Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. These can be activated by enzyme hydrolysis, which causes the sugar part to be broken off, making the chemical available for use. Many such plant glycosides are used as medications. In animals and humans, poisons are often bound to sugar molecules as part of their elimination from the body.A glycoside is a molecule consisting of a sugar and a non-sugar group, called an aglycone. The sugar group is known as the glycone and can consist of a single sugar group or several sugar groups. The sugars is in its cyclic form and is covalently attached to the aglycon through the hydroxyl group of the hemiactal function.
There are many different kinds of aglycones. It can be a terpene, a flavonoid, a coumarin or practically any other natural occurring product (se figure 1)
The glycone can be attached to the aglycon in many different ways. The most common bridging atom is oxygen (O-glycoside), but it can also be sulphur (S-glycoside), nitrogen (N-glycoside) or carbon (C-glycoside). In general, one distinguishes between α-Glycosides and β-glycosides, depending on the configuration of the hemiactal hydroxyl group. The majority of the naturally occurring glycosides are β-glycosidesGenerally glycosides are more polar than the aglycones and as a result glycoside formation usually increases water solubility. This may allow the producing organism to transport and store the glycoside more efficiently
Many biologically active compounds are glycosides. The pharmacological effects are largely determined by the structure of the aglycone.
Triterpene and steroid glycosides are known as widespread secondary metabolites in terrestrial higher plants. Although they are comparatively rare chemical constituents in the animal kingdom, numerous marine “animal” triterpene glycosides were also isolated from sea cucumbers (Holothurioidea, Echinodermata), in which they have been found in all the orders of this class of the phylum Echinodermata and possess a strict taxonomical specificity [1,2,3,4,5,6]. Steroid glycosides from starfish (Asteroidea, Echinodermata) compose another well-known group of bioactive marine glycosides, attracting much attention from chemists and pharmacologists for a long time [7,8,9]. Marine sponge glycosides were discovered later in comparison with those from echinoderms, but compounds of this chemical class were also shown to be a large and diverse class of bioactive amphiphilic natural products. Recently, some of them were partly reviewed along with different bioactive sesterterpenes and triterpenes isolated from sponges . In the present review, we attempt to cover literature data, concerning main directions in the studies on sponge glycosides, including structural diversity, physiological activities, taxonomic distribution and biological roles. Herein, we have processed and discussed the data about four subclasses of these substances: (i) tetracyclic triterpene glycosides; (ii) other triterpene glycosides; (iii) steroid glycosides, and (iv) glycosides of non-isoprenoid aglycones, but not of such specific groups as cerebrosides, nucleosides and tetramic acid glycosides from sponges.