The 2022 Nobel Prize in Chemistry—sweet!

On 2022 October 5, The Royal Swedish Academy of Sciences announced its decision to award the Nobel Prize in Chemistry 2022 to Carolyn R. Bertozzi (Stanford University), Morten Meldal (University of Copenhagen), and K. Barry Sharpless (Scripps Research) “for the development of click chemistry and bioorthogonal chemistry.” The glycobiology community was exultant! To understand why, one must appreciate how challenges in glycoscience research motivated the development of these transformative chemical tools. Glycoscience has a storied history contributing significant discoveries to hematology, metabolism, microbiology, and other fields of biomedical research throughout much of the 20th century. But when the 1970s and 80s ushered in new molecular biology methods that accelerated studies of nucleic acid and protein function, studies of glycans began to lag behind (National Research Council 2012; Agre et al. 2016). Fortunately, since then, vast improvements have been made in areas such as carbohydrate synthesis and glycan arrays (Rillahan and Paulson 2011; Li and Bennett 2022), availability of glyco-enzymes and small molecule inhibitors (Moremen et al. 2018; Almahayni et al. 2022), mass spectrometry analysis of glycans and glycopeptides (Ruhaak et al. 2018; Suttapitugsakul et al. 2020; Bagdonaite et al. 2022), as well understanding of biological systems (Varki 2017), all of which have facilitated studies of glycan function. Notwithstanding these and other important advances, glycan detection has presented a recurrent challenge. Indeed, much of the modern practice of biological research hinges on the ability to specifically address a biomolecule of interest so that it can be isolated, visualized, or quantified. For nucleic acids, this can often be achieved via hybridization, whereas specific proteins can be detected and tracked using antibodies or protein tagging methods. But genetically encoded tags cannot be used to track glycans, which are not primary gene products. For the glycobiologist, lectins and antibodies have been essential tools but can suffer from limitations relating to specificity and affinity (Cummings et al. 2022). Carolyn Bertozzi (Fig. 1) envisioned that the challenge of specifically tagging glycans could be solved through the use of what she termed bioorthogonal chemistry. In a prescient review article, her team posed this question: “Can a covalent chemical reaction compatible with a physiological environment achieve a level of selectivity comparable to that of antibody-antigen recognition?” They proposed that “such a reaction might enable the assembly of complex structures from simple components and the chemical targeting of biomolecules or even whole cells in vivo, opening the door to numerous opportunities in biotechnology and biomedical research.”(Lemieux and Bertozzi 1998). Carolyn Bertozzi, 2022 Nobel laureate in Chemistry. Photo by Christopher Michel. The vision of cell surface glycoconjugate labeling that inspired development of bioorthogonal chemistry. Bertozzi and colleagues devised a strategy to incorporate bioorthogonal chemical handles (e.g. azides) into cell surface glycans by supplementing non-natural monosaccharide analogs into systems that could convert these precursors into substrates used to build glycoconjugates. They then leveraged the potential energy of alkynes in strained rings (e.g. cyclooctyne) to drive a concerted [3 + 2] cycloaddition with azide groups in the labeled glycans, enabling various visualization or enrichment modalities to be appended to cell surface glycoconjugates with high specificity and without the need of toxic catalysts. This SPAAC was the genesis for bioorthogonal and “click” reactions that have inspired an entire discipline within the chemical biology field. This ingenious reaction, the prior work from Bertozzi group leading to this breakthrough, and the subsequent impact it had across multiple scientific enterprises earned Bertozzi recognition for the 2022 Nobel Prize in Chemistry along with Barry Sharpless and Morten Meldal for their work in developing the copper-catalyzed alkyne-azide cycloaddition. Image created with aid from BioRender.com, ChemDraw v21.0.0.28, and DrawGlycan-SNFG (Cheng et al. 2016). The concept of bioorthogonal chemistry revolves around 2 functional groups that react selectively and covalently with one another within a biological setting (Sletten and Bertozzi 2009). To use a bioorthogonal reaction to label glycans, one of these functional groups must be incorporated onto sugar residues of interest. The Bertozzi team drew inspiration from the work of the late Werner Reutter who showed that N-acyl-substituted mannosamines could be converted to the corresponding sialic acids and incorporated into glycoproteins in living organisms (Kayser et al. 1992). In 1997, they showed that this kind of metabolic labeling strategy could be used to incorporate a ketone functional group onto cell surface sialic acid residues and, further, that those sialic acid residues could be specifically labeled by reaction with a functionalized hydrazide (Mahal et al. 1997). While a significant proof-of-concept, the ketone-hydrazide ligation did not have the necessary features to be truly bioorthogonal; the reaction proceeds slowly at physiological pH and carbonyls present in endogenous biomolecules can lead to off-target labeling. Just 3 years later, the group reported an improved approach. They used metabolic labeling to incorporate an azide functional group onto sialic acid residues and demonstrated labeling via covalent ligation of the azide to a triarylphosphine reagent, in a new reaction inspired by the classic Staudinger reaction (Staudinger and Meyer 1919; Saxon and Bertozzi 2000). This reaction, now dubbed the Staudinger–Bertozzi ligation by some (Hosoya et al. 2004), met the criteria for bioorthogonality and, remarkably, could be used to label sialylated glycans in living animals (Prescher et al. 2004; Bednarek et al. 2020). At the same time, Morten Meldal and Barry Sharpless were each independently investigating the ability of the azide functional group to engage in chemoselective reactions. Meldal was seeking a reaction that could be used in solid-phase synthesis of peptides and other chemicals, while Sharpless was on the hunt for “a few good reactions” characterized by excellent yields due to high thermodynamic driving force (Kolb et al. 2001). Both identified a solution in the copper-catalyzed cycloaddition of terminal alkynes to azides (Rostovtsev et al. 2002; Tornoe et al. 2002; Wu 2022). This high-yielding reaction rapidly became a near-ubiquitous tool for in vitro labeling of biomolecules, and biologists everywhere became familiar with the concept of “click chemistry.” The copper-catalyzed azide–alkyne cycloaddition exhibited significantly faster reaction kinetics than the Staudinger–Bertozzi ligation, suggesting that it might offer an improved approach for glycan labeling. However, the toxicity of the catalyst limits its use in cell culture and in animals. To circumvent the need for the catalyst, the Bertozzi team devised a variant reaction: the strain-promoted [3 + 2] azide–alkyne cycloaddition (SPAAC), which relies on a highly reactive cycloalkyne (Agard et al. 2004). The Bertozzi team showed that copper-free versions of the azide–alkyne cycloaddition reaction could be used for glycan labeling in mice and glycan imaging in zebrafish, achieving the goal of chemically targeting specific glycans in vivo (Fig. 2; Laughlin et al. 2006; Chang et al. 2010). The reactions developed by Bertozzi, Meldal, and Sharpless have found widespread applications in both chemical and biological research labs and have inspired further innovation in the development of biorthogonal chemical reactions (Thirumurugan et al. 2013; Devaraj 2018; Pokorski and Smith 2018; Scinto et al. 2021). The applications of these reactions to glycobiology are myriad (Agrahari et al. 2021; Banahene et al. 2022; Cioce et al. 2021; Gilormini et al. 2018; Haiber et al. 2021; Hunter et al. 2018; Kim 2018; Lopez Aguilar et al. 2017; Rigolot et al. 2021; Riley et al. 2021). Along with the impact of bioorthogonal chemistry, the glycobiology community also celebrates other contributions that the laureates have made to glycoscience research. Prof. Meldal is a world leader in the development of methods for glycopeptide biosynthesis (St Hilaire and Meldal 2000). These molecules have been employed in a variety of biological studies, including cancer immunology research (Galli-Stampino et al. 1997). Prof. Bertozzi’s contributions span multiple areas of glycoscience; recent innovations include methods for glycoproteomics analysis (Riley et al. 2021), approaches to target Siglecs in cancer therapy (Smith and Bertozzi 2021), and design of synthetic glycoconjugates to direct targeted protein degradation (Ahn et al. 2021). Through the efforts of her research group and the biotechnology companies she has founded (Nobel laureate Bertozzi’s companies 2022), she is charting the course for translational and commercial applications of glycoscience, with new therapeutics on the horizon (Smith and Bertozzi 2021). In 2020, Prof. Bertozzi was the recipient of the President’s Innovator Award, recognizing her contributions to the Society for Glycobiology and the glycoscience community. She is a champion of glycoscience research both in person and on social media. A role model for women and LGBTQ scientists, she is a staunch advocate for diversity in science, often describing its positive impact on innovation in her research group. More personally, speaking for ourselves and on behalf of many former trainees in the Bertozzi lab, we are grateful to have experienced this intellectually vibrant training environment and to be the recipients of Carolyn’s continued support. The selection of this year’s Nobel Prize in Chemistry raises the visibility of glycoscience research and emphasizes its significance. Indeed, the Nobel committee noted that Bertozzi’s goal was to “[t]o map important but elusive biomolecules on the surface of cells – glycans.” With the development of bioorthogonal chemistry along with other tools to probe glycan function (Landhuis 2017), glycans will not remain elusive for long. Michael Boyce (Writing—original draft-Equal, Writing—review & editing-Equal), Stacy A. Malaker (Writing—original draft-Equal, Writing—review & editing-Equal), Nicholas M. Riley (Writing—original draft-Equal, Writing—review & editing-Equal), Jennifer Kohler (Writing—original draft-Equal, Writing—review & editing-Equal) Conflict of interest statement: S.A.M. is a consultant for InterVenn Biosciences.