Activities and locations of enzymes

Enzymes are specialized proteins that combine with a specific substrate and act to catalyze a biochemical reaction. In soils, enzyme activities are essential for energy transformation and nutrient cycling. The enzymes commonly extracted from soil, and their range of activities, are given in Table 3.7. Some enzymes (e.g., urease) are constitutive and routinely produced by cells; others such as cellulase are adaptive or induced, being formed only in the presence of a compatible substrate or some other initiator or in the absence of an inhibitor. Dehydrogenases are often measured because they are found only in living systems. Enzymes associated with proliferating cells occur in the cytoplasm, the periplasm, and the cell membrane. Figure 3.4 shows that soil enzymes are not only associated with proliferating cells but also associated, as extracellular enzymes, with humic colloids and clay minerals.

Standardized methods for a broad range of enzymes are described by Tabatabai (1994), Alef and Nannipieri (1995), and Schinner et al. (1996). A general introduction to enzymes in the environment and their activity, ecology, and applications is given by Burns and Dick (2002). This section will focus primarily on

TABLE 3.7 Some Enzymes Extracted from Soils, the Reactions They Catalyze, and Their Ranges of Activitya



Range of activity

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