Abstract
Galactose and sialic acids function as if they are “recognition saccharides” on cell surfaces. However, does this kind of “feeling” have some scientific basis? Also, glycans are often described as if they are extremely diverse. However, we ordinarily deal with only some 10 monosaccharides and no more than several hundred glycans. Nevertheless, there has been no satisfactory explanation. Thus, we lack fundamental knowledge on saccharides. To understand the essence of glycans, the third bio-informative macromolecule, as well as nucleic acids and proteins, the time has come to face a very basic question, i. e., the origin of saccharides. In this chapter, the author attempts to answer this question through investigation of saccharide structures, classic glycochemical reactions, biosynthetic features of monosaccharides, and recognition proteins (lectins). As a consequence, a possible scenario on the origin of saccharides is presented. In the hypothesis, fructose, glucose and mannose are defined as the “first triplet” saccharides having been generated before the birth of life, whereas the others are characterized as “late-comer” saccharides as a result of the development of metabolism. The latter saccharides include galactose, xylose, L-arabinose, L-fucose, L-rhamnose, sialic acids, uronic acids, and even ribose. They can be regarded as “bricolage products” as a result of derivatization of the first triplet saccharides. Here, special emphasis is made that galactose, being one of “accepted aldohexoses” together with glucose and mannose because of their thermodynamic stability, has a clearly distinct “history” from that of glucose and mannose. This should have provided galactose with a unique character as a recognition saccharide.
