Amino acids release from bacterial cells

Additional evidence for a bacterial contribution to HMW DOM proteins comes from molecular-level analyses of dissolved amino acids. Hydrolysis of HMW DON releases 11-29% of the nitrogen as amino acids (McCarthy et al., 1996). Specific amino acids include common protein amino acids, as well as /3-alanine and y-aminobutyric acid which are nonprotein amino acid degradation products. The distribution of amino acids is similar to that of fresh plankton cells, suspended particulate matter, and total dissolved amino acids. However, stereochemical analyses show HMW DOM amino acids to be elevated in the D-enantiomer, with d/l ratios for alanine, aspartic acid, glutamic acids, and serine ranging from 0.1 to 0.5 (McCarthy et al., 1998). Racemization of phytoplankton-derived L-amino acids is too slow at ocean temperatures to yield such high D/L ratios, but bacteria can synthesize D-amino acids, and it is likely that the D-amino acids in HMW DOM result from bacterial bioploymers rich in these particular amino acids. The high dA ratios of some amino acids and the abundance of amide nitrogen in HMW DOM N-NMR spectra led McCarthy et al. (1998) to... 

Apart from the immune system, there is little use for the degradation products released by the proteasome, and several peptidases exist which complete their conversion into free amino acids. One of them is tricorn peptidase, originally found in Thermoplasma cells, where it exists in the form of a giant icosahedral complex of approximately 15 MDa (Tamura et al., 1996 Walz et al., 1997). It has been shown in in vitro studies that tricorn in cooperation with an array of aminopeptidases converts oligopeptides of 8-12 residues efficiently into free amino acids (Tamura et al., 1998). Tricorn peptidase has been found in a number of archaeal and bacterial species, but it is not ubiquitous functional homologs are... 

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