Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438	Introductory Biochemistry	Spring 2007
Lecture Notes: 28 February	© R. Paselk 2006
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CARBOHYDRATES, cont.

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 polysaccharides contain glucose. Used for energy (food) storage (starches and glycogen) and structure (cellulose).

Starch (energy storage in plants). Two kinds

• Amylose: linear, alpha-1,4 glycosidic links. MW: 4,000-150,000. (Figure 8.22) [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 units - linear alpha-1,4 chain of 30 glu residues then alpha-1,6 branch point. of course get branches on branches as well. MW: ->500,000. (Figure 8.23, 8.24) [overhead 10-18, V&V] Broken down by alpha-amylase (pancreas and salivary glands; random cleavage of alpha-1,4 links) to give glucose and maltose; or beta-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: beta-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 8.25, 8.26) [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 beta-1,4 linkages, and is thus resistant to breakdown. (Figure 8.27)

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 8.28) which serves as a lubricant in joints and is a component of the vitreous humor. Again we see beta-1,4 linkages.

Also very important are the glycans conjugated to proteins and peptides to give proteoglycans (Figure 8.29).

LIPIDS

Recall the lipid definition: The portion of an organism which will partition into a non-polar solvent.

Types of Lipids: (Figure 9.1) [overhead 11.1, P]

Pathway Diagrams

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Lecture Notes

Last modified 28 February 2007