GLYCAN STRUCTURES, CLUES TO THE ORIGIN OF ...

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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: [email protected] (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 as well 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 and sugar 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 genes- multi 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 (C 6 H 12 O 6 ), are extremely simple in their compositions, reflecting formal name, "carbohydrates", which mean "C + H 2 O". In addition, only carbohydrates lack nitrogen, whereas amino acids and nucleotides 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 of naturally occurring saccharides and classic glycochemistries. Hence, he presents a possible scenario on the origin of saccharides 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 to convert fructose into glucose. The scenario clearly explains how and why only a few elementary saccharides were born and 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, the author refers to the essence of "glycome project", which is an emerging field of glycobiology along with the concept of post-genome science. ❆❖✔ ▼❍❅■✔ ❇❍■❆✔ ❍❆❂❄❏❊✔ ❂❅✔ ❄❅❆■❂❏❊✔ ❃❍❖✔ ❅❆✔ ✯❍❑❖❍❆❂❍❍❂❈❆❆❏✔ ❂❏❆❄❆❍ ❃❍❊❄❂❈❆✔❈❖❄❆

Transcript of GLYCAN STRUCTURES, CLUES TO THE ORIGIN OF ...

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: [email protected]

(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 as

well 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 and

sugar 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 genes-

multi 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 and

nucleotides 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 of

naturally occurring saccharides and classic glycochemistries. Hence, he presents a possible scenario on the origin of

saccharides 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 to

convert fructose into glucose. The scenario clearly explains how and why only a few elementary saccharides were born

and 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, the

author 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. 1. Structures of naturally occurring saccharides. For simplicity,all of the structures are drawn as β-anomers.

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Fig. 2. Basic pathways proposed for formose reaction [2]. In thepresence of an appropriate base catalyst, formaldehyde undergoesself-condensation reaction, yielding a series of saccharides. Amongthese, generation of C5 and C6 saccharides is estimated to be theresult of aldol condensation between C2 and C3, and between C3and C3 saccharides, respectively.

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Fig. 3. Friendly relationship between water and saccharide. Watermolecules tend to form a favored "tridymite" structure, which ismaintained by the presence of equatorial hydroxyl groups such asβ-glucose.

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Fig. 4. Various forms of D-glucose. By way of the intermediatelinear (aldehyde) form, D-glucose is convertible into either α/β-furanoses (left) or α/β−pyranoses (right).

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Fig. 5. Variation of glucose polymers. By the presence of variablelinkage patterns, i.e., α1-4/6, β1-3/4/6, different types of naturallyoccurring glucose polymers (glucans) are generated, of whichbiological functions are distinct. For example, α1-4 glucan(amylose) is soluble and an essential energy source, while β1-4glucan (cellulose) forms a mechanically stable insoluble fiberhardly digested by most organisms.

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Fig. 6. Image of the hierarchy presumed for three componentsaccharides, glucose, mannose and galactose. Glucose (mostly inthe form GlcNAc) is most fundamental, and mannose is second toit. On the other hand, galactose is distinct from these saccharides inboth chemical and biological aspects (see text). Sialic acids and L-fucose are rather optional, but actually play critical roles asmodifiers of galactose.

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Fig. 7. Linear forms and C1 chair forms of 8 possible D-aldohexoses. Naturally occurring glucose, mannose and galactoseare shown in bold face. In each linear form, OH group(s) epimericrelative to glucose are shown with solid box(es). In each chair form,axial OH-group(s) are shown in red for emphasis, and hydrogenatom(s) which cause thermodynamically unfavored 1, 3-diaxialinteractions are shown in bold bars (H is not shown). It is obviousthat the numbers of such interactions are minimal for glucose (0),mannose (1) and galactose (1). For simplicity, all of the pyranosestructures are drawn as C1 conformers of β-anomers.

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Hiroko ETO
- 124 -
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Viva Origino 29 (2001) 119 - 133

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Hiroko ETO
- 125 -
Hiroko ETO
Viva Origino 29 (2001) 119 - 133

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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 Glc3Mthey take completely different pathways. As a result, in yeast,"high-mannose type" structures lacking galactose are constructed,whereas in mammal the common Man9GlcNAc2 precursoroligosaccharide undergoes "processing" and subsequent transfer ofGlcNAc and galactose to convert the oligosaccharides into mature"complex-type" structures.

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Fig. 9. Structures of galactose-containing glycotope.Representative blood group antigens are shown. Data fromref. [19].

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Hiroko ETO
- 126 -
Hiroko ETO
Viva Origino 29 (2001) 119 - 133

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Fig. 10. Two alternative biosynthetic pathways for epimerization.(A) This type of epimerization proceeds via a common linearenediol 6-phosphate intermediate, from which both ketose(fructose) and aldoses (glucose and mannose) are generated. Thereaction essentially the same as Lobry de Bruyn-Alberda vanEckenstein rearrangement [17]. (B) This type of epimerizationproceeds via the 4-keto-intermediate of nucleotide-sugars. For theformation of the 4-keto-intermediate, NAD+ is utilized, and uponits reduction with NADH 4-epimerization is achieved. Such 4-keto-intermediates are variously utilized not only for epimerization of 4-OH but that of 3-OH and 5-CH2OH, as well as for the formation ofdeoxysaccharides, such as L- fucose, L-rhamnose, and variousdideoxyaldohexoses listed in Table 3.

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Hiroko ETO
- 127 -
Hiroko ETO
Viva Origino 29 (2001) 119 - 133

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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 ofglucose in saturated lime water at 35; for a week [11].

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Hiroko ETO
- 128 -
Hiroko ETO
Viva Origino 29 (2001) 119 - 133

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Fig. 12. Aldol condensation between DHA and GA as reverseglycolysis. Fischer et al [1] demonstrated that only fructose andsorbose having trans-configurations as regards 3-OH and 4-OH aregenerated 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, whereassorbose cannot yield such stable aldohexoses. If any, gulose oridose, neither of them naturally occurr.

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Hiroko ETO
- 129 -
Hiroko ETO
Viva Origino 29 (2001) 119 - 133

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Fig. 13. Summary of proposed scenario on the origin of saccharides.The figure is constructed originally on the basis of the previouspaper [19]. As a result of chemical evolution on the prebiotic earth,formose reaction generated small saccharides including GA andDHA, which subsequently undergo aldol condensation yielding fewketohexoses. Among the products, only fructose could be convertedinto the most stable aldohexose glucose and also mannose, and thus,these saccharides formed the first triplet. As the first metabolicpathways 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 beregarded as "late-comer" saccharide relative to glucose andmannose, both classified into the same aldohexose group.

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Hiroko ETO
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Viva Origino 29 (2001) 119 - 133

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Viva Origino 29 (2001) 119 - 133

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Hiroko ETO
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Hiroko ETO
Viva Origino 29 (2001) 119 - 133

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Hiroko ETO
- 133 -
Hiroko ETO
Viva Origino 29 (2001) 119 - 133