Proteins and Polypeptides

In living organisms most of the organic N is in the form of protein-bound amino acids. Proteins serve manifold purposes, e.g. as enzymes, transport proteins, regulators, storage substances or as structural proteins, and are usually composed of the 20 most frequent amino acids, which can be subdivided into basic, neutral or acidic amino acids (Fig. 2.1). In addition to the 20 major amino acids of proteins, bacterial cell walls also contain a series of unusual amino acids, linked in a two-dimensional structure which provides rigidity and elasticity to the bacterial cell wall (Koch 1990). The proteins from plant and microbial tissues can be decomposed by a multi-

neutral amino acids

HC-COOH H

CH3-C-COOH H

glycine alanine

CH3-CH-CH2-C-COOH leucine 1 H

CH3-CH-C-COOH valine 1 H CH3

OH-CH2-CH-COOH serine aromatic amino acids phenylalanine

-C-CH2-CH-COOH tryptophan „CH

-C-CH2-CH-COOH tryptophan „CH

secondary amino acids CH2-CH2

CH2 CH-COOH proline

acidic amino acids

NH2 i

HOOC-CH2-CH-COOH asparticacid

NH2 i

HOOC-CH2-C-COOH glutamic acid H

HO-CH-CH2

CH2 CH-COOH hydroxyproline ^NH"

basic amino acids amide bond nh2

HO H NH2-C-NH-CH2-CH2-CH2-CH-COOH arginine

R1 H R1 NH2

Fig. 2.1. Examples for the chemical structures of neutral, acidic, basic, aromatic and secondary amino acids and the amide bond tude of microorganisms and are considered to be less stable components with high turnover rates, due to the easily decomposable amide linkage (de Leeuw and Largeau 1993; Derenne and Largeau 2001). Nonetheless, there is growing evidence that enzymes and other protein-type materials may survive over extended time spans in soils (Nannipieri et al. 1990; Ladd et al. 1996; Amelung 2003; Knicker 2004).

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