Polysaccharides
`
`James N BeMiller, Purdue University, Indiana, USA
`
`Polysaccharides are polymers containing from 20 (usually 4 100) to as many as 60 000
`monosaccharide units. Polysaccharides have a range of general structures (from linear
`to various branched structures) and shapes. They are structural components of cell walls
`of bacteria, fungi, algae and higher plants, are energy- and carbon-storage substances,
`and serve various other functions as extracellular materials of plants, animals and
`microorganisms. They are the most abundant (by mass) of all organic substances in
`living organisms, comprising about two-thirds of the dry weight of the total biomass.
`Some have commercial value as isolated substances.
`
`Introductory article
`
`Article Contents
`
`. Nature, Occurrence and Classification
`
`. Determination of Structure
`
`. Higher Plant Cell Wall Polysaccharides
`
`. Energy Storage Polysaccharides
`
`. Chitin and Other Fungal Cell Wall Polysaccharides
`
`. Bacterial Exopolysaccharides
`
`. Glycosaminoglycans
`
`. Polysaccharides as Industrial Gums
`
`Nature, Occurrence and Classification
`
`Polysaccharides are polymers whose monomer units are
`simple aldose (aldehyde) and/or ketose (ketone) sugars
`(monosaccharides). The number of monosaccharide units
`in polysaccharides varies from about 35 (although it is
`usually at
`least 100)
`to approximately 60 000. The
`monosaccharide units are in either five-membered (fur-
`anosyl) or six-membered (pyranosyl) ring forms, most
`often the latter. These units are joined together in a head-
`to-tail fashion by glycosidic linkages, a glycosidic linkage
`being one-half of an acetal structure, the other half being
`that which forms the pyranosyl or furanosyl ring. An
`example of a glycosidic linkage is given in Figure 1.
`Polysaccharides may be linear or branched. There are a
`variety of branched structures, including structures with
`only a few, very long branches; linear structures with short
`branches regularly spaced,
`irregularly spaced, or in
`clusters; and branch-on-branch structures with branches
`clustered or positioned to produce bush-like structures
`with or without decoration with short branches. Repre-
`sentatives of the general kinds of branching found in
`polysaccharide molecules are shown in Figure 2. Each
`polysaccharide has one, and only one, reducing end, which
`is the end terminating in a hemiacetal (or carbonyl) group
`
`O
`HO
`
`CH OH2
`O
`
`HO
`O
`
`HO
`
`OH
`
`O
`
`CH OH2
`
`Figure 1 Repeating unit structure of cellulose showing how the b-D-
`glucopyranosyl units are joined by (1!4) glycosidic linkages. The carbon
`atoms and hydrogen atoms bonded to the carbon atoms are omitted for
`clarity. The carbon atoms of each glycosyl unit are numbered, C1 being the
`anomeric carbon atom (that are on the extreme right of each unit in this
`structure), C2 being the carbon atom attached to C1, C3 being the next
`and so on to C6 in this case. The oxygen atoms attached to each carbon
`atom are numbered O1, O2, O3, etc.
`
`(designated as f in Figure 2). Each branch generates an
`additional nonreducing end, which consists of a glycosyl
`unit that is attached to another through its anomeric
`hydroxyl group (see below) but has no glycosyl unit
`attached to it, making it a chain end with no carbonyl
`group. Therefore, a polysaccharide molecule may have
`many nonreducing ends. In terms of mass, linear poly-
`saccharides are the most abundant, being structural
`components of higher plants and marine algae. However,
`there are many more branched polysaccharides than linear
`ones.
`In addition to monosaccharide units, polysaccharides
`may contain ester, ether, and/or cyclic acetal moieties.
`Ester groups include acetate, glycolate, succinate, sulfate
`and phosphate groups on the polysaccharide’s hydroxyl
`groups. Methyl and ethyl ether groups may be present.
`Pyruvic acid may be present as a cyclic acetal. In those
`polysaccharides containing amino sugars,
`the amino
`groups are usually not free but are present as amido
`groups, with the acid moiety being acetic, glycolic or
`sulfuric acid. In polysaccharides containing uronic acid
`units (sugar units with a carboxylic acid group in place of
`the hydroxymethyl group), the carboxylic acid functions
`may be present as methyl esters.
`Glycose is the generic term for a monosaccharide (simple
`sugar). A glycosyl (saccharide) unit is the group formed by
`removing the OH group from the anomeric (originally the
`carbonyl) carbon atom of a pyranose or furanose ring form
`of the sugar. A glycosamine is a monosaccharide that
`contains an amino group in place of a hydroxyl group. In
`formal carbohydrate nomenclature, it is a deoxyamino
`sugar.
`Glycan is the generic term for a polysaccharide. For
`example, a xylan is a polysaccharide made up primarily of
`d-xylopyranosyl units (it may contain minor amounts of
`other sugars); a b-glucan is constructed of b-d-glucopyr-
`anosyl units; an arabinoxylan has l-arabinose and d-
`xylose as its monomeric units; a glucuronoxylomannan
`consists of d-glucuronic acid, d-xylose, and d-mannose;
`and so on. If the polysaccharide has a backbone structure,
`
`ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net
`
`1
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`Polysaccharides
`
`Figure 2 Portions of polysaccharide molecules showing the different
`kinds of branching. f indicates the reducing end.
`
`it forms the latter part of the systematic name; for example,
`galactomannans have a main mannan chain to which are
`attached d-galactopyranosyl units; the same is true of
`arabinoxylans. Polysaccharides that were named before
`the systematic names were developed may not follow either
`of these rules; examples are cellulose, amylose, pectin,
`glycogen and alginic acid.
`Polysaccharides may be attached to proteins, in which
`case they are known as protein-polysaccharides when they
`originate from plants, or proteoglycans when they
`originate from animals. Proteoglycans have different
`overall structures, but for the most part consist of a large
`number of polysaccharide (glycan) chains on one end of a
`polypeptide backbone to form a structure like a bottle
`brush, with the carbohydrate portion accounting for as
`much as 90% of the molecular weight of the proteoglycan.
`The polysaccharide chains contain amino sugars and are
`known as glycosaminoglycans. They are also acidic
`polysaccharides, containing uronic acid units and/or
`sulfate half-ester groups.
`
`2
`
`Lipopolysaccharides are cell wall components of Gram-
`negative bacteria (see Bacterial Exopolysaccharides be-
`low). Many bacteria also produce polysaccharides that are
`excreted outside the cell wall. They may be components of a
`capsule (capsular polysaccharides) or soluble in the
`extracellular medium. In either case, they are called
`exopolysaccharides.
`A polymer called peptidoglycan is present in most,
`perhaps all, bacterial cell walls as the major structural
`component. Other carbohydrate polymers present
`in
`bacterial cell walls are teichoic acids (polymers of alditol
`phosphates) and teichuronic acids, which are linked to
`peptidoglycan.
`Fungal, including yeast, cell walls may contain one or
`more of the polysaccharides chitin, cellulose, other b-
`glucans or mannans.
`Because their synthesis does not involve a template
`molecule, polysaccharides are polydisperse; that is, mole-
`cules of a specific polysaccharide from a single source are
`present in a range of molecular weights. In addition, the
`degree of polydispersity, the average molecular weight, and
`the range of molecular weights in a polysaccharide
`preparation vary from source to source. Most polysac-
`charides are also polymolecular;
`that
`is,
`their fine
`structures vary from molecule to molecule. With the
`exception of cellulose and a few other plant polysacchar-
`ides, only bacterial polysaccharides have repeating-unit
`structures. Structures of other polysaccharides, and even
`of bacterial polysaccharides with regard to noncarbohy-
`drate components, can vary between taxa and with growth
`conditions of the plant or microorganism and even
`between tissues of the same plant.
`Polysaccharides are present in most living organisms. In
`fact, polysaccharides comprise about 70% of the dry
`weight of the total biomass. They serve a variety of
`functions, not all of which are known. They are most
`abundant in higher plants (1) as structural components of
`primary and secondary cell walls and in the middle lamella,
`(2) as reserve food materials in leaves, seeds, stems, roots,
`tubers, rhizomes and other tissues, (3) as exudates of
`unknown function, and (4) as extractable material of
`unknown function. They are cell wall constituents, non-cell
`wall constituents, and storage materials in algae. In
`microorganisms, they may be extracellular, in addition to
`being cellular constituents. The polysaccharide chitin is a
`structural component in the exoskeletons of crustaceans
`and insects.
`There is no ideal system of polysaccharide classification.
`The best system should be that based on chemical
`structure. However, because of the polymolecularity,
`which limits descriptions to statistical structures in many
`cases, and because of the great variety of structures,
`classifying polysaccharides in this way has limitations.
`Combinations of the following categories are used:
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`(1) Branching: (a) unbranched; (b) linear with short
`branches or side units (i) regularly spaced or (ii)
`irregularly spaced; (c) branches in clusters; (d) highly
`branched (branch-on-branch) structures
`(2) Different kinds of monomer units: (a) one (homo-
`glycan); (b) two (diheteroglycan); (c) three (triheter-
`oglycan);
`(d)
`four
`(tetraheteroglycan);
`(e) five
`(pentaheteroglycan)
`(3) Charge: (a) neutral; (b) anionic (acidic); (c) cationic
`
`Specific sugars and linkage types may then be used within
`each of these general groups. There may also be variability
`in any noncarbohydrate constituents. Polysaccharides
`used industrially are most often classified by source.
`Examples of a very limited number of polysaccharides
`classified in these two ways are found in Table 1 and Table 4.
`
`Determination of Structure
`
`Because polysaccharides other than bacterial polysacchar-
`ides and a very few plant polysaccharides are chemically
`heterogeneous (i.e. their structures vary in proportions of
`monosaccharide constituents and/or in proportions of
`linkage types and in noncarbohydrate groups, if present,
`from molecule to molecule, as well as in molecular weight),
`a variety of chemical and instrumental methods are used
`together to determine structures, and a most probable
`statistical structure is deduced.
`Polysaccharide preparation, whether in the laboratory
`for characterization or in commercial production, begins
`with extraction from the source in the case of a plant
`polysaccharide, or with isolation from a fermentation
`culture medium in the case of a bacterial polysaccharide. In
`laboratory preparations, extractions from a plant tissue
`are usually preceded by removal of interfering substances
`such as lipids and lignin. Extraction may be done with
`water in a few cases, but most often involves an alkaline
`solution. Both extraction and recovery from a fermenta-
`tion medium are followed by purification and fractiona-
`tion to separate
`the desired polysaccharide
`from
`noncarbohydrate materials, such as proteins, and from
`other polysaccharides. Purification most often involves
`precipitation, sometimes fractional precipitation. Precipi-
`tation is usually achieved by addition of a water-soluble
`alcohol such as ethanol (in the laboratory) or 2-propanol
`(industrially).
`Structural analysis of a polysaccharide may be under-
`taken once it is obtained in an acceptable degree of purity.
`Polysaccharides have a great variety of structures, the only
`common feature being that each is composed, at least
`primarily, of monosaccharide units. Structural character-
`ization involves determination of (1) monosaccharide
`composition; (2) linkage types; (3) ring size, i.e. pyranose
`or furanose; (4) anomeric configurations, i.e. configuration
`
`Polysaccharides
`
`at carbon atom 1 (C1) of an aldose or at carbon atom 2 (C2)
`of a ketose; (5) presence and location of substituent groups;
`and (6) degree of polymerization/molecular weight.
`Because there is such great variability in structures, there
`is some variability in methods used; however, some
`generalities can be described.
`Determination of
`the monosaccharide composition
`begins with acid-catalysed hydrolysis. The monosacchar-
`ides released are then determined both qualitatively and
`quantitatively by high-performance liquid chromatogra-
`phy (HPLC) or by gas–liquid chromatography (GLC)
`after conversion to volatile,
`thermostable derivatives
`(often alditol acetates).
`Linkages are determined by methylation analysis, which
`can reveal the linkage position, the ring size and the nature
`of
`the monosaccharide. In methylation analysis, all
`hydroxyl groups of the polysaccharide are converted into
`methyl ethers. Hydrolysis of the completely methylated
`polysaccharide then exposes the hydroxyl groups involved
`in glycosidic linkages as free (unmethylated) hydroxyl
`groups. The anomeric hydroxyl group of each unit will
`always be involved in a glycosidic bond, so only the other
`hydroxyl groups are significant. Each of the other hydroxyl
`groups involved in a glycosidic linkage before hydrolysis
`will be unmethylated, a characteristic that marks its
`location. Units
`that are nonreducing end-units are
`completely methylated, i.e. methyl ethers are formed at
`all hydroxyl groups of the pyranose or furanose ring form
`of the sugar unit. Thus, methylation analysis indicates the
`position of linkages to each monosaccharide unit, but not
`the sequence or any other structural information except
`which units are at nonreducing termini.
`To obtain information about sequences, the polysac-
`charide is partially depolymerized using specific enzyme-
`catalysed and/or acid-catalysed hydrolysis to yield oligo-
`saccharides, the structures of which are then determined.
`
`Higher Plant Cell Wall Polysaccharides
`
`Differences in cell wall compositions occur within phyla,
`classes, families and genera of plants; with location within
`a given plant
`(because different cells have different
`functions and exist in different environments); and with
`stage of development. Nevertheless, some general features
`can be described.
`Polysaccharides are the primary constituents of plant
`cell walls. Because cellulose is the principal component of
`the cell walls of higher plants, it is the most abundant
`organic compound on Earth. Cellulose is also present in
`brown, red and green algae and in certain fungi and slime
`moulds and is excreted extracellularly by certain bacteria.
`The amount of cellulose in a plant varies greatly from
`species to species. Wood, which is about half cellulose on a
`dry weight basis (db), has the highest percentage of
`
`3
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`Polysaccharides
`
`Table 1 Classification of unmodified polysaccharides by
`structure, with examples
`
`I. Linear molecules
`A. Unbranched
`1. Neutral homoglycans
`a. Cellulose
`b. Laminaransa
`c. Yeast glucans
`d. Cereal b-glucans
`e. Amyloses
`f. Inulins
`g.Yeast mannans
`2. Neutral diheteroglycans
`a. Konjac mannan
`b. Agarose component of agar
`3. Anionic/acidic homoglycans
`a. Lambda-carrageenans
`b. Pectins, pectic acidsb
`4. Anionic/acidic diheteroglycans
`a. Algins/alginates
`b. Kappa-carrageenans
`c. Iota-carrageenans
`5. Anionic/acidic triheteroglycan
`a. Gellan gum
`6. Cationic/basic homoglycan
`a. Chitosan
`B. Linear with short branches/side units
`1. Branches irregularly spaced
`a. Neutral homoglycans
`i. Fungal (mushroom) b-glucans
`b. Neutral diheteroglycans
`i. Galactomannans (guar gum, locust bean gum,
`tara gum)
`ii. Flour arabinoxylans
`iii. Larch arabinogalactan
`iv. Neutral tetraheteroglycans
`- Xyloglucans
`2. Branches regularly spaced
`a. Neutral homoglycan
`i. Yeast mannan
`b. Anionic/acidic triheteroglycan
`i. Xanthan gum
`
`II. Nonlinear molecules
`A. Branches in clusters, homoglycan
`1. Amylopectins
`B. Highly branched/branch-on-branch-on-branch struc-
`tures, anionic/acidic
`1. Tetraheteroglycans
`a. Gum karayas
`b. Okra gum
`2. Pentaheteroglycans
`a. B-type hemicelluloses of cereal brans, etc.
`(arabinoxylans)
`
`4
`
`continued
`
`Table 1 – continued
`
`b. Gum arabic
`c. Psyllium seed gum
`a Some laminarans contain d-mannitol units.
`b Principal structure in commercial high- and low-methoxyl pectins.
`Pectins are actually complex, heterogeneous polysaccharides
`containing some percentage of a-l-rhamnopyranosyl units in the
`main chain and perhaps arabinogalactan side-chains.
`
`cellulose, except for the fibrous seed hairs of cotton, which
`are  90% cellulose. In wood, the majority of the cellulose
`is found in thickened, secondary cell walls. Even certain
`animals (tunicates and protozoans) synthesize cellulose.
`Thus cellulose is synthesized by both plants and animals,
`though almost exclusively by plants, and by both
`prokaryotes and eukaryotes, but primarily by eukaryotes.
`Cellulose differs from most other polysaccharides in being
`a homoglycan with a single type of linkage and in occurring
`naturally as a paracrystalline, but largely crystalline,
`polymer. It also has a higher average molecular weight
`than most other polysaccharides. In its native state,
`cellulose usually exists in the form of strong fibres.
`Chemically, all celluloses are the same; they all are
`polymers of b-d-glucopyranosyl units linked (1!4), but
`these (1,4)-b-glucans differ in physical organization into
`fibrillar structures from source to source.
`In both primary and secondary cell walls, cellulose is
`mixed with hemicellulose(s) (Table 2) and lignin. The term
`hemicellulose indicates a polysaccharide closely associated
`with cellulose, not structurally but physically, in cell walls.
`Hemicelluloses may be largely homoglycans (e.g. d-xylan),
`but are usually heteroglycans. Their structures vary from
`linear to highly branched and bush-like. The hemicellulose
`content of woods varies from  17% to  23% (db).
`Pectic substances, another family of polysaccharides,
`are located in middle lamellae and primary cell walls. Pectic
`substances comprise 1–4% of woody tissue. Xyloglucans
`are also cell wall components of many higher plant cells.
`The polysaccharides found in cell walls are specific to
`specific cell types. Most flowering plants have type I cell
`walls. These walls have a cellulose–xyloglucan interlocking
`framework (  50% of the wall mass) embedded in a matrix
`of pectic polysaccharides (  30%, rhamnogalacturonan I
`plus lesser amounts of galacturonans/partially methyl
`esterified poly(d-galacturonic acids)). Grasses have type II
`cell walls. The characteristic components of these walls are
`cellulose, glucuronoarabinoxylans, galacturonans, and
`mixed-linkage b-glucans/(1,3:1,4-b-glucans).
`Structural polysaccharides of algae vary greatly between
`phyla. Included as cell wall components of algae are
`cellulose, alginates, sulfated galactans (carrageenans,
`agars, etc.), lichenan (a 1,3:1,4-b-glucan), xylans, mannans
`and pectic substances.
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`Table 2 Cell wall polysaccharidesa
`
`Table 3 Energy storage polysaccharides
`
`Polysaccharides
`
`Higher land plants
`Cellulose
`Hemicelluloses
`Arabinoxylans
`Galactoglucomannans
`b-Glucans
`Glucomannans
`Mannans
`Xylans
`Xyloglucans
`Pectic polysaccharides
`Arabinans
`Arabinogalactans
`Galactans
`Galacturonans
`Rhamnogalacturonans
`
`Marine algae
`Algins
`Cellulose
`l-Fucans
`Galactans
`Agars
`Carrageenans
`Furcellarans
`b-Glucans
`Mannans
`Xylans
`
`Fungi and yeasts
`Cellulose
`Chitin
`b-Glucans
`a-Glucans
`Mannans
`
`aMany of these polysaccharides may contain monosaccharide units
`other than those indicated in the name. For example, xylans often
`contain uronic acid units, and polysaccharides named l-fucans
`may contain, in addition to the principal sugar (l-fucose), d-
`galactose, d-glucuronic acid, d-mannose and d-xylose.
`
`Energy Storage Polysaccharides
`
`Acting as a reserve energy supply is one function of
`polysaccharides in living systems (Table 3). Glycogen, a
`polysaccharide that serves in this role, is ubiquitous in
`mammals, fish, molluscs, insects, other animals, bacteria,
`fungi and some plants, and exists in different forms and
`amounts in these organisms. Glycogen is present in most
`mammalian tissues; the liver contains the highest concen-
`tration; skeletal muscle contains the greatest amount.
`Glycogen is well constructed to be an energy storage
`molecule: it stores a large number of units of d-glucose
`with negligible increase in osmotic pressure because of its
`high molecular weight and/or in viscosity because of its
`
`Higher land plants
`Fructans
`Galactans
`Galactomannans
`Glucomannans
`Starches
`Xyloglucans
`
`Marine algae
`Fructans
`a-Glucansa
`b-Glucans
`Xylans
`
`Freshwater algae
`a-Glucansa
`b-Glucans
`
`Fungi and yeasts
`a-Glucansa
`b-Glucans
`
`aStarch-like and glycogen-like polymers.
`
`compactness; and its highly branched, bush-like structure
`makes a large number of glucosyl units simultaneously
`available to enzyme molecules that release them. Liver
`glycogen is used to maintain blood glucose levels. Muscle
`glycogen is used as a supply of energy, even under less than
`fully aerobic conditions. Glycogen pools are generally in
`dynamic states.
`Starch is the principal carbohydrate energy storage
`substance of higher plants and, after cellulose, is the second
`most abundant carbohydrate end product of photosynth-
`esis. As well as being a reserve substance for most higher
`plants, starch is an energy source for animals that feed on
`these plants. Starch is found in leaves, where it serves as a
`transient d-glucose storage material, and in seeds (espe-
`cially those of cereal grains), fruits, roots, rhizomes, stems,
`tubers and trunks for long-term storage. Starches provide
`at least 70% of human caloric intake on a worldwide basis.
`Starch is unique among carbohydrates in that it occurs in
`discrete particles called granules. Starch granules are
`relatively dense and insoluble. Most starch granules
`contain two polymers: an essentially linear polysaccharide
`called amylose and a highly branched polysaccharide
`called amylopectin. Each starch from each plant source
`and tissue is unique in terms of granule morphology,
`composition, polysaccharide structures and characteristic
`properties.
`Other energy storage polysaccharides include inulin and
`other fructans in roots, tubers and stems; galactomannans
`in legume seeds; mannans in palm seeds; and starch-type
`and laminaran-type polysaccharides of green and brown
`algae, respectively. (Laminaran is a (1,3)-b-glucan.)
`
`5
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`Polysaccharides
`
`Table 4 Classification of commercial polysaccharides by
`source
`
`I. Plant polysaccharidesa
`A. Higher plants
`1. Cell wall constituents
`a. Cellulose
`b. Native and extracted/commercial pectins
`2. Storage polysaccharides
`a. Starches (amyloses and amylopectins)
`b. Galactomannans (guar gum, locust bean/carob gum,
`tara gum)
`c. Konjac mannan
`d. Inulin
`3. Other non-cell wall polysaccharides
`a. Exudate gums
`i. Gum arabics
`ii. Gum tragacanths
`iii. Gum ghattis
`iv. Gum karayas
`b. Extractives
`i. Larch arabinogalactan
`ii. Psyllium seed gum
`B. Algae
`1. Cell wall constituents
`a. Agars
`b. Alginates
`c. Carrageenans
`d. Furcellarans (Danish agars)
`2. Non-cell wall constituent
`a. Laminaran
`C. Microorganisms
`1. Extracellular
`a. Xanthans
`b. Gellan
`c. Curdlan
`
`II. Animal
`A. Crustacean
`1. Chitinb
`
`III. Derived from native polysaccharides
`A. From cellulose
`1. Carboxymethylcelluloses
`2. Cellulose acetates
`3. Cellulose acetate butyrates
`4. Cellulose acetate propionates
`5. Ethylcelluloses
`6. Hydroxyethylcelluloses
`7. Hydroxypropylcelluloses
`8. Hydroxypropylmethylcelluloses
`9. Methylcelluloses
`10. Microcrystalline celluloses
`B. From starch
`1. Starch acetates
`2. Starch 1-octenylsuccinates
`3. Starch phosphates
`
`6
`
`Table 4 – continued
`
`4. Starch succinates
`5. Starch adipates
`6. Hydroxyethylstarches
`7. Hydroxypropylstarches
`8. Dextrins
`C. From guar gum
`1. Carboxymethylguar gum
`2. Carboxymethylhydroxypropylguar gum
`3. Hydroxypropylguar gum
`4. 2-Hydroxy-3-(trimethylammonium chloride)-propyl-
`guar gum
`D. From native pectins
`1. Pectic acids (low-methoxyl pectins)
`2. Amidated pectins
`E. Propylene glycol alginates
`F. Chitosan
`
`aGenerally restricted to those polysaccharides isolated in at least a
`semipurified form. Does not include those that are present as
`constituents of other materials; for example does not include the
`arabinoxylans of flours or the b-glucans of brans.
`bNot produced in the native form, but rather in the deacetylated form
`(chitosan, III.F).
`
`Chitin and Other Fungal Cell Wall
`Polysaccharides
`
`Fungal cell walls often contain 80–90% (db) polysacchar-
`ide. Fungal cell walls have been categorized into eight types
`by polysaccharide composition: cellulose–glycogen, cellu-
`lose–b-glucan, cellulose–chitin, chitosan–chitin, chitin–b-
`glucan (the most common), mannose-containing polysac-
`charide–b-glucan, mannan–chitin, galactosaminoglycan–
`chitin. The primary cell wall component of many species is
`chitin. Chitin is also a primary matrix constituent of mollusc
`shells and the exoskeletons of insects. Its structure is
`identical to that of cellulose, with the exception of having
`an acetamido (–NH–CO–CH3) group in place of the
`hydroxyl group at C2 of each monomer unit. Thus, chitin
`is (1!4)-linked poly(N-acetyl-b-d-glucosamine) or more
`properly a poly(2-acetamido-2-deoxy-b-d-glucopyranose).
`Like cellulose, chitin is highly crystalline and insoluble.
`Yeast cell walls may contain a (1,3)-b-glucan, a (1,6)-b-
`glucan, various mannans or a galactan. Other cell wall
`components of certain fungi include rhamnomannans,
`glucomannans, galactomannans, xylomannans, glucuro-
`noxylomannans, and other polysaccharides.
`
`Bacterial Exopolysaccharides
`
`Carbohydrate-containing polymers are components of
`bacterial cell walls and capsules and are also secreted into
`the culture medium. Of the cell wall polysaccharides, the
`
`continued
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`Petitioner Ex. 1025 - Page 6
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`lipopolysaccharides of Gram-negative bacteria should be
`mentioned. These materials consist of three parts: a
`compound called lipid A, a core polysaccharide section,
`and O-specific side-chains, which are attached to the core
`polysaccharide. The O-specific chains also are polysac-
`charides and are immunogenic. Lipopolysaccharide struc-
`tures are quite diverse and often contain sugar units that
`are rarely or never found in other polysaccharides. Gram-
`positive bacteria do not synthesize lipopolysaccharides of
`the type present in Gram-negative bacteria, but do make
`other types of cell wall polysaccharides. Some also produce
`capsules of polysaccharide. Intracellular polysaccharides
`are present in some bacteria.
`As with eukaryotes, the great diversity of types of
`bacteria leads to a great diversity in polysaccharide
`structures, but the overwhelming majority of bacterial
`polysaccharides are heteroglycans that are polymers of
`oligosaccharide repeating units. These repeating units
`generally contain 3–8 glycosyl units. The polymers often
`also contain noncarbohydrate components.
`Bacterial exopolysaccharides (extracellular polysac-
`charides) include those that are made from sucrose, viz.
`dextrans and fructans. Dextrans are branched a-glucans
`containing (1!3), (1!6) and occasionally (1!2) linkages.
`Fructans contain b-d-fructofuranosyl units linked (2!6)
`or (2!1).
`Several extracellular microbial polysaccharides are
`commercial products (see Polysaccharides as Gums and
`Thickeners below).
`
`Glycosaminoglycans
`
`Glycosaminoglycans are found primarily in animal tissues,
`mostly in connective tissue, and often as part of a larger
`supermolecular structure known as a proteoglycan. As the
`name implies, they contain amino sugars. These polysacchar-
`ides usually contain alternating units of an amino sugar (2-
`amino-2-deoxy-d-glucose (d-glucosamine) or 2-amino-2-
`deoxy-d-galactose (d-galactosamine)) and a uronic acid (d-
`glucuronic acid or l-iduronic acid); they usually contain N-
`acetyl or N- or O-sulfate groups. An exception to this overall
`general structure is keratan sulfate, which has a d-
`galactopyranosyl unit
`in place of a uronic acid unit
`alternating with an amino sugar. The glycosaminoglycan
`heparin, unlike the others, originates in mast cells in many
`nonconnective tissues. Glycosaminoglycans that do have the
`usual structure and are present in connective tissue, either free
`or as part of a proteoglycan, are hyaluronic acid, chondroitin
`4- and 6-sulfates, dermatan sulfate and heparan sulfate.
`
`Polysaccharides as Industrial Gums
`
`Many polysaccharides are used as they are obtained from
`plants or microorganisms in various industrial applica-
`
`Polysaccharides
`
`tions, including in food products, and they may also be
`modified before use. Their common characteristic is their
`hydrophilic nature. Many are water soluble; those that are
`not, like cellulose, absorb water (hydrate). The water-
`soluble polysaccharides and modified polysaccharides
`used in industrial applications are known as gums. When
`used in food products, they (and also the protein gelatin)
`may be referred to as hydrocolloids. Gums/hydrocolloids
`are used primarily to thicken and/or gel aqueous solutions
`and otherwise to modify and/or control the rheological
`properties of aqueous systems, but they may also be used
`for a wide range of other attributes, including, but not
`limited to, their ability to function as adhesives, crystal-
`lization inhibitors, emulsion and suspension stabilizers,
`film formers and texturing agents. Table 4 lists commercial,
`water-soluble polysaccharides and polysaccharide deriva-
`tives by source.
`
`Further Reading
`
`Aspinall GO (ed) (1982) The Polysaccharides, vol. 1. New York:
`Academic Press.
`Aspinall GO (ed) (1983) The Polysaccharides, vol. 2. New York:
`Academic Press.
`Aspinall GO (ed) (1985) The Polysaccharides, vol. 3. New York:
`Academic Press.
`BeMiller JN (1999) Structure–property correlations of non-starch food
`polysaccharides. Macromolecular Symposia 140: 1–15.
`(1985) Biochemistry of Storage
`Dey PM and Dixon RA (eds)
`Polysaccharides in Green Plants. London: Academic Press.
`Fishman ML and Jen JL (eds) (1986) Chemistry and Function of Pectins.
`Washington, DC: American Chemical Society.
`Fuchs A (ed) (1993) Inulin and Inulin-containing Crops. Amsterdam:
`Elsevier.
`Klemm D, Philipp B, Heinze T, Heinze U and Wagenknecht W (1998)
`Comprehensive Cellulose Chemistry, vols. 1 and 2. Weinheim,
`Germany: Wiley-VCH.
`Muzzarelli R, Jeuniaux C and Gooday GW (eds) (1986) Chitin in Nature
`and Technology. New York: Plenum Press.
`Pinto BM (ed) (1999) Comprehensive Natural Products Chemistry, vol. 3,
`Carbohydrates and Their Derivatives Including Tannins, Cellulose, and
`Related Lignins. Amsterdam: Elsevier.
`Preiss J (ed) (1988) The Biochemistry of Plants, vol. 14, Carbohydrates.
`New York: Academic Press.
`Shimizu K (1991) Chemistry of hemicelluloses. In: Hon DN-S and
`Shiraishi N (eds) Wood and Cellulosic Chemistry. New York: Marcel
`Dekker.
`Suzuki M and Chatterton NJ (eds) (1993) Science and Technology of
`Fructans. Boca Raton, FL: CRC Press.
`Stone BA and Clarke AE (1992) Chemistry and Biology of (1!3)-b-
`Glucans. Victoria, Australia: La Trobe University Press.
`Varki A, Cummings R, Esko J et al. (1999) Essentials of Glycobiology.
`Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.
`Walter RH (ed) (1991) The Chemistry and Technology of Pectin. San
`Diego: Academic Press.
`Whistler RL and BeMiller JN (1984) Starch: Chemistry and Technology.
`Orlando: Academic Press.
`
`7
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`Pharmacosmos A/S v. American Regent, Inc.
`Petitioner Ex. 1025 - Page 7
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