Lectins--proteins with a sweet tooth: functions in cell recognition.

Lectins, non-enzymic proteins that bind mono- and oligosaccharides reversibly and with high specificity, occur widely in nature. They come in a variety of sizes and shapes, but can be grouped in families with similar structural features. The combining sites of lectins are also diverse, although they are similar in the same family. The specificities of lectins are determined by the exact shape of the binding sites and the nature of the amino acid residues to which the carbohydrate is linked. Small changes in the structure of the sites, such as the substitution of only one or two amino acids, may result in marked changes in specificity. The carbohydrate is linked to the protein mainly through hydrogen bonds, with added contributions from van der Waals contacts and hydrophobic interactions. Coordination with metal ions may occasionally play a role too. Microbial surface lectins serve as a means of adhesion to host cells of viruses (e.g. influenza virus), bacteria (e.g. E. coli) and protozoa (e.g. amoeba): a prerequisite for the initiation of infection. Blocking the adhesion by carbohydrates that mimic those to which the lectins bind prevents infection by these organisms. The way is thus open for the development of anti-adhesive therapy against microbial diseases. Lectin-carbohydrate mediated interactions between leucocytes and endothelial cells are the first step in the recirculation of lymphocytes and in the migration of neutrophils to sites of inflammation. Such interactions may also feature highly in the formation of metastases. Studies of these processes are expected to lead to the development of carbohydrate-based anti-adhesion drugs for the treatment of inflammatory diseases as well as cancer.