ABSTRACT
To utilize the structural diversity of sugar structures in the biological information transfer network, another type of biomolecules-lectins-was evolved by nature of nonimmune origin. Lectins are proteins or glycoproteins having specicity for sugars, but unlike
the sugar binding enzymes, they retain the structural integrity of the sugar even after binding. Thus, lectin has a carbohydrate recognition domain (CRD) without any catalytic activity. Since its rst historical identi- cation as hemagglutinin, new ndings of lectin-sugar interplay in glycobiology have upgraded the knowledge base of a range of complex biological mechanisms associated with molecular and cellular crosstalks relevant in immunology, developmental biology, hostpathogen interaction, and so on. Lectins are ubiquitous in nature and much research endeavor has gone into their identication and biological characterization. However, only a comparatively small volume of literature is available for lectin-sugar interactions evolved in marine invertebrates. Many of these lectins are protective molecules against the predators or pathogens and can be promising sources of new biologically active molecules. Among the lectins from marine invertebrates, sponge-derived lectins and their biological and biopharmaceutical activities will be discussed in this chapter. The natural habitat of marine invertebrates often has a very high microbial load of 106 bacteria and 109 viruses per milliliter of seawater. Lacking the adaptive immunological machinery, marine invertebrates such as sponges have to solely depend on their innate immunity. Invertebrate lectins are found to play an essential role in their innate immune system. Sponges, the primitive metazoans, are lter feeders and do not have any true circulatory system other than water
38.1 Introduction ................................................................................................................................................................... 503 38.2 Biological activity of the sponge lectins .................................................................................................................... 504
38.2.1 Sponge lectins and their immunological aspects ........................................................................................ 504 38.2.2 Sponge lectin in protection against pathogens ............................................................................................ 505 38.2.3 Sponge lectin in anti-predation ...................................................................................................................... 506 38.2.4 Sponge lectin for symbiosis ............................................................................................................................. 506 38.2.5 Sponge lectin as antioxidant or protection from ultraviolet damage ....................................................... 507 38.2.6 Sponge lectin having glutamate receptor activity ....................................................................................... 507 38.2.7 Sponge lectins with antitumor activity ......................................................................................................... 508
38.3 Summary ........................................................................................................................................................................ 508 References ................................................................................................................................................................................ 509
circulatory systems. Therefore, they have to circulate enormous amounts of water through their aquiferous system and thus are exposed to a high risk of infection. Therefore, they have evolved a chemical defense system to protect themselves. It has been found that lectins are the major components of this chemical defense shield (Proksch 1994). Sponges are also equipped with efcient humoral and cellular immune mechanisms that show astoundingly high sequence similarity with human immune molecules, and this acts as an impetus for research in these areas with the hope that sponge lectins might have a similar type of biological response and relevance in humans (Muller et al. 1999). Table 38.1 summarizes some of the lectins isolated from marine sponge species along with their carbohydrate specicity and structural units.
