Oh H H Oh

FIGURE 7-15 Amylose and amylopectin, the polysaccharides of starch. (a) A short segment of amylose, a linear polymer of d-glucose residues in (a1n4) linkage. A single chain can contain several thousand glucose residues. Amylopectin has stretches of similarly linked residues between branch points. (b) An (a1n6) branch point of amylopectin. (c) A cluster of amylose and amylopectin like that believed

Nonreducing ends

Amylopectin

Amylose

Reducing ends

Amylopectin

to occur in starch granules. Strands of amylopectin (red) form double-helical structures with each other or with amylose strands (blue). Glucose residues at the nonreducing ends of the outer branches are removed enzymatically during the mobilization of starch for energy production. Glycogen has a similar structure but is more highly branched and more compact.

whereas in amylose, amylopectin, and glycogen the glucose is in the a configuration. The glucose residues in cellulose are linked by (¡1n4) glycosidic bonds, in contrast to the (a1n4) bonds of amylose, starch, and glycogen. This difference gives cellulose and amylose very different structures and physical properties.

Glycogen and starch ingested in the diet are hy-drolyzed by a-amylases, enzymes in saliva and intestinal secretions that break (a 1n4) glycosidic bonds between glucose units. Most animals cannot use cellulose as a fuel source, because they lack an enzyme to hydrolyze the (¡1n4) linkages. Termites readily digest cellulose

FIGURE 7-16 The structure of cellulose. (a) Two units of a cellulose chain; the d-glucose residues are in (¡1n4) linkage. The rigid chair structures can rotate relative to one another. (b) Scale drawing of segments of two parallel cellulose chains, showing the conformation of the d-glucose residues and the hydrogen-bond cross-links. In the hex-ose unit at the lower left, all hydrogen atoms are shown; in the other three hexose units, the hydrogens attached to carbon have been omitted for clarity as they do not participate in hydrogen bonding.

(/31n4)-linked D-glucose units (a)

FIGURE 7-17 Cellulose breakdown by wood fungi. A wood fungus growing on an oak log. All wood fungi have the enzyme cellulase, which breaks the (01n4) glycosidic bonds in cellulose, such that wood is a source of metabolizable sugar (glucose) for the fungus. The only vertebrates able to use cellulose as food are cattle and other ruminants (sheep, goats, camels, giraffes). The extra stomach compartment (rumen) of a ruminant teems with bacteria and protists that secrete cellulase.

FIGURE 7-17 Cellulose breakdown by wood fungi. A wood fungus growing on an oak log. All wood fungi have the enzyme cellulase, which breaks the (01n4) glycosidic bonds in cellulose, such that wood is a source of metabolizable sugar (glucose) for the fungus. The only vertebrates able to use cellulose as food are cattle and other ruminants (sheep, goats, camels, giraffes). The extra stomach compartment (rumen) of a ruminant teems with bacteria and protists that secrete cellulase.

(and therefore wood), but only because their intestinal tract harbors a symbiotic microorganism, Tricho-nympha, that secretes cellulase, which hydrolyzes the (01n4) linkages. Wood-rot fungi and bacteria also produce cellulase (Fig. 7-17).

Chitin is a linear homopolysaccharide composed of ^-acetylglucosamine residues in 3 linkage (Fig. 7-18). The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acety-lated amino group. Chitin forms extended fibers similar to those of cellulose, and like cellulose cannot be digested by vertebrates. Chitin is the principal component of the hard exoskeletons of nearly a million species of arthropods—insects, lobsters, and crabs, for example— and is probably the second most abundant polysaccha-ride, next to cellulose, in nature.

Steric Factors and Hydrogen Bonding Influence Homopolysaccharide Folding

The folding of polysaccharides in three dimensions follows the same principles as those governing polypeptide structure: subunits with a more-or-less rigid structure dictated by covalent bonds form three-dimensional macromolecular structures that are stabilized by weak interactions within or between molecules: hydrogen-bond, hydrophobic, and van der Waals interactions, and, for polymers with charged subunits, electrostatic interactions. Because polysaccharides have so many hydroxyl groups, hydrogen bonding has an especially important influence on their structure. Glycogen, starch, and cellulose are composed of pyranoside subunits (having six-membered rings), as are the oligosaccharides of glycoproteins and glycolipids to be discussed later. Such molecules can be represented as a series of rigid pyra-nose rings connected by an oxygen atom bridging two carbon atoms (the glycosidic bond). There is, in princi-

CH3 C=O

CH3 C=O

Weight Loss Funnel

Weight Loss Funnel

Who Else Wants To Discover The 3 Most Effective Fat Burning Methods The Weight Loss Industry Does NOT Want You To Know About.

Get My Free Ebook


Post a comment