Humic enzyme complexe

The main portion of the input of organic material into the sediment is particulate organic carbon which has to be enzymatically decomposed, at least partly, prior to incorporation into cells. In this process, extracellular enzymes are involved which are secreted by living cells (Corpe and Winters, 1972) or liberated during the lysis of dead and decaying cells. As shown by Burns (1980) some of these enzymes may retain their activity outside the cells by the formation of humic-enzyme complexes bound to clay particles. 

Ruggiero, P., and Radogna, V. M. (1988). Humic acids-tyrosinase interactions as a model of humic-enzyme complexes. Soil Biol. Biochem. 20,353-359. 

FIGURE 6 Potential interactive pathways and processes of humic substances emanating from decomposition products of higher plant tissues with extracellular and surface-bound enzymes and photolytic reactions, particularly with UV irradiance. Humic acid-enzyme complexes can be stable for long periods (weeks and months) and subsequently reactivated upon exposure to weak UV light. Further photolysis can cleave simple compounds from the macromolecules for subsequent utilization by microbes. 

The relatively recalcitrant dissolved poly-phenolic compounds resulting from breakdown of higher plants (fulvic and humic acids) complex with many bacterial and algal enzymes, but particularly phosphatases (Wetzel, 1992). The formation of such complexes inactivates phosphomonoesterase (Boavida and Wetzel, 1998), which is inhibited both competitively and non-competitively (Wetzel, 1992). Phosphorus-limited cells therefore need to expend more energy on phosphatase synthesis, and enhanced phosphomonoesterase activity has been reported for waters with increased humic materials (Stewart and Wetzel, 1992b). Wetzel (1992) pointed out these results with frequent observations (e.g. Jones, 1990) that the primary production in humic-rich waters is consistently lower than in clear waters with comparable loadings and light availability. 

Model enzyme complexes. Early studies by Ladd and Butler show ed that soil humic acids and trypsin reacted ionically to form insoluble precipitates with initially decreased enzyme activity, compared with unreacted trypsin, but with enhanced stability against autodigestion. In this example the enzyme and humic acid could be easily separated. Later, Rowell et prepared complexes of trypsin and Pronase by reacting the... 

These studies on model enzyme complexes imply that the organic compounds which stabilize exocellular enzymes in soil are brown-coloured humic materials, probably aromatic in character. However, knowledge of the nature of the organic ligands and of the manner in which they may complex active enzymes, rests in part on the extraction of enzymes from soil, preferably in high yield and without artefact formation, and their characterization after fractionation and purification. 

MnP is the most commonly widespread of the class II peroxidases [72, 73], It catalyzes a PLC -dependent oxidation of Mn2+ to Mn3+. The catalytic cycle is initiated by binding of H2O2 or an organic peroxide to the native ferric enzyme and formation of an iron-peroxide complex the Mn3+ ions finally produced after subsequent electron transfers are stabilized via chelation with organic acids like oxalate, malonate, malate, tartrate or lactate [74], The chelates of Mn3+ with carboxylic acids cause one-electron oxidation of various substrates thus, chelates and carboxylic acids can react with each other to form alkyl radicals, which after several reactions result in the production of other radicals. These final radicals are the source of autocataly tic ally produced peroxides and are used by MnP in the absence of H2O2. The versatile oxidative capacity of MnP is apparently due to the chelated Mn3+ ions, which act as diffusible redox-mediator and attacking, non-specifically, phenolic compounds such as biopolymers, milled wood, humic substances and several xenobiotics [72, 75, 76]. 

Silver tends to form complexes with inorganic chemicals and humic substances in soils (Boyle 1968). Since silver is toxic to soil microorganisms and inhibits bacterial enzymes (Domsch 1984), biotransformation is not expected to be a significant process. 

A major experimental issue to be addressed is the rate and means by which particles are hydrolyzed and solubilized to provide substrates for heterotrophic bacteria, and the role of free enzymes in this process. Burns (1982) reviewed the possible locations and origin of enzyme activities in soils, and particularly underscored the potential importance of enzyme-humic complexes in microbial catalysis of substrates. As Burns (1982) discussed, enzymes associated with soil particles or humic substances are not subject to the same biochemical and physical restraints as are enzymes newly produced by microbial cells. Soil-held (or sediment-held) enzymes may therefore play a catalytic trigger role in substrate degradation, providing critical signals about substrate availability to the local microbial community. The conceptual model presented by Vetter et al. (1998) suggested that release of free enzymes into the environment may in fact represent... 

Wetzel, R. G. 1993. Humic compounds from wetlands Complexation, inactivation, and reactivation of surface-bound and extracellular enzymes. Internationale Vereinigung fiir Theoretische und Angewandte Limnologie Verhandlungen 25 122—128. 

Humic acids have been shown to slightly inhibit tyrosinase activity by complexing the enzyme (Ruggiero and Radogna, 1988). Allison (2006b) also demonstrated that the addition of humic acid to a soil significantly decreased the polyphenoloxidase activity of the soil. 

In the past the mineral matrix was considered as inert, only providing stabilization support for enzymes and humic substances however, due to the overwhelming amount of evidence at the molecular level, there is no doubt that minerals participate in abiotic catalysis of humification reactions in soils. Naidja et al. (2000) referred to mineral particles as the Hidden Half of enzyme-clay complexes, which not only prolong the activity of immobilized enzymes but also are readily able to participate in electron transfer reactions. Many environmental factors can negatively affect the... 

The more soluble humic substances ean coat insect bodies, fecal material, and plant materials in lower horizons of the soils. Complexation of enzyme proteins by humic substances (Burns, 1978) inhibits their catalytic activities. An envelope of humic substances around even readily decompos-... 

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