Enzymes, abiontic

The persistence of some enzymic activities in soils, and the possibility that active abiontic enzymes may be stabilized by combination with soil colloids, has led a minority of workers to attempt the characterization of enzymes in soil extracts and in other fractions. Elucidation of the mechanisms by which abiontic enzymes are stabilized in soil may be important in the wider context of understanding the processes which confer biological resistance on soil organic N. 

The persistence of soil enzymes active against low molecular weight substrates only is less predictable in that reactions may be catalysed solely by intracellular enzymes, or partly by intracellular and partly by exocellular (abiontic) enzymes. Activities by exocellular enzymes may be less dependent on energy source availability because of contributions from accumulated, protected enzymes. 

Active enzymes have been extracted from soils by shaking suspensions for periods ranging from 5 min to 24 h, and at temperatures, sometimes ambient or below, but more usually at 37-40°C (Table 3). Generally conditions are chosen to avoid extensive damage to live cells and to favour extraction of abiontic enzymes. Mostly, soil extracts have been assayed for hydrolases, some have been fractionated and the organic matter associated with enzymic activity has been partly characterized. Kinetic constants, stabilities. 

Enzymic activities of crude soil suspensions have been demonstrated to follow Michaelis-Menten kinetics. Calculated Km values have varied for different soils and for active fractions of soil extracts. The extent to which kinetic constants of soil enzymes are influenced by the state in which the enzymes occur in soils, is unknown. For some activities, two Km values have been distinguished for the one crude soil extract. Fractionation has revealed that enzymes may exist in tightly- and loosely- bound complexes with soil coloured humic compounds. Enzymes freed of coloured materials may nevertheless still be bound in complexes by their association with carbohydrates. These may not only influence enzyme kinetic properties but evidence suggests that they may also confer some degree of stability on the enzymes in soil. Whereas early speculation on the mechanism(s) by which enzymes are protected in soil tended, in the case of abiontic enzymes, to focus on the role of clay or humic colloids, fractionation studies have drawn attention to the potentially important role of soil carbohydrates. The manner in which carbohydrates are bonded to the enzymes in soil has not as yet been established and may be a fruitful line of enquiry. 

Comparisons of the properties (Km, pH-activity profiles) of soil enzymes with those of "free" enzymes, ie. enzymes purified from known plant or microbial sources, have served to reinforce concepts of the mechanisms by which abiontic enzymes exist in soil, but otherwise they also have limited value. Not only may the soil enzymes differ from each other and from the selected reference enzymes, both in their origins and in their intrinsic properties, but the reference enzyme itself may exhibit different kinetic properties according to its state of purification. 

Abiontic, involving free extracellular enzymes or solubilizing agents, enzymes bound to soil surfaces, enzymes within dead or non-proliferating cells, or enzymes associated with dead cell fragments. Extracellular enzymes are important in the initial stages of organic matter oxidation, in which polysaccharides and proteins are hydrolysed to soluble compounds that can be absorbed by microbial cells and further oxidized in biotic processes. 

Skujins, J. 1978. History of abiontic soil enzyme research, pp. 1-49 in R.G. Burns, ed. Soil Enzymes. Academic Press, London. 

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