Apparent kinetic properties of soil phosphomonoesterase and β-glucosidase are disparately influenced by pH

Extracellular enzymes catalyze organic matter transformations in soils, yet the study of soil enzymes does not typically evaluate how enzyme kinetic properties that govern activity in situ may shift with environmental parameters such as pH. To test this, we quantified changes in apparent kinetic properties (maximum catalysis rate [^AppV_max] and substrate concentration that produces 50% of V_max [^AppK_m]) of soil phosphomonoesterase (PME) and β-glucosidase (BG) as a function of pH by using dual gradients of buffer pH (3–12) and substrate concentrations in a selected set of diverse soils. A third kinetic property, apparent specificity constant (^AppK_a = ^AppV_max/^AppK_m) was also evaluated as a function of pH. Results indicated widely differing pH optima of soil PME and BG ^AppV_max and ^AppK_m, decoupling of which entailed unpredictable pH optima of ^AppK_a. Additionally, changes in soil enzyme activity with pH may not be driven by shifts in the specificity constant because pH–^AppV_max relationships are largely unrelated to co-occurring changes in ^AppK_a. These results invalidate the assumption that K_m of a soil enzyme is constant across pH and emphasize the importance of assaying soil enzyme activities at sufficiently high (5 × K_m) substrate concentrations at a given pH to avoid the mischaracterizing pH optima.