Infection and the Glycome─New Insights into Host Response.

Glycans play critical roles in the host-pathogen interactions leading to infection. However, we still understand very little about the dynamic nature of glycosylation in response to infection and its function in modulating host immunity. Many of the host proteins involved in immune defense are glycoproteins. Furthermore, the innate immune system recognizes glycans. The glycoform of a protein can impact proteolytic stability, receptor interactions, serum half-life, and other aspects. New, cutting-edge chemical biology tools are shedding light on the interplay between infection and the host glycome. In this review, we highlight new work on the importance of dynamic glycosylation of host proteins in the innate and adaptive immune pathways in response to infection. These include recent findings on altered glycoprofiles of mucins, complement components, and antibodies.

A "biphasic glycosyltransferase high-throughput screen" identifies novel anthraquinone glycosides in the diversification of phenolic natural products.

The sugar moieties of many glycosylated small molecule natural products are essential for their biological activity. Glycosyltransferases (GTs) are enzymes responsible for installing these sugar moieties on a variety of biomolecules. Many GTs active on natural products are inherently substrate promiscuous and thus serve as useful tools in manipulating natural product glycosylation to generate new combinations of sugar units (glycones) and scaffold molecules (aglycones) in a process called glycodiversification. It is important to have an effective screening tool to detect the activity of promiscuous enzymes and their resulting glycoside products. Toward this aim, we developed a strategy for screening natural product GTs in a high-throughput fashion enabled by rapid isolation and detection of chromophoric or fluorescent glycosylated natural products. This involves a solvent extraction step to isolate the resulting polar glycoside product from the unreacted aglycone acceptor substrate and the detection of the formed glycoside by the innate absorbance or fluorescence of the aglycone moiety. Using our approach, we screened a collection of natural product GTs against a panel of precursors to therapeutically important molecules. Three GTs showed previously unreported promiscuity toward anthraquinones resulting in novel ε-rhodomycinone glycosides. Considering the pharmaceutical value of clinically used anthraquinone glycosides that are biosynthesized from an ε-rhodomycinone precursor, and the significance that the sugar moiety has on the biological activity of these drugs, our results are of particular importance toward the glycodiversification of therapeutics in this class. The GTs identified and the novel compounds they produce show promise toward new biocatalytic tools and therapeutics.

Engineering the enzyme toolbox to tailor glycosylation in small molecule natural products and protein biologics.

Many glycosylated small molecule natural products and glycoprotein biologics are important in a broad range of therapeutic and industrial applications. The sugar moieties that decorate these compounds often show a profound impact on their biological functions, thus biocatalytic methods for controlling their glycosylation are valuable. Enzymes from nature are useful tools to tailor bioproduct glycosylation but these sometimes have limitations in their catalytic efficiency, substrate specificity, regiospecificity, stereospecificity, or stability. Enzyme engineering strategies such as directed evolution or semi-rational and rational design have addressed some of the challenges presented by these limitations. In this review, we highlight some of the recent research on engineering enzymes to tailor the glycosylation of small molecule natural products (including alkaloids, terpenoids, polyketides, and peptides), as well as the glycosylation of protein biologics (including hormones, enzyme-replacement therapies, enzyme inhibitors, vaccines, and antibodies).

In Vitro Reconstitution of the dTDP-l-Daunosamine Biosynthetic Pathway Provides Insights into Anthracycline Glycosylation.

Many small molecule natural products are decorated with sugar moieties that are essential for their biological activity. A considerable number of natural product glycosides and their derivatives are clinically important therapeutics. Anthracyclines like daunorubicin and doxorubicin are examples of valuable glycosylated natural products used in medicine as potent anticancer agents. The sugar moiety, l-daunosamine (a highly modified deoxyhexose), plays a key role in the bioactivity of these molecules as evidenced by semisynthetic anthracycline derivatives such as epirubicin, wherein alteration in the configuration of a single stereocenter of the sugar unit generates a chemotherapeutic drug with lower cardiotoxicity. The nucleotide activated sugar donor that provides the l-daunosamine group for attachment to the natural product scaffold in the biosynthesis of these anthracyclines is dTDP-l-daunosamine. In an in vitro system, we have reconstituted the enzymes in the daunorubicin/doxorubicin pathway involved in the biosynthesis of dTDP-l-daunosamine. Through the study of the enzymatic steps in this reconstituted pathway, we have gained several insights into the assembly of this precursor including the identification of a major bottleneck and competing reactions. We carried out kinetic analysis of the aminotransferase that catalyzes a limiting step of the pathway. Our in vitro reconstituted pathway also provided a platform to test the combinatorial enzymatic synthesis of other dTDP-activated deoxyhexoses as potential tools for "glycodiversification" of natural products. To this end, we replaced the stereospecific ketoreductase that acts in the last step of dTDP-l-daunosamine biosynthesis with an enzyme from a heterologous pathway with opposite stereospecificity and found that it is active in the in vitro pathway, demonstrating the potential for the enzymatic synthesis of nucleotide-activated sugars with regio- and stereospecific tailoring.