective generation of these unnatural amino acids was successfully applied to probe the contributing roles (e.g., size, H-bonding) of His residues toward activity in the model enzymes subtilisin protease from Bacillus lentus and Mycobacterium tuberculosis pantothenate synthetase.

We have developed an expeditious procedure to yield large amounts of orthogonally protected Gal-β1,3/4-GlcNAc, which allowed for the systematic introduction of a sulfate group onto the C3/C6 positions of Gal and/or the C6 position of GlcNAc. In particular, the disaccharide precursors were prepared in five or six steps and high overall yield from para-tolyl-6-O-tert-butyldiphenylsilyl-1-thio-β-D-galactopyranoside. After deprotection and sulfation steps, the final products were characterized by using several NMR methods to unambiguously confirm the location of each introduced sulfate group and they were examined for their binding specificity of human galectin-1 and galectin-8.

In a recent directed-evolution study, Escherichia coli D-sialic acid aldolase was converted by introducing eight point mutations into a new enzyme with relaxed specificity, denoted RSaldolase (also known formerly as L-3-deoxy-manno-2-octulosonic acid (L-KDO) aldolase), which showed a preferred selectivity toward L-KDO. To investigate the underlying molecular basis, we determined the crystal structures of D-sialic acid aldolase and RS-aldolase. All mutations are away from the catalytic center, except for V251I, which is near the opening of the (␣/␤) 8 -barrel and proximal to the Schiff base-forming Lys-165. The change of specificity from D-sialic acid to RS-aldolase can be attributed mainly to the V251I substitution, which creates a narrower sugar-binding pocket, but without altering the chirality in the reaction center. The crystal structures of D-sialic acid aldolase⅐L-arabinose and RS-aldolase⅐hydroxypyruvate complexes and five mutants (V251I, V251L, V251R, V251W, and V251I/V265I) of the D-sialic acid aldolase were also determined, revealing the location of substrate molecules and how the contour of the active site pocket was shaped. Interestingly, by mutating Val251 alone, the enzyme can accept substrates of varying size in the aldolase reactions and still retain stereoselectivity. The engineered D-sialic acid aldolase may find applications in synthesizing unnatural sugars of C 6 to C 10 for the design of antagonists and inhibitors of glycoenzymes.Carbohydrates on cell surfaces play a pivotal role in the molecular recognition processes in various cellular interactions. Interfering with the recognition processes between pathogens and hosts may present therapeutic strategies for infectious diseases (1, 2). D-Sialic acids are widely found in nature, from bacteria to plants and animal tissues. Cell-surface glycoproteins and glycolipids frequently contain D-sialic acids at the termini of the oligosaccharide chains. The invading bacteria and viruses also target the cell-surface sialic acids of their hosts, which constitutes an important step for infectivity (3). On the other hand, the monosaccharide D-3-deoxy-manno-2-octulosonic acid (D-KDO) 2 has never been identified in vertebrates, but it is an important component in Gram-negative bacteria (4 -6). Because D-KDO is vital to the structural stability of bacterial outer membrane, targeting D-KDO and the related biosynthetic enzymes may be a new strategy for future antibacterial agent discovery.Currently, unnatural enantiomeric compounds become increasingly important in pharmaceutical implementations. Enantiomers (D-form) of natural peptides have been used to target cell-surface D-sugars. These D-peptide cell-surface sugar binders might serve as new drug candidates for infectious disease (7) and as tools for studying the interactions between proteins and carbohydrate. Owing to the resistance to hydrolases and high binding affinity to sugars, the unnatural D-peptides can become potential therapeutic agents. Through mirror-image phage display, natural L-peptides can be...