Amino Sugar Polymers

The basic unit of the cell walls of fungi and also of the exoskeleton of insects is chitin (Peberdy 1990). It is a highly ordered polysaccharide of (1-4)-linked 2-acetamido-2-deoxy-ß-d-glucose (N-acetylglucosamine; Fig. 2.2). Chitosan is composed of the deacetylated form (2-amino-2-deoxy-ß-d-glucose).

Eubacterial cell walls are composed of a peptidoglycan (also called murein or mucopeptide), which contains carbohydrate as well as amino sugar and amino acid elements (Fig. 2.3). Whereas glucosamine is also found in insects and fungi, galactosamine, muramic acid and diaminopi-melic acid are only found in bacteria (Stevenson 1994).

Mureins are peptidoglycans consisting of polysaccharide chains with short peptides attached (Fig. 2.4). The carbohydrate backbone of murein

NHCOCH3 CH2OH NHCOCH3 CH2OH NHCOCH3

NHCOCH3 CH2OH NHCOCH3 CH2OH NHCOCH3

chitin ch2oh

"A

nh-c-ch3 O

ß-N-acetyl-D-galactosamine nh2

meso-diaminopimelic acid

Fig. 2.3. Structures of major amino sugar compounds (glucosamine, galactosamine, mu-ramic acid) and diaminopimelic acid found in soils

hon nh-c-ch3 II O

ß-N-acetyl-D-glucosamine nh-c-ch3 II O

ß-N-acetyl-D-glucosamine nh

1 I ch3-ch-cooh ch3

N-acetyl muramic acid

N-acetyl glucosamine

N-acetyl muramic acid

N-acetyl glucosamine

N-acetyl muramic acid

alanine diaminopimelic acid

Fig. 2.4. Structure of a peptidoglycan (murein) subunit alanine diaminopimelic acid

Fig. 2.4. Structure of a peptidoglycan (murein) subunit is composed of alternating units of 2-acetamido-2-deoxy-3-d-glucose (N-acetylglucosamine) and 2-acetamido-3-O-(1-carboxyethyl)-2-deoxy-^-d-glucose (N-acetylmuramic acid). The cross-linking of the chains occurs via peptide bridges (de Leeuw and Largeau 1993). The composition of the peptides varies considerably between Gram-positive and Gram-negative bacteria, and also between individual species. They contain common amino acids, but also specific amino acids, such as d-glutamic acid and d-alanine, diaminopimelic acid and d-lysine (Koch 1990). Cell walls of Gram-positive bacteria contain approximately 20-40 murein layers, whereas the cell walls of Gram-negative bacteria are composed of fewer, even possibly only one murein layer. Therefore, murein amounts to approximately 50% of the dry weight of the Gram-positive cell wall, but only 10% of the dry weight of the cell wall of Gram-negative bacteria (Koch 1990).

Organisms use nucleic acids for the storage of genetic information (DNA) and for the translation of this genetic information for protein synthesis (RNA). The nucleotides forming DNA and RNA are composed of a phosphate, sugar and heterocyclic N-containing base unit, i.e. the purine units adenine, guanine and the pyrimidine units uracil, cytosine, and thymine (Fig. 2.5). Outside living cells, RNA and DNA are considered to be biode-graded very easily (de Leeuw and Largeau 1993). In soils they are susceptible to adsorption onto clay minerals as well as humic materials. These surface interactions allow them to persist in the soil environment, but their bioac-tivity maybe diminished (Chenu and Stotzky2002). For further details, see Chaps. 6 and 7.

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