Carolyn R. Bertozzi

The study of biomolecules in their native environments is a challenging task because of the vast complexity of cellular systems. Technologies developed in the last few years for the selective modification of biological species in living systems have yielded new insights into cellular processes. Key to these new techniques are bioorthogonal chemical reactions, whose components must react rapidly and selectively with each other under physiological conditions in the presence of the plethora of functionality necessary to sustain life. Herein we describe the bioorthogonal chemical reactions developed to date and how they can be used to study biomolecules.

Selective chemical reactions that are orthogonal to the diverse functionality of biological systems have become important tools in the field of chemical biology. Two notable examples are the Staudinger ligation of azides and phosphines and the Cu(I)-catalyzed [3 + 2] cycloaddition of azides and alkynes ("click chemistry"). The Staudinger ligation has sufficient biocompatibility for performance in living animals but suffers from phosphine oxidation and synthetic challenges. Click chemistry obviates the requirement of phosphines, but the Cu(I) catalyst is toxic to cells, thereby precluding in vivo applications. Here we present a strain-promoted [3 + 2] cycloaddition between cyclooctynes and azides that proceeds under physiological conditions without the need for a catalyst. The utility of the reaction was demonstrated by selective modification of biomolecules in vitro and on living cells, with no apparent toxicity.

General principles - historical background and overview saccharide structure and nomenclature evolution of glycan diversity protein-glycan Interactions exploring the biological roles of glycans biosynthesis, metabolism, and function - monosaccharide metabolism N-glycans O-glycans glycosphingolipids glycophospholipid anchors proteoglycans and glycosaminoglycans other classes of golgi-derived glycans nuclear and cytoplasmic glycosylation the O-GlcNAc modification sialic acids structures common to different types of glycans glycosyltransferases degradation and turnover of glycans glycosylation in model organisms glycobiology of plant cells bacterial polysaccharides proteins that recognize glycans - discovery and classification of animal lectins P-type lectins I-type lectins C-type lectins selectins S-type lectins (galectins) microbial glycan-binding proteins glycosaminoglycan-binding proteins plant lectins glycans in genetic disorders and disease - genetic disorders of glycosylation in cultured cells naturally occurring genetic disorders of glycosylation in animals determining glycan function using genetically modified mice glycosylation changes in ontogeny and cell activation glycosylation changes in cancer glycobiology of protozoal and helminthic parasites acquired glycosylation changes in human disease methods and applications - structural analysis and sequencing of glycans chemical and enzymatic synthesis of glycans natural and synthetic inhibitors of glycosylation glycobiology in biotechnology and medicine.

Selective chemical reactions enacted within a cellular environment can be powerful tools for elucidating biological processes or engineering novel interactions. A chemical transformation that permits the selective formation of covalent adducts among richly functionalized biopolymers within a cellular context is presented. A ligation modeled after the Staudinger reaction forms an amide bond by coupling of an azide and a specifically engineered triarylphosphine. Both reactive partners are abiotic and chemically orthogonal to native cellular components. Azides installed within cell surface glycoconjugates by metabolism of a synthetic azidosugar were reacted with a biotinylated triarylphosphine to produce stable cell-surface adducts. The tremendous selectivity of the transformation should permit its execution within a cell's interior, offering new possibilities for probing intracellular interactions.

Chemical tools have proven indispensable for studies in glycobiology. Synthetic oligosaccharides and glycoconjugates provide materials for correlating structure with function. Synthetic mimics of the complex assemblies found on cell surfaces can modulate cellular interactions and are under development as therapeutic agents. Small molecule inhibitors of carbohydrate biosynthetic and processing enzymes can block the assembly of specific oligosaccharide structures. Inhibitors of carbohydrate recognition and biosynthesis can reveal the biological functions of the carbohydrate epitope and its cognate receptors. Carbohydrate biosynthetic pathways are often amenable to interception with synthetic unnatural substrates. Such metabolic interference can block the expression of oligosaccharides or alter the structures of the sugars presented on cells. Collectively, these chemical approaches are contributing great insight into the myriad biological functions of oligosaccharides.