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Czech J. Food Sci. Vol. 23, No. 4: 129–144

Biosynthesis of Food Constituents: Saccharides. 1. , , and Related Compounds – a Review

JAN VELÍŠEK and KAREL CEJPEK

Department of Food Chemistry and Analysis, Faculty of Food and Biochemical Technology, Institute of Chemical Technology Prague, Prague, Czech Republic

Abstract

VELÍŠEK J., CEJPEK K. (2005): Biosynthesis of food constituents: Saccharides. 1. Monosaccharides, oligosaccharides, and related compounds – a review. Czech J. Food Sci., 23: 129–144.

This review article presents a survey of selected principal biosynthetic pathways that lead to the most important monosaccharides, oligosaccharides, alcohols, and cyclitols in foods and in food raw materials and informs non- specialist readers about new scientific advances as reported in recently published papers. Subdivision of the topics is predominantly via biosynthesis. Monosaccharides are subdivided to sugar phosphates, sugar nucleotides, nucleotide- interconversion pathway , nucleotide- interconversion pathway sugars, and aminosugars. The part concerning oligosaccharides deals with saccharose, , , and biosynthesis. The part devoted to sugar alcohols and cyclitols includes the biosynthetic pathways leading to glucitol, inositols, and pseudosaccharides. Extensively used are reaction schemes, sequences, and mechanisms with the involved and detailed explana- tions employing sound chemical principles and mechanisms.

Keywords: biosynthesis; monosaccharides; sugar acids; sugar alcohols; cyclitols; sugar phosphates; sugar nucleotides; aminosugars; oligosaccharides; pseudooligosaccharides

The main pathways of saccharides biosynthesis saccharides by the acetate, shikimate, mevalonate, and degradation comprise an important component and 1-deoxy-D- pathways (VOET & VOET of primary that is essential for all liv- 1990; DEWICK 2002). ing organisms. Saccharides are the first products formed in photosynthesis, and are the products 1 Monosaccharides from which plants synthesise their own reserves, structural components of cell walls (VELÍŠEK & 1.1 Sugar phosphates and sugar nucleotides CEJPEK 2005) as well as a variety of conjugates including glycoproteins, proteoglycans, and glycoli- Six- sugars () and five-carbon pids. These materials then become the foodstuffs sugars () are the most frequently encoun- 1 of other organisms. Secondary metabolites are tered monosaccharides in nature. Photosynthesis also ultimately derived from the metabolism of produces initially the three-carbon sugar D-glycer-

Partly supported by the Ministry of Education, Youth and Sports of the Czech Republic, Project MSM 6046137305.

129 Vol. 23, No. 4: 129–144 Czech J. Food Sci. aldehyde 3-phosphate (3-phosphoglyceraldehyde), D- 3-phosphate giving rise to D-ribo- which isomerises to 1,3- phos- se 5-phosphate and D-xylulose 5-phosphate. These phate (glyceron phosphate). These two molecules two pentoses form D- 5-phosphate which are used to synthesise D- 1,6-bisphosphate. is further phosphorylated to D-ribulose 1,5-bis- D-Fructose 1,6-bisphosphate is converted into phosphate and starts a new round of the Calvin D-fructose 6-phosphate by splitting off phosphate. cycle (VOET & VOET 1990). D-Fructose 6-phosphate has two alternative fates. The overview of sugar phosphates and sugar One molecule of D-fructose 6-phosphate is con- nucleotides relevant to the synthesis of cell wall verted into D-glucose 6-phosphate; the other mol- polymers and storage products in higher plants ecule of D-fructose 6-phosphate disintegrates is given in Figure 1 (REITER 2002). into D-xylulose 5-phosphate and acetyl phosphate One of the two molecules of D-fructose 6-phos- that forms D- 4-phosphate together phate (D-Fru6P) formed in the Calvin cycle is the with D-glyceraldehyde 3-phosphate. D-Erythrose net yield of photosynthesis. It reversibly isomer- 4-phosphate is coupled to one molecule of 1,3-di- ises either to D-glucose 6-phosphate (D-Glc6P), hydroxyacetone phosphate and the result is one by a sequence which effectively achieves the re- molecule of D- 1,7-bisphosphate verse of the glycolytic reactions, or to D-mannose (D-altro-hept-2-ulose 1,7-bisphosphate). After 6-phosphate (D-Man6P). The 6-phosphates splitting off one phosphate molecule, the D-se- (D-Glc6P and D-Man6P) are reversibly converted doheptulose 7-phosphate formed reacts with to the corresponding 1-phosphates (D-Glc1P and

Respiration Photosynthesis

Glycolysis Gluconeogenesis GDP-L-

D-glucose 6-phosphate D-fructose 6-phosphate D-mannose 6-phosphate

D-glucose 1-phosphate UDP-L- D-mannose 1-phosphate GDP-D-mannose

D-fructose

saccharose UDP-D-glucose UDP-D-galacturonic acid GDP-L-

UDP-D-apiose

UDP-D-galactose UDP-D-glucuronic acid UDP-D-

D-galactose 1-phosphate D-glucuronic acid 1-phosphate UDP-D-xylose L-ascorbic acid

D-galactose D-glucuronic acid UDP-L-

myo-inositol L-arabinose 1-phosphate L-arabinose

Figure 1

1 Photosynthesis is the process by which light energy is converted into chemical energy stored in ATP and NADPH. The light-independent Calvin cycle (also known as Calvin-Benson cycle), a series of biochemical reactions taking place

in photosynthetic organisms, uses the energy from these energy carriers to convert CO2 into organic compounds that can be used by the organism.

130 Czech J. Food Sci. Vol. 23, No. 4: 129–144

CH2OH CH2OP CH2OP CH2OP CH2OH O OH O O O O HO OH OH OH OH HO OH OH HO CH2OH HO OPPU HO HO HO OP OH OH EC 5.4.2.2 OH EC 5.3.1.9 EC 5.3.1.8 EC 5.4.2.8

D-glucose 1-phosphate D-glucose 6-phosphate D-fructose 6-phosphate D-mannose 6-phosphate D-mannose 1-phosphate

Figure 2

D-Man1P) by suitable phosphomutases (Figure 2). phosphate group is covalently bound to serine) Most of the enzymes involved interact metaboli- forming the intermediate dephosphoenzyme bound cally with glycolysis and gluconeogenesis through to D-glucose 1,6-bisphosphate (D-Glc1,6P) which the reversible actions of phosphomannose isomer- leads to the formation of D-Glc1P and the phos- ase (EC 5.3.1.8), phosphoglucose (EC phoenzyme is regenerated (VOET & VOET 1990) 5.3.1.9), phosphoglucomutase (EC 5.4.2.2), and (Figure 4). phosphomannomutase (EC 5.4.2.8) (Figure 2). The 1-phosphate sugars are further converted Conversions of aldose-aldose and aldose- by the reaction with the respective 5´-nucleo- are base-catalysed reactions that proceed through side triphosphate (SEIFERT 2004) to metaboli- the open-chain endiolate stabilised by hydrogen cally active forms, nucleotide sugars, such as bonds (VELÍŠEK 2002) (Figure 3). adenosinediphospho-D-glucose (ADP-D-Glc), These isomerisations are also metabolically frequent cytidinediphospho-D-glucose (CDP-D-Glc), guani- reactions catalysed by specific . For exam- dinediphospho-D-glucose (GDPD-Glc), and uridine- ple, the glycolytic phosphoglucose isomerase diphospho-D-glucose (UDP-D-Glc) (Figure 5). The (EC 5.3.1.9) catalyses isomerisation of D-Glc6P to reaction is catalysed by suitable nucleotidylyltrans- D-Fru6P and vice versa. The enzyme obvi- ferases that have an important role not only for ates the need for a strong base (B:) to achieve the the initial activation of sugars in the production of formation of the endiolate anion (DEWICK 2002). nucleotide sugars, but also for the scavenging or Transformation of D-Glc6P to D-Glc1P and vice recycling of the sugars released from glycosides, versa is a reaction catalysed by phosphogluco- oligosaccharides, and . The type mutase (EC 5.4.2.2). The C-1 hydroxyl group of of nucleotide employed depends on the biosyn- Glc6P attacks the phosphorylated enzyme (its thetic pathway and the that is synthesised.

1,2-enolisation BH CH2OP B B CH2OP OH O CHO CH O CH O H CH OH O H H OH OH H C OH C O C O CO CH2OH HO O H R R R R OH OH

�-D-glucopyranose (Z)-endiolate �-D-fructofuranose

Figure 3

enzyme CH2OP enzyme CH2OH enzyme CH2OP

CH2OP CH2OP CH2OH O O O OH OH OH HO OH HO OP HO OP OH OH OH

D-glucose 6-phosphate D-glucose 1,6-bisphosphate D-glucose 1-phosphate

Figure 4

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CH OH O CH2OH UTP PP 2 O O NH OH OH OO HO HO OP EC 2.7.7.9 O P OPO CH2 O N O OH OH OHOH

D-glucose 1-phosphate uridinediphospho-D-glucose OHOH

CH2OH ATP PP CH2OH NH2 O O N OH OH OO N HO HO O P EC 2.7.7.27 O P OPO CH2 O N N OH OH OHOH

D-glucose 1-phosphate adenosinediphospho-D-glucose OHOH CH OH 2 GTP PP CH2OH O O O N OH HO OH HO OO NH HO OP HO O P OPO CH N EC 2.7.7.22 2 O N NH2 OHOH D-mannose 1-phosphate guanidinediphospho-D-mannose OHOH

Figure 5

UDP-sugars are the nucleotide sugars most fre- pathway for the synthesis of UDP-D-Glc is catalysed quently employed. by the degradative enzyme synthase (EC The two major points of direct biosynthesis of 2.4.1.13). This enzyme converts UDP and sac- nucleotide sugars involved in the synthesis of the charose into UDP-D-Glc and D-Fru. The sucrose cell wall material from phosphorylated mono- synthase-catalysed metabolism is thus linked with saccharides are the synthesis of UDP-D-Glc and biosynthetic processes (i.e. cell wall or storage the synthesis of GDP-D-Man (REITER & VANZIN products) (KOCH 2004). 2001; REITER 2002; SEIFERT 2004). UDP-D-Glc is Free monosaccharides and alduronic acids formed from D-Glc1P and uridine-5´-triphosphate formed from homoglycosides (oligosaccharides, (UTP) in a reaction catalysed by UDP-D-glucose polysaccharides) and heteroglycosides by hydro- pyrophosphorylase (EC 2.7.7.9) and the formation lases and/or by transglycosylation reactions can of GDP-D-mannose (GDP-D-Man) from D-Man1P be transformed to the corresponding phosphates is catalysed by GDP-D-mannose pyrophosphory- by the sequential action of ki- 2 lase (EC 2.7.7.22) . Analogously, ADP-D-glucose nases and nucleotide sugar pyrophosphorylases pyrophosphorylase (EC 2.7.7.27) catalyses the via so-called salvage pathways (SEIFERT 2004). conversion of D-Glc1P and adenosine-5´-triphos- Although a general C-6 (EC 2.7.1.1) phate (ATP) into inorganic diphosphoric acid may operate on all available sugars, C-6 (PP) and ADP-D-Glc which is involved in for specific sugars do exist. Thus, formation of biosynthesis (Figure 5). Diphosphoric acid can be D-Glc6P from D-Glc is catalysed by hexokinase hydrolysed to two molecules of phosphoric acid (EC 2.7.1.1) and, in vertebrates and , by by diphosphatase (EC 3.6.1.1). (EC 2.7.1.2), formation of D-Fru6P In plant cells that grow rapidly and thus synthe- from D-Fru by (EC 2.7.1.4), D-Man6P sise the cell wall material, including those under- from D-Man by hexokinase (EC 2.7.1.1) and man- going secondary wall thickening, an alternative nokinase (EC 2.7.1.7).

2Analogous pyrophosphorylases act e.g. on D-Gal1P (UTP- 1-phosphate uridylyltransferase, EC 2.7.7.10), D-Xyl1P (UTP-xylose 1-phosphate uridylyltransferase, EC 2.7.7.11) and other sugars.

132 Czech J. Food Sci. Vol. 23, No. 4: 129–144

ATP ADP CH OH CH2OH 2 1.2 D-Galactose, uronic acids, pentoses, HO HO O O and L-rhamnose (UDP-glucose OH OH OH interconversion pathway) OP EC 2.7.1.6 The activation of D-Glc via UDP-D-Glc pyro- OH OH (EC 2.7.7.9) or D-galactose D-galactose 1-phosphate (EC 2.4.1.13) yields a cytoplasmatic pool of UDP- D-Glc which functions, e.g., as a donor of glucose Figure 6 for the synthesis of and . UDP- D-Glc also serves as a for the synthesis D-Galactose 1-phosphate (D-Gal1P) forms from of other nucleotide sugars and nucleotide uronic D-galactose (D-Gal) in a reaction catalysed by ga- acids required for the formation of cell wall matrix lactokinase (EC 2.7.1.6) (Figure 6), the formation components, e.g. UDP-D-Gal, UDP-D-glucuron- of L-fucose 1-phosphate (L-Fuc1P) from L-fucose ic acid (UDP-D-GlcA), UDP-D-galacturonic acid (L-Fuc) is catalysed by fucokinase (EC 2.7.1.52), (UDP-D-GalA), and UDP-L-rhamnose (UDP-L-Rha) L-arabinose 1-phosphate (L-Ara1P) arises from (SEIFERT 2004) (Figure 7). L-arabinose (L-Ara) under the catalysis of arab- UDP-D-glucose can be converted to UDP-D- inokinase (EC 2.7.1.46). D-Glucuronic acid 1-phos- Gal via a freely reversible 4-epimerisation reac- phate (D-GlcA1P) forms from D-glucuronic acid tion involving an enzyme-bound (UDP-D-glucose (D-GlcA) in a reaction catalysed by glucuronokinase 4-epimerase, EC 5.1.3.2) 4-oxo intermediate, (EC 2.7.1.43) and D-galacturonic acid 1-phos- UDP-D-xylo-hexopyranos-4-ulose (called UDP- phate (D-GalA1P) forms from D-galacturonic acid 4-dehydro-D-glucose in biochemical literature) (D-GalA) under the catalysis with galacturonoki- (SEIFERT 2004). Most of the UDP-D-Gal is ulti- nase (EC 2.7.1.44). For simplicity, only D-Gal and mately needed for the synthesis of arabinogalactan- L-Ara salvage pathways are shown in Figure 1 (SEI- proteins and cell wall polysaccharides including FERT 2004). xyloglucan, rhamnogalacturonan I, rhamnoga- However, the primary route of synthesis for most lacturonan II, and the storage ga- nucleotide monosaccharides and the correspond- lactomannan (e.g. in guar gum). In green tissues, ing uronic acids is the modification of the sugar substantial amounts of UDP-D-Gal are also needed moiety in UDP-Glc or GDP-Man by oxidation, for the synthesis of chloroplast galactolipids. reduction, isomerisation, and/or decarboxylation L-Rhamnose (L-Rha) is a predominant 6-de- reactions. oxyhexose in most higher plants, and represents

CH2OH CH3 NADPH + H NADP HO O O HO O OH OH O CH3 OPPU OPPU OPPU OH OH OH OH

UDP-D-galactose UDP-6-deoxy-D-xylo-hexos-4-ulose UDP-L-rhamnose

NADP

EC 5.1.3.2 H2O EC 4.2.1.76

NADPH + H

2 NADH + H COOH CH2OH NADPH + H NADP CH2OH 2 NAD O O O O OH OH OH OPPU EC 5.1.3.2 HO OPPU EC1.1.1.22 HO OPPU OH OH OH

UDP-D-xylo-hexos-4-ulose UDP-D-glucose UDP-D-glucuronic acid

Figure 7

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COOH ATP ADP UTP PP COOH O2 H2O COOH OH O O O OH HO OH OH OH OH HO HO HO HO OPPU EC 1.13.99.1 OH EC 2.7.1.43 O P EC 2.7.7.44 OH OH OH OH

myo-inositol D-glucuronic acid D-glucuronic acid 1-phosphate UDP-D-glucuronic acid

Figure 8 a major component of the pectic polysaccharide UDP-D-glucuronic acid represents a major branch- rhamnogalacturonan I and rhamnogalacturonan II. ed-point in the biosynthesis of several other nu- L-Rhamnosyl residues are also often conjugated to cleotide sugars (SEIFERT 2004). Higher plants secondary metabolites yielding the correspond- incorporate large amounts of D-GalA residues in ing rhamnosides (SEIFERT 2004). The irreversible the backbones of pectic material and the intercon- conversion of UDP-D-Glc to UDP-L-Rha proceeds version between UDP-D-GlcA and UDP-D-GalA via UDP-6-deoxy-D-xylo-hexopyranos-4-ulose is freely reversible (Figure 9). It is catalysed by (UDP-4-dehydro-6-deoxy-D-glucose or UDP-4-de- UDP-D-galacturonic acid 4-epimerase (EC 5.1.3.6). hydro-D-chinovose) by L-rhamnose synthase, The 4-epimerisation reaction proceeds via a 4-oxo which hypothetically consists of sequentially acting intermediate, UDP-D-xylo-hex-4-ulopyranosic UDP-D-glucose 4,6-dehydratase (EC 4.2.1.76) and acid (UDP-4-dehydro-D-glucuronic acid), which is UDP-6-deoxy-D-xylo-hexos-4-ulose 3,5-epime- stereospecifically reduced by the enzyme prosthetic rase-4-reductase (UDP-4-oxo-6-deoxy-D-glucose group NAD(P)H + H+ to UDP-D-GalA. 3,5-epimerase-4-reductase). UDP-D-glucuronic acid also serves as the precur- Higher plants incorporate large amounts of D-GluA sor for the synthesis of UDP-D-xylose (UDP-D-Xyl), residues into cell wall glucuronoarabinoxylans. UDP-L-arabinose (UDP-L-Ara), and UDP-D-apiose D-GluA forms irreversibly from UDP-D-Glc by (UDP-D-Api) (Figure 9). UDP-D-glucuronate de- UDP-D-glucose dehydrogenase (EC 1.1.1.22) carboxylase (synonymous with UDP-D-xylose (Figure 7) (DUMVILLE & FRY 2000). Biogenesis synthase, EC 4.1.1.35) converts UDP-D-GluA into of UDP-D-GlcA can also occur via an alternative UDP-D-Xyl in an essentially irreversible reaction. pathway from myo-inositol, which involves the The interconverting enzyme uses an NAD(P)+ co- action of myo-inositol oxygenase (EC 1.13.99.1) factor to generate a transient 4-oxo intermediate, and the sequential action of D-glucuronokinase UDP-D-xylo-hex-4-ulopyranosic acid (UDP-4-de- (EC 2.7.1.43) and UDP-glucuronic acid pyrophos- hydro-D-glucuronic acid). This intermediate loses β phorylase (EC 2.7.7.44) (Figure 8). CO2 in an elimination reaction typical of -oxo

COOH COOH CO2 HO O O O O OPPU OH OH OH + CH2OH OPPU EC 5.1.3.6 HO OPPU HO OPPU OH OH OH OHOH

UDP-D-galacturonic acid UDP-D-glucuronic acid UDP-D-xylose UDP-D-apiose

CO2

O HO O OH OH

HO OPPU EC 5.1.3.5 OPPU OH OH

UDP-D-xylose UDP-L-arabinose

Figure 9

134 Czech J. Food Sci. Vol. 23, No. 4: 129–144

CO NAD(P) NAD(P)H + H O 2 COOH O H O O O OH O OH HO OH HO OPPU OPPU OPPU OH OH OH

UDP-D-glucuronic acid UDP-D-xylo-hex-4-ulosic acid

NAD(P)H + H NAD(P) O O OH O OH HO OPPU OPPU OH OH

UDP-D-xylose UDP-L-threo-pentos-4-ulose

Figure 10 acid and forms UDP-L-threo-pentopyranos-4- UDP-D-xylose is reversibly converted to UDP-L- ulose (UDP-4-dehydro-D-xylose), which is then Ara by UDP-D-xylose 4-epimerase (EC 5.1.3.5) via a stereospecifically reduced to yield UDP-D-Xyl common 4-oxo intermediate UDP-L-threo-pentos-4- (Figure 10). ulose (UDP-4-dehydro-D-xylose) (SEIFERT 2004). The second bifunctional decarboxylase UDP-D- apiose/UDP-D-xylose synthase (also referred to as 1.3 L-Galactose, L-, and L-fucose UDP-D-glucuronate cyclase) converts UDP-D-GlcA (GDP-mannose interconversion pathway) into approximately equimolar amounts of UDP-D- Xyl (Figure 9) and UDP-D-Api (MøLHøJ et al. 2003). GDP-D-mannose (GDP-D-Man) is an activated In most higher plants, D-Api is specifically found form of D-mannose for the incorporation into in complex polysaccharide rhamnogalacturonan N-linked glycans and mannose-containing cell II. This transformation proceeds via UDP-L-threo- wall components such as glucomannans and ga- pentopyranos-4-ulose (Figure 11). It is believed that lactomannans. GDP-D-mannose also serves as a it may involve aldol cleavage between C-2 and C-3 substrate for nucleotide sugar interconversion followed by isomerisation and aldol condensation enzymes yielding GDP-L-fucose (GDP-L-Fuc) and between C-2 and C-4. The aldehyde group at C-3 GDP-L-galactose (GDP-L-Gal) (Figure 12). The of the resulting intermediate would be converted latter compound serves as the donor for glyco- to hydroxymethyl group by the transiently reduced syltransferases but it also plays a key role in the NAD(P)H+ of the enzyme. biosynthesis of L-ascorbic acid (SEIFERT 2004).

H H

O O O O O OH O O OPPU OPPU OPPU HO OPPU OH OHO OHO O O

UDP-L-threo-pentos-4-ulose

NAD(P)H + H NAD(P) O CH O O O O CH CH2OH HO OPPU OPPU OPPU O OH OH OH OH

UDP-D-apiose

Figure 11

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CH2OH O O O CH OH CH2OH OH HO 2 OH HO HO HO OPPG EC 5.1.3.18 HO OPPG EC 5.1.3.18 HO OPPG OH GDP-GDP-LL-gulose-gulose GDP-GDP-DD-mannose-mannose GDP-GDP-LL-galactose-galactose

EC 4.2.1.47 H2O

CH3 NADPH + H NADP O O OH HO CH3 O HO OPPG EC 1.1.1.271 HO OPPG OH

GDP-6-deoxy-D-lyxo-hexos-4-ulose GDP-L-fucose

Figure 12

GDP-L-fucose formation requires subsequent rial cell walls (mureins), and activities of GDP-D-mannose 4,6-dehydratase (mucopolysaccharides) that function in various (EC 4.2.1.47) and a bifunctional GDP-L-fucose syn- biological systems as components of proteogly- thase (EC 1.1.1.271). The intermediate, UDP-6-de- cans (hyaluronic acid and dermatan sulfate). oxy-D-lyxo-hexopyranos-4-ulose (4-dehydro-6-de- N-Acetyl-D-galactosamine (GalNAc) is a building oxy-D-mannose), formed by the 4,6-dehydratase unit of proteoglycans (chondroitin sulfate and reaction, undergoes a 3,5-epimerisation followed dermatan sulfate). by a C-4 reduction yielding L-Fuc. The fucosylated The starting compound for the synthesis of side chains of xyloglucans are believed to enhance N-acetyl-D-glucosamine (2-acetamido-2-deoxy-D- the rate of formation of strong xyloglucan-cel- glucose, UDP-GlcNAc) is D-Fru6P (Figure 13). It is lulose interactions. first converted to D-glucosamine 6-phosphate in a GDP-L-galactose, on the other hand, is formed reaction with L-glutamine catalysed by glucosamine by GDP-D-mannose 3,5-epimerase (EC 5.1.3.18), 6-phosphate synthase (EC 2.6.1.16)3. D-Glucosamine which also generates GDP-L-gulose (GDP-L-Gul). 6-phosphate is acetylated and forms N-acetyl- This conversion represents a 3,5-epimerisation via D-glucosamine 6-phosphate using acetyl-CoA enol intermediates. L-Galactose is incorporated as the acetyl group donor (glucosamine 6-phos- into xyloglucans and N-linked glycans. phate N-acetyltransferase, EC 2.3.1.4), N-acet- yl-D-glucosamine 6-phosphate is converted to 1.4 Aminosugars N-acetyl-D-glucosamine 1-phosphate by phos- phoacetylglucosamine mutase (EC 5.4.2.3). 1.4.1 Aldosamines and related compounds α-D-Glucosamine 1,6-phosphomutase (EC Aminosugars such as D-glucosamine (2-ami- 5.4.2.10) catalyses the isomerisation of D-glu- no-2-deoxy-D-glucose), D-galactosamine (2-ami- cosamine 6-phosphate to D-glucosamine 1-phos- no-2-deoxy-D-galactose), and D-mannosamine phate in the pathway leading to bacterial cell wall (2-amino-2-deoxy-D-mannose) and their N-acetyl peptidoglycan and lipopolysaccharide biosyntheses. derivatives occur as principal components of vari- D-Glucosamine 1-phosphate is then acetylated to ous biologically active oligosaccharides and bi- N-acetyl-D-glucosamine 1-phosphate by D-glu- opolymers (VOET & VOET 1990; DEWICK 2002; cosamine 1-phosphate N-acetyltransferase (EC VELÍŠEK 2002). For instance, N-acetyl-D-glucos- 2.3.1.157). N-acetyl-D-glucosamine 1-phosphate amine (GlcNAc) is a building unit of milk oli- reacts with UTP yielding UDP-N-acetyl-D-glu- gosaccharides, , peptidoglycans of bacte- cosamine (UDP-GlcNAc pyrophosphorylase, EC

3The reverse reaction, i.e. hydrolysis of N-acetyl-D-glucosamine to D-fructose 6-phosphate and ammonia is catalysed by glucosamine 6-phosphate deaminase (EC 3.5.99.6); hydrolysis of N-acetyl-D-glucosamine 6-phosphate to D-glu- cosamine 6-phosphate is provided by N-acetylglucosamine 6-phosphate deacetylase (EC 3.5.1.25).

136 Czech J. Food Sci. Vol. 23, No. 4: 129–144

L-glutamine L-glutamic acid CH2OP CH OP 2 OH O O OH HO OH HO CH2OH EC 2.6.1.16 OH NH2

D-fructose 6-phosphate D-glucosamine 6-phosphate acetyl-CoA

EC 5.4.2.10 EC 2.3.1.4 HS-CoA

CH2OH CH2OP O OH OH OH HO OP HO NH2 NH

D-glucosamine 1-phosphate O EC 2.3.1.157 EC 5.4.2.3 CH3 CH OH acetyl-CoA 2 N-acetyl-D-glucosamine 6-phosphate O OH

HS-CoA HO OP NH

O CH3 N-acetyl-D-glucosamine 1-phosphate

UTP

EC 2.7.7.23

PP CH OH CH2OH CH2OH 2 O HO O O O CH OH OH OH HN 3 HO OPPU EC 5.1.3.7 HO OPPU EC 5.1.3.14 OPPU NH NH

O O CH3 CH3

UDP-N-acetyl-D-galactosamine UDP-N-acetyl-D-glucosamine UDP-N-acetyl-D-mannosamine

2 NAD + H2O EC 1.1.1.136

2 NADH + 2 H COOH O OH

HO OPPU NH

O CH3 UDP-N-acetyl-2-amino-2-deoxy-D-glucuronic acid

Figure 13

2.7.7.23). This activated form of N-acetyl-D-glu- 6-dehydrogenase, EC 1.1.1.136), UDP-N-acetyl-D-ga- cosamine can be further converted to UDP-N-acetyl- lactosamine, i.e. UDP-2-acetamido-2-deoxy-D-ga- D-glucuronic acid (UDP-N-acetylglucosamine lactose (UDP-N-acetylglucosamine 4-epimerase,

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EC 5.1.3.7), and UDP-N-acetyl-D-mannosamine, residues one or various acetyl groups may occur. i.e. UDP-2-acetamido-2-deoxy-D-mannose (UDP- In mammals, sialic acids are found at the distal N-acetylglucosamine 2-epimerase, EC 5.1.3.14). ends of cell surface conjugates, and thus are ma- Finally, various are involved in the jor determinants of specific biological functions biosynthesis of biopolymers containing amino- such as cellular adhesion, formation or masking sugars (VOET & VOET 1990). of recognition determinants and stabilisation of glycoprotein structures (SCHAUER 2004). In cer- 1.4.2 Acetylmuramic acid tain strains of pathogenic bacteria, they occur as a Bacterial cell walls contain an unusual saccharide, homopolysaccharide (polysialic acid) with α-(2→8) N-acetyl-β-muramic acid, i.e. 2-acetamido-3-O- and/or α-(2→9) ketosidic linkages in capsular [(R)-1-carboxyethyl]-2-deoxy-β-D-glucopyranose polysaccharides that mask the organism against (β-MurAc), bound in peptidoglycan structures the immune system (REVILLA-NUIN et al. 1998). called murein (DEWICK 2002; VOET & VOET 1990). N-acetylneuraminic (5-acetamido-3,5-di-deoxy- The peptidoglycan chains are composed of alter- D-glycero-D-galacto-non-2-ulopyranosonic) acid nating β-(1→4) linked N-acetyl-D-glucosamine (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc), and and N-acetylmuramic acid residues that are cross- N-acetyl-9-O-acetylneuraminic acid (Neu5,9Ac2) linked via peptide structures (through the lactyl are the three most frequently occurring members group of the N-MurAc to link the peptide via of the sialic acids family. Only Neu5Ac is ubiqui- amide/peptide bond). The peptidoglycan building tous, while the others are not found in all species stone, UDP-N-MurAc, forms from UDP-N-acetyl- (SCHAUER 2004). D-glucosamine and phosphoenolpyruvic acid N-acetyl-α-, a constituent of food via the intermediate UDP-N-acetyl-3-O-(1-car- glycoproteins (e.g. milk κ-caseins) and glycolipids boxyvinyl)-D-glucosamine (UDP-N-acetyl-D-glu- (e.g. gangliosides), forms by aldol-type reaction cosamine 1-carboxyvinyl , EC 2.5.1.7), from N-acetyl-D-mannosamine and pyruvic acid which is reduced to UDP-N-MurAc by UDP-N- (Figure 15). acetylmuramate dehydrogenase (EC 1.1.1.158) The reaction is more complex in the enzyme-cata- (Figure 14). lysed version (LUCKA et al. 1999). The two enzymes initiating the biosynthesis of Neu5Ac from N- 1.4.3 Acetylneuraminic acid acetyl-D-mannosamine (mammals), the hydrolysing Sialic acids are a family of nine carbon α-oxo acids UDP-N-acetylglucosamine 2-epimerase (EC 5.1.3.14) that play a wide variety of biological roles in higher and N-acetylmannosamine (Mg2+/K+-de- animals and some microorganisms. Sialic acids pendent enzyme, EC 2.7.1.60), are parts of one comprise about 50 members which are derivatives bifunctional enzyme that catalyses hydrolysis of of neuraminic (5-amino-3,5-dideoxy-D-glycero-D- UDP-N-acetyl-D-glucosamine, its isomerisation to galacto-non-2-ulopyranosonic) acid (Neu). They N-acetyl-D-mannosamine, and phosphorylation of carry various substituents at the amino or hydroxyl N-acetyl-D-mannosamine to N-acetyl-D-man- groups. The amino group of Neu is acetylated or nosamine 6-phosphate. The next step, aldol-type glycolated, while at all non-glycosidic hydroxyl condensation of N-acetyl-D-mannosamine 6-phos-

H2C COOH

OP H2CCOOH H3CCOOH phosphoenolpyruvic acid NADH + H NAD OH P O O

CH2OH CH2OH CH2OH HO HO HO

HN O EC 2.5.1.7 HN O EC 1.1.1.158 HN O OPPU OPPU OPPU H3CO H3CO H3CO

UDP-N-acetyl-D-glucosamine UDP-N-acetyl-3-O-(1-carboxyvinyl)-D-glucosamine UDP-N-acetylmuramic acid

Figure 14

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phosphoenolpyruvic acid

HOOC O P 1 2 HOOC CO HOOCOHC CH 2 3 CH2 CH2 P 4 CHO HOHC HOHC H C H3C H3C 5 3 NH CH NH CH NH CH O O 6 O HO CH H2O HO CH O CH 7 H C OH H C OH H C OH 8 H C OH H C OH H C OH 9 CH OH CH2OH CH2OH 2

N-acetyl-D-mannosamine N-acetylneuraminic acid N-acetyl-�-neuraminic acid (open-chain form) (open-chain form)

Figure 15 phate (open-chain form) with phosphoenolpyruvic 2 Oligosaccharides acid to N-acetylneuraminic acid 9-phosphate, is catalysed by N-acetylneuraminate 9-phosphate The formation of glycosides, oligosaccharides, synthetase (EC 2.5.1.57). Hydrolysis of this phos- and polysaccharides is dependent on the activation phate by N-acetyl-neuraminate 9-phosphatase of the respective sugar by its binding to a nucle- (EC 3.1.3.29) yields Neu5Ac. N-Acetylneuraminate oside diphosphate. Nucleophilic displacement of synthase (EC 2.5.1.56) generates Neu5Ac from the respective nucleoside diphosphate-leaving N-acetyl-D-mannosamine and phosphoenolpyruvic group by a suitable nucleophile then generates 4 acid (bacteria) (Figure 16). the new sugar derivative . This will be a glycoside

ATP ADP CH OH H2O UDP 2 CH2OH O CH2OP O O O O OH OH HN CH3 OH HN CH3 HO OPPU EC 5.1.3.14 HO OH EC 2.7.1.60 HO OH NH N-acetyl-D-mannosamine N-acetyl-D-mannosamine 6-phosphate O CH3 H2C COOH H2C COOH UDP-N-acetyl-D-glucosamine + H2O + H2O OP OP EC 2.5.1.56 phosphoenolpyruvic acid EC 2.5.1.57 phosphoenolpyruvic acid

P P

CH2OH CH2OP H OH H OH P H2O OH H OH OH H OH O O COOH COOH OH OH EC 3.1.3.29 NH NH

H CO 3 H3CO N-acetylneuraminic acid N-acetylneuraminic acid 9-phosphate

Figure 16

4Synthesis of nucleoside triphosphates from nucleoside diphosphates is realised by nucleoside-diphosphate kinases. Many ribo- and deoxyribonucleoside triphosphates can act as donors, the most usual one is ATP. For example, UTP from UDP is formed by uridine diphosphate kinase (EC 2.7.4.6) that carries the phosphate residue from ATP to UDP.

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UDP (VELÍŠEK 2002). Most photosynthetic CH2OH CH2OH H and some specific prokaryotes synthesise saccha- O RO H O OR OH OH rose. In higher plants, saccharose occupies a unique HO HO H position, comparable only to glucose in the animal OPPU OH OH world. It is a major product of photosynthesis, with a central role as a primary transport sugar, in the UDP-�-D-glucose �-D-glucoside growth, development, storage, signal transduction, Figure 17 and stress (WINTER & HUBER 2000; CUMINO et al. 2002). if the nucleophile is a suitable aglycone molecule The biosynthetic enzymes sucrose-phosphate or if the nucleophile is another synthase (EC 2.4.1.14) and sucrose-phosphate sugar molecule. phosphatase (EC 3.1.3.24), and the degradative

This reaction, if mechanistically of SN2 type, enzymes invertase (EC 3.2.1.26) and sucrose syn- 5 should give an inversion of configuration at C-1 thase (EC 2.4.1.13) have been well character- in the electrophile, generating a product with ised in plants and unicellular eukaryotes (KOCH β-configuration and nucleoside diphosphate (Fig- 2004). Biosynthesis of saccharose starts with the ure 17). Many of the linkages formed between glucose donor UDP-D-Glc that reacts with the sugar monomers actually have α-configuration, sugar acceptor β-D-Fru6P and forms saccharose F F and it is believed that a double SN2 mechanism 6 -phosphate and UDP. Saccharose 6 -phosphate operates involving also a nucleophilic group of is then hydrolysed to saccharose and phosphate the enzyme (DEWICK 2002) (Figure 18). (Figure 19).

2.1 Saccharose 2.2 Trehalose The non-reducing saccharose, β- α,α-Trehalose, α-D-fructofuranosyl-(1↔1)- D-fructofuranosyl-(2↔1)-α-D-glucopyranoside, α-D-glucopyranoside, is another non-reducing is one of the most abundant products in nature disaccharide that occurs in plants and a wide range

Enzyme Enzyme UDP CH2OH CH2OH CH2OH H X O X O O H OH OH OH HO HO HO OR OPPU H RO H OH OH OH

UDP-�-D-glucose �-D-glucoside

Figure 18

CH2OH CH2OH UDP O O OH H O CH2OH 2 P OH POCH2 O O OH HO HO OH + HO OH OH CH2O P O CH2OH O CH OH HO OPPU 2 O O OH OH EC 2.4.1.14 HO HO

CH2OH CH2OH OH OH

UDP-�-D-glucose �-D-fructose 6-phosphate saccharose 6F-phosphate saccharose

Figure 19

5The only known enzymatic paths of saccharose cleavage in plants are catalysed by invertase (EC 3.2.1.26) and reversible sucrose synthase (EC 2.4.1.13) reactions. Invertase-catalysed hydrolysis of saccharose into D-glucose and D-fructose has been associated with cell expansion, whereas sucrose synthase-catalysed decomposition into D-fructose and UDP-D-Glc has been linked with biosynthetic processes such as starch biosynthesis.

140 Czech J. Food Sci. Vol. 23, No. 4: 129–144

UDP OH OH CH2OH CH2O P CH2OH H2O P CH2OH O O O O + OH OH OH OH OH OH OH POH2C HOH C O O 2 OPPU O OH O OH HO HO EC 2.4.1.15 HO HO OH OH OH EC 3.1.3.12 OH

UDP-D-glucose D-glucose 6-phosphate �,�-trehalose 6-phosphate �,�-trehalose

Figure 20 of organisms such as bacteria, fungi, nematodes, 2.3 Raffinose and crustaceans. In addition to its function as a storage and transport sugar, α,α-trehalose plays Biosynthesis of raffinose, α-D-ga- an important role in the protection against stress lactopyranosyl-(1→6)-α-D-glucopyranosyl-(1↔2)- especially during heat stress and dehydration β-D-frucofuranoside, starts with UDP-D-Gal that, (WINGLER 2002). Biosynthesis of α,α-trehalose under catalysis with galactinol synthase (EC is a two-step reaction. It starts with UDP-D-Glc 2.4.1.123), reacts with myo-inositol and forms pseu- that reacts with D-Glc6P and forms α,α-trehalose dooligosaccharide galactinol (1-O-α-D-galacto- 6-phosphate (trehalose 6-phosphate synthase, pyranosyl-1L-myo-inositol) (LOEWUS & MURTHY EC 2.4.1.15). Trehalose 6-phosphate phosphatase 2000). The subsequent reaction of galactinol with (EC 3.1.3.12) catalyses the phosphate hydrolysis saccharose catalysed by galactinol-sucrose ga- to α,α-trehalose (Figure 20). lactosyltransferase (EC 2.4.1.82) yields raffinose

CH2OH HO O OH

O OH UDP myo-inositol saccharose myo-inositol CH2 CH2OH CH2OH OH HO O HO O OH O OH OH OH HO HO OPPU O OH OH EC 2.4.1.123 OH OHOH EC 2.4.1.82 O HOCH 2 O UDP-D-galactose galactinol HO

CH2OH raffinose OH Figure 21

UDP CH OH CH2OH CH2OH CH2OH 2 HO O O HO O O OH + OH OH OH OH OH OPPU HO EC 2.4.1.22 O OH OH OH OH

UDP-D-galactose D-glucose lactose

Figure 22

6Lactose synthase (EC 2.4.1.22) is a complex composed of two distinct protein components, a catalytic and a regulatory subunit. The catalytic subunit, UDP-galactose-glycoprotein (EC 2.4.1.38), does not catalyse the biosynthesis of lactose. It participates in the biosynthesis of oligosaccharide chains of secretory and membrane- bound glycoconjugates (e.g. glycoproteins and glycopeptides) by catalysing the transfer of Gal from UDP-Gal to the terminal N-acetyl-β-D-glucosaminyl residue. The regulatory subunit (i.e. the modifier protein α-lactalbumin) in the lactose synthase complex, reversibly binds to UDP-galactose-glycoprotein galactosyltransferase and thus promotes, in the presence of Mn2+ ions, glucose binding and facilitates the biosynthesis of lactose.

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CH O CH OH and myo-inositol is released (Figure 21). Another NAD(P)H + H NAD(P) 2 galactosyltransferase (galactinol-raffinose galac- H OH H OH tosyltransferase or synthase, EC 2.4.1.67) HO H HO H HOH HOH catalyses galactosyl transfer from galactinol to raffi- EC 1.1.1.21 nose resulting in the formation of HOH HOH stachyose, α-D-galactopyranosyl-(1→6)-α-D-ga- CH2OH CH2OH lactopyranosyl-(1→6)-α-D-glucopyranosyl-(1↔2)- D-glucose D-glucitol β -D-frucofuranoside. Figure 23

2.4 Lactose 3.2 Inositols Lactose, β-D-galactopyranosyl-(1→4)-D-gluco- Biosynthetic conversion of D-glucose 6-phos- , the major reducing disaccharide in most phate to free myo-inositol involves two enzymatic mammalian milks, is synthesised from UDP-D-Gal steps. The first step is the irreversible cyclisation of and D-glucose exclusively in the lactating mam- D-Glc6P to 1L-myo-inositol 1-phosphate (1D-myo- mary glands (BERGER & ROHRER 2003). The reac- inositol 3-phosphate). This reaction is catalysed tion is catalysed by the enzyme lactose synthase 7 6 by inositol 1-phosphate synthetase (EC 5.5.1.4) . (EC 2.4.1.22) (Figure 22). The second step, the loss of phosphate catalysed by inositol phosphatase (EC 3.1.3.25), releases free 3 Sugar alcohols and cyklitols myo-inositol (Figure 24). Altogether, this scheme constitutes the sole pathway of myo-inositol bio- 3.1 Glucitol synthesis in cyanobacteria, algae, fungi, plants, can be converted to sugar alcohols by and animals, and occupies the central role in their the widely specific (EC 1.1.1.21). cellular metabolism (LOEWUS & MURTHY 2000). This NAD(P)H-dependent enzyme acts e.g. on Functionally, the conversion of D-Glc6P to D-glucose, D-galactose, L-arabinose, D-xylose and 1L-myo-inositol 1-phosphate involves three sub some other sugars and forms the correspond- steps: NAD+-coupled oxidation of D-Glc6P at ing sugar alcohols, i.e. D-glucitol (D-), C-5, aldol condensation between C-1 and C-6 galactitol, L-arabinitol, and , respectively of D-xylo-5-hexulose 6-phosphate (5-oxo-D-glu- (Figure 23). cose 6-phosphate), NADH catalysed reduction of Several other acting with NAD+ 1L-2-myo-inosose 1-phosphate (1D-2-myo-inosose or NADP+ as acceptors catalyse the oxidation of 3-phosphate, D-2,3,6/3,5-pentahydroxycyclohexane sugar alcohols to the corresponding parent sug- 2-phosphate) to yield 1L-myo-inositol 1-phosphate ars and vice versa. For instance, NAD+-depend- (1D-myo-inositol 3-phosphate). ent (EC 1.1.1.14) also acts Only myo-inositol is biosynthesised de novo from on D-glucitol (giving D-fructose) and other closely D-Glc6P. Metabolic processing of myo-inositol then related sugar alcohols. NAD+-dependent mannitol produces many biologically important products dehydrogenase (EC 1.1.1.67) acts on D-mannitol (LOEWUS & MURTHY 2000). The oxidation of giving D-fructose (DEWICK 2002). myo-inositol gives D-GlcA (p. 134), conjugation

CH2OP NAD NADH + H O O NADH + H NAD OH 2 1 O OH CH2OP OH OH OH HO OH OH CH O OH PO 3 OH PO OH OH 5 6 HO HO EC 5.5.1.4 HO HO EC 5.5.1.4 HO 4 OH OH OH OH OH EC 3.1.3.25

D-glucose 6-phosphate 1L-2-myo-inosose 1-phosphate 1L-myo-inositol 1-phosphate myo-inositol

Figure 24

7This enzyme requires NAD+, which dehydrogenates the -CHOH- group to -CO- at C-5 of the glucose 6-phosphate, changing C-6 into an active methylene, able to condense with the –CH=O group at C-1. Finally, the enzyme-bound NADH reconverts C-5 into the -CHOH- form. The enzyme has a preference for the β-anomeric form of D-glucose 6-phosphate.

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OH OH OHOH OH OH

3 OCH3 OH OH 2 4 OH 1 OH OCH3 6 HO 5 H3CO HO OH OH OH D-bornesitol D-ononitol D-sequoyitol (1D-1-O-methyl-myo-inositol) (1D-4-O-methyl-myo-inositol) (1D-5-O-methyl-myo-inositol)

OH OH H3CO OH 3 3 2 2 4 OH 1 1 HO 4 6 6 H3CO OH HO OH 5 5 OH OH

D-pinitol L-quebrachitol (1D-4-O-methyl-chiro-inositol) (1L-2-O-methyl-chiro-inositol) Figure 25 with UDP-D-Gal forms galactinol (p. 141), the the need for counter-ions, and to avoid the mecha- galactosyl donor for biosynthesis in the raffinose nistic confusion. and galactopinitol series of pseudosaccharides. Ac acetyl The isomerisation of myo-inositol produces other ADP adenosine-5´-diphosphate stereo-forms of inositol. The methylation of myo- Api apiose inositol and other isomeric (scyllo-, chiro-, muco-, Ara arabinose and neo-) inositols leads to O-methyl inositols (bor- ATP adenosine-5´-triphosphate nesitol, ononitol, sequoyitol, pinitol, quebrachitol, CDP cytidine-5´-diphosphate etc.), which participate in stress-related responses, CoA coenzyme A as a part of a thioester storage of seed products, and the production of Fru fructose inositol-glycosides (Figure 25). Fuc fucose The research on inositol-linked stress-relat- Gal galactose ed processes in plants is still in its pioneering GalA galacturonic acid stage. For example, the biochemical conversion GalNAc N-acetyl-D-galactosamine of myo-inositol to D-pinitol is catalysed by O-me- GDP guanosine-5´-diphosphate thyl transferase yielding D-ononitol. The enzyme GTP guanosine-5´-triphosphate catalysing the next step, epimerisation of C-l of Glc glucose ononitol, has yet to be examined. This reaction GlcA glucuronic acid proceeds via 1D-4-O-methyl-1-myo-inosose as GlcNAc N-acetylglucosamine the intermediate and yields D-pinitol as the final Gul gulose product. Furthermore, myo-inositol is involved in Man mannose biosynthesis of phytic acid, phosphatidylinositol, Mur muramic acid its polyphosphates, and other lipids. Neu neuraminic acid P phosphoric acid EC (Enzyme Commission) numbers PP diphosphoric acid and some common abbreviations Rha rhamnose UDP uridine-5´-diphosphate EC (Enzyme Commission) numbers, assigned UTP uridine-5´-triphosphate by IUPAC-IUBMB, were taken from KEGG: Kyoto Xyl xylose Encyclopedia of Genes and Genomes, http://www. biologie.uni-hamburg.de. In many structures, the Reference s abbreviation P is used to represent the phosphate group and PP the diphosphate group. At physi- BERGER E.G., ROHRER J. (2003): Galactosyltransferase-still ological pH, these and some other groups will be up and running. Biochemie, 85: 261–274. ionised, but in pictures the unionised forms are CUMINO A., CURATTI L., GIARROCCO L., SALERNO G.L. depicted to simplify the structures, to eliminate (2002): Sucrose metabolism: Anabaena sucrose-phos-

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Corresponding author:

Prof. Ing. JAN VELÍŠEK, DrSc., Vysoká škola chemicko-technologická v Praze, Fakulta potravinářské a biochemické technologie, Ústav chemie a analýzy potravin, Technická 5, 166 28 Praha 6, Česká republika tel.: + 420 220 443 177, fax: + 420 233 339 990, e-mail: [email protected]

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