ADSORPTION OF ALKALINE PHOSPHATES ON PALYGORSKITE AND SEPIOLITE: A TRADEOFF BETWEEN ENZYME PROTECTION AND INHIBITION
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ADSORPTION OF ALKALINE PHOSPHATES ON PALYGORSKITE AND SEPIOLITE: A TRADEOFF BETWEEN ENZYME PROTECTION AND INHIBITION MEHRAN SHIRVANI, BANAFSHE KHALILI * , MAHMOUD KALBASI, HOSSEIN SHARIATMADARI, AND FARSHID NOURBAKHSH 1
Department of Soil Science, College of Agriculture, Isfahan University of Technology, Isfahan 8415683111, Iran
Abstract—Enzymes adsorbed on clay minerals and soil colloids may exhibit lower activities compared to those of free enzymes. A particular toxic metal may affect the activity of the adsorbed enzyme less critically than that of the free form, however. This information is necessary for predicting catalytic performances of clay-immobilized enzymes in natural soils as well as in food, pharmaceutical, and chemical systems. The objective of the present study was to find out how adsorption on palygorskite and sepiolite minerals modifies the catalytic activity and the Michaelis–Menten kinetics of alkaline phosphatase (ALP). Inhibition kinetics of adsorbed ALP by Cd was also compared to that of the free enzyme. The results revealed that the affinity to the substrate and the maximum reaction velocity of ALP decreased upon adsorption on the fibrous clay minerals. The ALP adsorbed maintained a reasonably high activity recovery (AR) compared to the free enzyme. The AR of the adsorbed ALP ranged from 76.9 to 92.5% for palygorskite and from 71.2 to 90.2% for sepiolite, depending on the substrate concentration applied. The presence of Cd decreased the affinity to the substrate of both the free and the adsorbed ALP, while the maximum reaction velocity remained nearly unchanged, indicating that the inhibitory effects of Cd on both the free and adsorbed ALP activities were competitive in nature. The adsorbed enzyme, however, was inhibited less severely by Cd compared to the free enzyme. The adsorption of ALP on the fibrous clay minerals, therefore, maintains the ALP activity to a great extent and provides more resistance for the enzyme against the inhibitory effects of Cd. Keywords—Enzyme kinetics . Enzyme immobilization . Fibrous clay minerals . Heavy metals INTRODUCTION Alkaline phosphatase is one of the most important hydrolase exoenzymes in the soil environment. Soil ALP is thought to arise solely from microorganisms and is responsible for phosphate ester bond cleavage and dephosphorylation of organic substrates (Stevenson and Cole 1999). Hence, ALP is central to the decomposition of plant and microbial detritus organic phosphorus and is increasingly incorporated into soil biogeochemical models as a driver for detritus breakdown (Nannipieri et al. 2011; Schimel et al. 2017). ALP has also been used as a sensitive indicator for assessing soil-quality changes in response to environmental stresses and management practices (Gülser and Erdoğan 2008; Vandana et al. 2012; Xin et al. 2016). Extensive research has been carried out over recent decades on the interactions of exoenzymes with soil minerals, which has helped to advance understanding of the behavior of enzymes in soils. An established and tested hypothe
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