Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431	Biochemistry	Fall 2001
Lecture Notes:: 1 October	© R. Paselk 2001
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CARBOHYDRATES, cont.

Animals can't digest b-bonds, essentially only bacteria can break this bond. Cool biological examples: Desert Iguana consume feces to maintain culture; Rabbits eat and reprocess first pass feces (soft) to take advantage of fermentation; Multiple stomachs in Ruminants; Ultimate symbiosis in some termites: protozoans in gut have bacteria in gut, and use spirochetes as "cilia" (rowers).

Sucrose, Sucrase (Invertase), and the magic of liquid filled chocolate covered cherries.

POLYSACCHARIDES

Can have both homo- and heteropolysaccharides. We will focus on homopolysaccharides as most central, but will mention some heteropolysaccharides to illustrate their functions. Homopolysaccharides have a single type of residue. Most common contain glucose. Used for energy (food) storage (starches and glycogen) and structure (cellulose).

Starch (energy storage in plants). Two kinds

• Amylose: linear, a-1,4 glycosidic links. MW: 4,000 - 150,000. (Figure 8.10 p 206) [overhead 8.21 MvH] (Gives char. deep blue color with iodine due to coiled complex enclosing iodine-color is lost with heating, returns as cooled).
• Amylopectin: branched every 30 or so units­linear a-1,4 chain of 30 glu residues then a-1,6 branch point. of course get branches on branches as well. MW: ->500,000. (Figure p207) [overhead 10-18, V&V] Broken down by a-amylase (pancreas and salivary glands; random cleavage of a-1,4 links) to give glucose and maltose; or b-amylase (plants; hydrolyses from reducing ends to give maltose). When either of these enzymes attack amylopectin they are blocked when they reach or are near a branch, thus end up with a "limit dextrin."

Glycogen: animal starch. Just like amylopectin, but more highly branched (every 8-12 residues). This allows more free ends for more rapid breakdown-important in animals.

STRUCTURAL POLYSACCHARIDES

Cellulose: b-1,4 linkages, thus resistant to breakdown (including acid hydrolysis) as want for structure (don't want to digest self). Multiple, extended strands come together as fibrils held together with H-bonds (Figure 8-9, p 205) [overhead 10.15, V&V], laid down in cell wall in criss-cross pattern, glued together with polyalcohols (lignin).

Chitin: Serves similar role to cellulose, but in animals (crustaceans and insects), fungi, and some algae. Homopolymer of N-acetyl-D-glucosamine. Like cellulose , it has b-1,4 linkages, and is thus resistant to breakdown. (Figure p 205)

Among the heteropolysaccharides are the glycans such as Hyaluronic acid, an alternating polysaccharide of D-glucuronic acid and N-acetyl-D-glucosamine; MW to 5,000,000 (Figure 8-12, p 208) which serves as a lubricant in joints and is a component of the vitreous humor. Again we see b-1,4 linkages.

Also very important are the glycans conjugated to proteins and peptides to give proteoglycans (Figure 8-13, p 209).

Pathway Diagrams

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Last modified 1 October 2001