GLYCAN STRUCTURES, CLUES TO THE ORIGIN OF SACCHARIDES
Jun Hirabayashi
Department of Biological Chemistry, Faculty of Pharmaceutical Sciences, Teikyo University
Sagamiko, Kanagawa 199-0195, Japan
Tel: 0426-85-3741

Fax: 0426-85-3742

E-mail: j-hira@pharm.teikyo-u.ac.jp(Received 1 August 2001, Accepted 11 September 2001)

Abstract
Glycans are carbohydrate chains that are considered to be one of the most essential bio-informative macromolecules aswell as nucleic acids and proteins. Although biological significance and actual states of glycans are not yet fully
understood, they certainly play fundamental roles in various recognition phenomena, such as microbe-parasite
infections, cell proliferation and differentiation, fertilization, apoptosis, cancer metastasis, etc. Distinct from nucleic
acids, glycans are expressed on cell surfaces and in extracellular matrices as various forms of glycoconjugates. They are
indispensable to cover vital cells and to protect against physical and biochemical attacks. Distinct from proteins,glycans are indirect products of so-called glycogenes, i.e., genes that encode glycosyltransfearses, glycosidases andsugar nucleotide transporters involved in glycan biosynthesis. Since individual steps of these processes are not complete,a series of glycans are produced simultaneously as a consequence of collaboration of glycogenes. Here, a"multi genesmulti glycans" principle is applied for glycan biosynthesis instead of the "one gene-one enzyme" principle for nucleic
acids and proteins.. As another unique feature of glycans, they have a number of linkage and branching isomers.
Nevertheless, sugar units, e.g., glucose (C6H12O6), are extremely simple in their compositions, reflecting formal name,"carbohydrates", which mean "C + H2O". In addition, only carbohydrates lack nitrogen, whereas amino acids andnucleotides contain this atom. On the other hand, saccharides have chirality like amino acids. Naturally occurring
saccharides are basically defined as "D-enantiomers", while L-fucose, L-rhamnose and some other L-sugars are actually
biosynthesized from either D-mannose or D-glucose. Important notation is that only few component saccharides, i.e.,
D-glucose, D-mannose and D-galactose are utilized in nature among possible 16 aldohexoses. This observation implies
that the first living organisms could make use of a relatively small number of simple saccharides that had been
sufficiently available on the prebiotic earth. In this article, the author reviews structural and metabolic features ofnaturally occurring saccharides and classic glycochemistries. Hence, he presents a possible scenario on the origin ofsaccharides consisting of i) formose reaction to generate the smallest (C3) sugars, ii) aldol condensation between
glyceraldehyde (GA) and dihydroxyacetone (DHA) to yield few ketohexoses, and iii) Lobry de Bruyn rearrangement toconvert fructose into glucose. The scenario clearly explains how and why only a few elementary saccharides were bornand selected. On the other hand, galactose is categorized into "late-comer" saccharides together with many other"bricolage" saccharides, such as ribose, sialic acid and more unusual deoxy- and dideoxyaldohexoses. In the end, theauthor refers to the essence of "glycome project", which is an emerging field of glycobiology along with the concept of
post-genome science.
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Fig. 9. Structures of galactose-containing glycotope.
Representative blood group antigens are shown. Data fromref. [19].

Fig. 8. Biosynthetic pathways of protein N-glycosylation(asparagine-linked saccharides) in yeast (upper) and mammal(lower). Both organisms utilize the same starting material, i.e.,
Glc3Man9GlcNAc2-dolicol, but after the removal of outer Glc3M
they take completely different pathways. As a result, in yeast,"high-mannose type" structures lacking galactose are constructed,
whereas in mammal the common Man9GlcNAc2 precursor
oligosaccharide undergoes "processing" and subsequent transfer of
GlcNAc and galactose to convert the oligosaccharides into mature"complex-type" structures.

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Viva Origino 29 (2001) 119 - 133
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¢/£¦Âô{nr|5Fig. 10. Two alternative biosynthetic pathways for epimerization.
(A) This type of epimerization proceeds via a common linear
enediol 6-phosphate intermediate, from which both ketose(fructose) and aldoses (glucose and mannose) are generated. The
reaction essentially the same as Lobry de Bruyn-Alberda van
Eckenstein rearrangement [17]. (B) This type of epimerization
proceeds via the 4-keto-intermediate of nucleotide-sugars. For theformation of the 4-keto-intermediate, NAD+ is utilized, and upon
its reduction with NADH 4-epimerization is achieved. Such 4-ketointermediates are variously utilized not only for epimerization of 4OH but that of 3-OH and 5-CH2OH, as well as for the formation of
deoxysaccharides, such as L- fucose, L-rhamnose, and various
dideoxyaldohexoses listed in Table 3.

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Fig. 11. Lobry de Bruyn-Alberda van Eckenstein rearrangement.
According to Wolform (1928), he obtained glucose (63.5%),
mannose (2.5%) and fructose (31%) as a result of reaction of
glucose in saturated lime water at 35; for a week [11].

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lóI(b¡³Û/^ocþÀ÷Fig. 12. Aldol condensation between DHA and GA as reverse
glycolysis. Fischer et al [1] demonstrated that only fructose andsorbose having trans-configurations as regards 3-OH and 4-OH are
generated as a result of aldol condensation between DHA and GA.
Importantly, fructose can be converted into stable aldohexoses,
glucose or mannose by Lobry de Bruyn rearrangement, whereas
sorbose cannot yield such stable aldohexoses. If any, gulose or
idose, neither of them naturally occurr.

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27/D

Fig. 13. Summary of proposed scenario on the origin of saccharides.
The figure is constructed originally on the basis of the previous
paper [19]. As a result of chemical evolution on the prebiotic earth,formose reaction generated small saccharides including GA and
DHA, which subsequently undergo aldol condensation yielding few
ketohexoses. Among the products, only fructose could be converted
into the most stable aldohexose glucose and also mannose, and thus,
these saccharides formed the first triplet. As the first metabolic
pathways were developed by the first living organisms, glucose andmannose were maximally utilized, and a number of "bricolage"
saccharides were produced that included galactose as a"recognition" saccharide (glycotope) and ribose as a framesaccharide in nucleic acids. In this context, galactose should be
regarded as "late-comer" saccharide relative to glucose andmannose, both classified into the same aldohexose group.

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