Polisakarida

Storage Polysaccharides

polymer is called a heteropolysaccharide. Even those storage and structural poly-saccharides that are heteropolymers are rarely complex; usually no more than twokinds of residues are involved. In further contrast to protein and nucleic acid mol-ecules, which are almost always of defined length, polysaccharide chains grow torandom lengths. And, as mentioned earlier, glycans are distinctive in that they canform branched chains. The ability to branch, plus the numbers of functionalgroups on each monomer that can participate in bond formation, give polysac-charides amazing structural diversity, and this undoubtedly contributes to thelarge number of roles played by polysaccharides.

The functional reasons for the distinctions from proteins and nucleic acidsare not hard to find. A storage material, such as starch, needs neither to conveyinformation nor to adopt a complicated three-dimensional form. It is simply abin in which to put away glucose molecules for future use. Many structural poly-saccharides (like structural proteins) form extended, regular secondary structures,well suited to the formation of fibers or sheets. Often a regular repetition of somesimple monosaccharide or disaccharide motif will serve this function. (Recall, forcomparison, the simple and repetitive amino acid sequences of collagen and silkfibroin described in Chapter 6.) The only glycan polymers in which well-definedand complex sequences are found are some of the oligosaccharides attached tocell surfaces or those attached to specific glycoproteins. Because these oligomersserve to identify cells or molecules, they must convey information. This functionrequires precisely defined “words” in the polysaccharide language, just as nucleicacid sequences spell out information in their own language.

Storage PolysaccharidesThe principal storage polysaccharides are amylose and amylopectin, whichtogether constitute starch in plants, and glycogen, which is stored in animal andmicrobial cells. Both starch and glycogen are stored in granules within cells (Figure9.17). Starch is found in almost every kind of plant cell, but grain seeds, tubers, andunripe fruits are especially rich in this material. Glycogen is deposited in the liver,which acts as a central energy storage organ in many animals. Glycogen is also abun-dant in muscle tissue, where it is more immediately available for energy release.

330 CHAPTER 9 CARBOHYDRATES: SUGARS, SACCHARIDES, GLYCANS

(a) Chloroplast granules (b) Tuber cell granules

(c) Liver granules

FIGURE 9.17

Storage of starch and glycogen in granules.In each case a representative granule is indi-cated by an arrow. (a) Starch granules in a plantleaf chloroplast. (b) Starch granules in potatotuber cells. (c) Glycogen granules in liver.

(a) From Science Source, © BiophotoAssociates/Photo Researchers, Inc.; (b) Dr. LloydM. Beidler/Science Photo Library; (c) Medimage/Science Photo Library.

Glycogen and the components of starch—amylose and amylopectin—are storage poly-saccharides. Amylose is linear; amylopectinand glycogen are branched.

polymer is called a heteropolysaccharide. Even those storage and structural poly-saccharides that are heteropolymers are rarely complex; usually no more than twokinds of residues are involved. In further contrast to protein and nucleic acid mol-ecules, which are almost always of defined length, polysaccharide chains grow torandom lengths. And, as mentioned earlier, glycans are distinctive in that they canform branched chains. The ability to branch, plus the numbers of functionalgroups on each monomer that can participate in bond formation, give polysac-charides amazing structural diversity, and this undoubtedly contributes to thelarge number of roles played by polysaccharides.

The functional reasons for the distinctions from proteins and nucleic acidsare not hard to find. A storage material, such as starch, needs neither to conveyinformation nor to adopt a complicated three-dimensional form. It is simply abin in which to put away glucose molecules for future use. Many structural poly-saccharides (like structural proteins) form extended, regular secondary structures,well suited to the formation of fibers or sheets. Often a regular repetition of somesimple monosaccharide or disaccharide motif will serve this function. (Recall, forcomparison, the simple and repetitive amino acid sequences of collagen and silkfibroin described in Chapter 6.) The only glycan polymers in which well-definedand complex sequences are found are some of the oligosaccharides attached tocell surfaces or those attached to specific glycoproteins. Because these oligomersserve to identify cells or molecules, they must convey information. This functionrequires precisely defined “words” in the polysaccharide language, just as nucleicacid sequences spell out information in their own language.

Storage PolysaccharidesThe principal storage polysaccharides are amylose and amylopectin, whichtogether constitute starch in plants, and glycogen, which is stored in animal andmicrobial cells. Both starch and glycogen are stored in granules within cells (Figure9.17). Starch is found in almost every kind of plant cell, but grain seeds, tubers, andunripe fruits are especially rich in this material. Glycogen is deposited in the liver,which acts as a central energy storage organ in many animals. Glycogen is also abun-dant in muscle tissue, where it is more immediately available for energy release.

330 CHAPTER 9 CARBOHYDRATES: SUGARS, SACCHARIDES, GLYCANS

(a) Chloroplast granules (b) Tuber cell granules

(c) Liver granules

FIGURE 9.17

Storage of starch and glycogen in granules.In each case a representative granule is indi-cated by an arrow. (a) Starch granules in a plantleaf chloroplast. (b) Starch granules in potatotuber cells. (c) Glycogen granules in liver.

(a) From Science Source, © BiophotoAssociates/Photo Researchers, Inc.; (b) Dr. LloydM. Beidler/Science Photo Library; (c) Medimage/Science Photo Library.

Glycogen and the components of starch—amylose and amylopectin—are storage poly-saccharides. Amylose is linear; amylopectinand glycogen are branched.

Starch• Pati berfungsi sebagai cadangan makanan pada tanaman

• Pati disimpan dalam bentuk granula yang tak larut di kloroplast pada daun atau di amiloplast

• Penyimpanan glukosa dalam bentuk pati mengurangitekanan osmosis intra sel

• Pati merupakan campuran glukan, yaitu amilosa dan amilopektin

Scanning electron micrographs of starch granules from nine different botanical sources: A, potato; B, rice; C, wheat; D, mung bean; E, maize; F, waxy maize; G, tapioca; H, shoti; J, leaf starch. Magnification 1500X for each starch

Amilosa• Polimer linier tersusun atas beberapa ribu glukosa

yang berikatan a(1⟶4) glikosidik

• Amilosa memiliki struktur sekunder berbentukheliks dengan 6 residu glukosa setiap putarannya

Amilopektin• Polimer bercabang dengan monomer glukosa

dengan jumlah residu glukosa sekitar 106 untuksetiap moekulnya

• Memiliki ikatan glikosidik a(1⟶4) dan a(1⟶6)

• Percabangan terbentuk setiap 24 – 30 residuglukosa

Glikogen• Merupakan cadangan makanan pada hewan

• Berupa granula

• Sebagian besar disimpan di hati dan otot. (kandungan glikogen pada hati bisasampai 10% beratnya)

• Memiliki struktur mirip seperti amilopektin tetapi lebih bercabang(percabangan terjadi setiap 8 – 14 residu glukosa)

Structural Polysaccharides

Selulosa• Merupakan glucan linier dengan ikatan b(1⟶4) glikosidik

• Satu molekul selulosa bisa mengandung sampai 15 000 residu glukosa

• Selulosa merupakan komponan structural utama pada dinding sel tanaman

Selulosa• Residu glukosa yang kedua berputar 180o dari

residu glukosa sebelumnya membentuk strukturlembaran.

• Serat selulosa mengandung 40 lembaran glucan yang tersusun secara vertikal.

• Ikatan hydrogen menstabilkan susunan lembarantersebut.

Struktur Kayu

Hemiselulosa

Kitin• Merupakan komponen utama pada rangka luar invertebrata

(serangga, laba laba), sebagian jamur dan alga

• Selulosa dan kitin memiliki strukur yg mirip.

• Monomer kitin adalah N-acetyl-b-D-glucosamine,329

11.3 Glycoproteins

In addition to being a key component of structural tissues, glycosami-noglycans are common throughout the biosphere. Chitin is a glycosami-noglycan found in the exoskeleton of insects, crustaceans, and arachnids and is, next to cellulose, the second most abundant polysaccharide in nature (Figure  11.22). Cephalopods such as squid use their razor sharp beaks, which are made of extensively crosslinked chitin, to disable and consume prey.

Mucins are glycoprotein components of mucus

A third class of glycoproteins is the mucins (mucoproteins). In mucins, the protein component is extensively glycosylated at serine or threonine resi-dues by N -acetylgalactosamine (Figure 11.10). Mucins are capable of form-ing large polymeric structures and are common in mucous secretions. These glycoproteins are synthesized by specialized cells in the tracheobronchial, gastrointestinal, and genitourinary tracts. Mucins are abundant in saliva where they function as lubricants.

A model of a mucin is shown in Figure 11.23A. The defining feature of the mucins is a region of the protein backbone termed the variable number of tandem repeats (VNTR) region, which is rich in serine and threonine residues that are O -glycosylated. Indeed, the carbohydrate moiety can account for as much as 80% of the molecule by weight. A number of core carbohydrate structures are conjugated to the protein component of mucin. Figure 11.23B shows one such structure.

Mucins adhere to epithelial cells and act as a protective barrier; they also hydrate the underlying cells. In addition to protecting cells from

environmental insults, such as stomach acid, inhaled chemicals in the lungs, and bacterial infections, mucins have roles in fertilization, the immune response, and cell adhesion. Mucins are overexpressed in bronchitis and cystic fibrosis, and the overexpression of mucins is characteristic of adeno-carcinomas—cancers of the glandular cells of epithelial origin.

FIGURE 11.23 Mucin structure. (A) A schematic representation of a mucoprotein. The VNTR region is highly glycosylated, forcing the molecule into an extended conformation. The Cys-rich domains and the D domain facilitate the polymerization of many such molecules. (B) An example of an oligosaccharide that is bound to the VNTR region of the protein. See Figure 11.16 for the carbohydrate key. [Information from A. Varki et al. (Eds.), Essentials of Glycobiology, 2d ed. (Cold Spring Harbor Press, 2009), pp. 117, 118.]

α2α3

α3

α6αβ4

β6

β3

β3Ser/Thr

D domainCys rich

Cys richO-Glycans

VNTR

(B)

(A)

β4

β4

FIGURE 11.22 Chitin, a glycosamino-glycan, is present in insect wings and the exoskeleton. Glycosaminoglycans are components of the exoskeletons of insects, crustaceans, and arachnids. [FLPA/Alamy.]

FIGURE 11.21 Structure of proteoglycan from cartilage. (A) Electron micrograph of a proteoglycan from cartilage (with false color added). Proteoglycan monomers emerge laterally at regular intervals from opposite sides of a central filament of hyaluronate. (B) Schematic representation. G 5 globular domain. [(A) Courtesy of Dr. Lawrence Rosenberg. From J. A. Buckwalter and L. Rosenberg. Collagen Relat. Res. 3:489–504, 1983.]

300 nm

HyaluronateKeratan sulfate

Aggrecan

Chondroitin sulfate

G3

G2G1 G3

G2

G1

G3 G2 G1

G3

G3

G2 G1

G2

G1

A B

Kitosan

Karagenan• Dihasilkan oleh alga merah

• Merupakan linier glikan yang memiliki gugus sulfat.

• Perbedaan ketiga jenis karagenan terletak pada jumlah gugus sulfat yang dimilikinya.

• Semakin banyak gugus sulfat menurunkan kekuatan gel yang dihasilkan

Alginat• Merupakan komponen penyusun dinding sel alga coklat

• Alginat adalah kopolimer linier antara homopolymer M (monomer D-mannuronat yang berikatan β (1→4) ) denganhomopolymer G (monomer L-guluronate yang berikatan a (1→4) ).