Starch

Starch is a plant polymer synthesized from glucose and stored in plastids as grains of 1 to 100 nm in diameter (see Fig. 12.6 for glucose structure). It consists

P-L-arabinose (Ara)

FIGURE 12.6 Common sugars of the plant cell wall and their interconversions. The modifications to convert D-glucose into other sugars are highlighted in light blue.

P-L-arabinose (Ara)

FIGURE 12.6 Common sugars of the plant cell wall and their interconversions. The modifications to convert D-glucose into other sugars are highlighted in light blue.

of two glucose polymers, amylose and amylopectin. Amylose contains long unbranched chains of (1-4)-glucose units. For most plants, amylose can account for up to 30% of the total starch. Amylopectin has a similar structure, linked every 20 to 30 glucose residues by (1-6)-glucose bonds (Fig. 12.7). A class of enzymes known as amylases degrades starch readily. a-Amylase catalyzes the cleavage of glucosyl bonds, producing small glucans called dextrins along with glucose and maltose. ^-Amylase cleaves maltose residues from the nonreducing end of the starch molecule. The final degradation of maltose, short-chained glucans and dextrins to glucose, is achieved by ,5-glycosidase. Many bacteria and fungi produce these enzymes to convert insoluble starch effectively into easily metabolized glucose.

Amylose

Nonreducing Q end

Nonreducing Q end

OOOO├žu^poO Reducing end

Branched chain

Amylopectin

Reducing end

Nonreducing ends

Branched chain

Amylopectin

Reducing end

Nonreducing ends

FIGURE 12.7 The structure of starch showing the (A) a(1-4) linkage of amylose and (B) a(1-6) branched linkage of amylopectin.

FIGURE 12.7 The structure of starch showing the (A) a(1-4) linkage of amylose and (B) a(1-6) branched linkage of amylopectin.

HEMICELLULOSES, PECTINS, AND CELLULOSE

The majority of plant carbohydrates are found as polysaccharides as part of the cytoskeleton in the primary and secondary plant cell wall. Polysaccharides are long chains of sugars that are covalently linked through H bonds. The vast majority are aldoses with the empirical formula (CH2O)n. All of the monosaccharides in the plant cell wall are derived from glucose and upon alteration form a variety of 5- and 6-C sugars (Fig. 12.6). The sugars are locked into polymerized chains by pyranose or furanose rings to form either hemicellulose/pectic or cellulose microfibrils.

Hemicelluloses and pectin are composed of 5-C sugars or glycans. The majority of glycans of flowering plants consist of D-xylose, D-glucuronic acid, and D-arabinose. A common cross-linked structure is (1-4)-D-glucan and (1-4)-D-xylose found in all dicotyledons and about half of monocots. In many grasses, the major cross-linked glycan is glucuronoarabinoxylan. Cereals and grasses also contain a mixed linkage of 1-3(1-4)-D-glucans as a distinguishing component of the cross-linked glycan-cellulose microfibril network. A mixture of heterogeneous, branched, highly hydrated polysaccharides composed mainly of D-galacturonic acid is called pectin. Pectins are thought to influence cell wall porosity, pH, and cell-to-cell adhesion. Hemicelluloses are reduced to simple sugars by several enzymes known collectively as pectinases. For this reason, the degradation of pectin and hemicel-lulose is thought to occur together. The study of hemicellulose/pectin degradation has received much attention because of its importance as a point of attack by pathogens and as a means for symbionts such as rhizobia and mycorrhiza to gain access to the middle lamella.

The pectinase enzyme system includes three stages of substrate degradation. The degradation of hemicellulose/pectin is similar to cellulose degradation except that more enzymes are involved. The decoupling of the hemicellulose and pectin cross-linked glycan structure is thought to provide access for other cell wall-degrading enzymes. The release of oligogalacturonides and simple sugars represses the enzyme system and is thought to control microbial succession during litter decomposition. Pectinolytic soil bacteria include species of Erwinia, Arthobacter, Pseudomonas, and Bacillus. Common pectinolytic fungi are species of Aspergillus, Rhyzopus, Fusarium, Sclerotina, and Penicillium. Yeast such as Conida and Kluyveromyces also exhibit pectinase activity.

Cellulose is the most abundant plant polysaccharide and accounts for 15 to 30% of the primary cell wall dry mass and a greater percentage of the secondary cell wall, especially of woody species. It consists of glucose units linked by ,5(1-4) bonds to form D-glucan chains (Fig. 12.8). The chains are cross-linked by H bonds to form paracrystalline assemblages called microfibrils. The average microfibril is composed of 36 individual glucan chains and several thousand individual glucan molecules to reach a length of 2 to 3 mm. Cellulose microfibrils are cross-linked into a network or scaffold with glycans or hemicellulose. In addition to cellulose, plants can synthesize callose by linking glucans in the ,(1-3) configuration similar to that found in yeast and fungi. Callose found in phloem sieve plates, pollen tubes, cotton fibers, and other specialized cells is produced in response to wounding such as fungal hypha penetration of the primary cell wall.

Cellulose microfibrils are decomposed by the enzyme system termed cellulase, composed of endoglucanase, exoglucanase, and , -glucosidase (also known

Cellulose

CH2OH

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