Abstract
Protein modules that bind specific oligosaccharides are found across all kingdoms of life from single-celled organisms to man. Different, overlapping and evolving designations for sugar-binding domains in proteins can sometimes obscure common features that often reflect convergent solutions to the problem of distinguishing sugars with closely similar structures and binding them with sufficient affinity to achieve biologically meaningful results. Structural and functional analysis has revealed striking parallels between protein domains with widely different structures and evolutionary histories that employ common solutions to the sugar recognition problem. Recent studies also demonstrate that domains descended from common ancestors through divergent evolution appear more widely across the kingdoms of life than had previously been recognized.
Highlights
Proteins that seem to have a primary function of binding sugars are often referred to as lectins, a term used initially in the context of plant seed proteins and broadened to include examples from a wider range of species [1]
It is clear that there is no single set of unifying principles that describe carbohydrate recognition across all the kingdoms of life
The examples described in this short review illustrate that some of the solutions to the sugar recognition problem go back very far in evolution and that mechanisms for binding sugars based on the chemical properties of the sugar ligands can be implemented in the context of many different protein folds
Summary
Proteins that seem to have a primary function of binding sugars are often referred to as lectins, a term used initially in the context of plant seed proteins and broadened to include examples from a wider range of species [1]. Ligation of sugars to Ca2+ ions was first described for the C-type carbohydraterecognition domains in animal lectins [11], but has recently been identified in several other groups of sugar-binding proteins with carbohydrate-recognition domains from different fold families (Figure 1). A proliferation of secondary binding sites A further interesting comparison of convergent sugarbinding sites is that, within fold families, there are often common mechanisms of binding to a core monosaccharide in a primary binding site, but diversity in binding of oligosaccharide and glycoconjugate ligands is achieved through extended and secondary binding sites that are unique to individual members of the family Such extensions can involve interactions with additional sugar residues in an oligosaccharide ligand, but an increasing number of examples demonstrate binding to other modifications of the sugars.
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