POLYSACCHARIDES



Storage polysaccharides:

All of the common storage polysaccharides are homopolymers of -D-glucopyranose, which is glucose in its six-membered ring configuration.

Amylose and amylopectin are found in plant as starch; amylose is a linear polymer of glucose linked a(1->4).

Glycogen is the animal storage form of glucose; like amylopectin, glycogen occurs as branched chains (some glycosidic bonds are a 1 -> 4; others are a 1 -> 6).

Amylose (starch) can bind molecular iodine due to the fact that sugar can from a helix, into which iodine fits to form a complex that produces a deep blue color. This chemical characteristic allows starch to be used as an indicator for titrations involvi ng the reduction of iodine (I2 --> 2 I-).

Glucose is stored as polymer, rather than individual molecules, in part to reduce problems related to osmotic pressure (recall Raoult's law).

Structural polysaccharides:

Cellulose, a linear homopolymer of glucose (b1 -> 4), is found in woody and fibrous plants, where it serves a structural function because plants don't use fibrous proteins like collagen and keratins for structure.

Humans and most other animals can't hydrolyze this form of glucose because they don't have cellulases, which are present in some fungi, ruminant bacteria, and termites.

Cellulose exists as extended chains (no helix); each glucose is flipped by 180o with respect to its neighbor; the hydrogen bonding that occurs between strands (like -sheet of silk fibroin) is strong, but has no elasticity.

Chitin is a polysaccharide found in the exoskeletons of arthropods and mollusks; it is a homopolymer of N-acetyl--D-glucosamine.


Glycosaminoglycans:

These compounds, which were formerly called mucopolysaccharides, are all repeating disaccharide units in which one of the sugars is either N-acetylgalactosamine or N-acetylglucosamine.

Examples:

a. chondroitin sulfates: found in connective tissue, consists of glucuronic acid + GalNAc-6-sulfate
b. keratin sulfates: found in connective tissue, consist of Gal + GalNAc-6-sulfate
c. dermatan sulfates: found in skin, contain hyaluronic acid
d. synovial fluid: found in the joints between bones and the vitreous humor of the eye, contains glucuronic acid + GalNAc
e. heparin: acts as an anticoagulant, constructed of sulfated carbohydrates


Bacterial cell walls:

Most unicellular organisms have a cell wall to protect them from osmotic stress.

Microbiologists have taken advantage of differences in cell wall construction among bacteria to create a classification system. Bacteria can be easily divided into Gram-positive and Gram negative based on their response to a simple staining procedure. Gram stain is a mixture of gentian violet (a dye) and iodine.

Gram-positive bacteria have a multilayered polysaccharide-peptide complex called the peptidoglycan (see below), whereas Gram-negative strains have a single layer of peptidoglycan covered by outer lipid membrane.

This lipid membrane prevents the Gram stain from reaching the carbohydrate with which it would react. During the staining procedure it is washed away and no staining results.

Peptidoglycan:

The peptidoglycan is an alternating copolymer of N-acetylgalactosamine and N-acetylmuramic acid that is linked to a tetrapeptide consisting of (L-Ala)-(D-Glu)-(L-Lys)-(D-Ala), which is in turn cross-linked by a pentapeptide of Gly.

The peptidoglycan is anchored to the cell membrane by lipoteichoic acids.

Human sleep requires muramic acid (see above) which apparently comes from E. coli found in our digestive tract. This is one reason why sleep is more difficult when someone has an intestinal upset.

Penicillin and ampicillin are antibiotics that inhibit bacterial growth by interfering with the formation of the peptidoglycan layer.

These antibiotics are beta lactams (cyclic amides) that resemble amino acids but don't react like them. Bacterial resistance to these antibiotics comes from gaining a plasmid (extrachromosomal DNA) coding for the enzyme beta lactamase, which can hydrolyze the amide bond of the antibiotic.