Interaction with Soil Constituents Determines the Environmental Impact of Proteins

Proteins are important compounds in soils, made up of monomeric amino acids joined by peptide bonds, and there are several reasons for studying their fate in soil: (i) their amino acids are an important source of soil nitrogen, (ii) enzymes secreted by soil microorganisms, plant roots and soil invertebrate guts can be involved in the biogeochemical cycles of soil organic matter (Quiquampoix, 2000; Quiquampoix and Mousain, 2005), (iii) pathogenic proteins such as prions involved in transmissible spongiform encephalopathies (Brown and Gajdusek, 1991; Revault et al., 2005) or insecticidal toxins expressed in transgenic plants (Tapp and Stotzky, 1998; Pagel-Wieder et al;, 2004) represent a growing environmental concern. Proteins generally have a strong affinity for all types of interfaces found in soil, both the solid–liquid interfaces of soil minerals and organic colloids and the liquid–gas interfaces developed in a microporous system and dependent on the soil pore-water content. The affinity for various types of interfaces originates in the flexibility of the polypeptide chain and in the diversity of the 20 amino acids that can be classified on an electrical scale as positively, neutrally or negatively charged and on a hydrophobic scale from polar to non-polar. These properties give rise to a large variety of interactions with soil surfaces, relationships that may be dominated by enthalpic or entropic contributions to the free energy. The strong and often largely irreversible adsorption of proteins on the mineral phase of the soil has important consequences not only on their mobility, but also on their resistance to breakdown (proteins as N source in soil) and functional activity (catalylic for enzymes, infectious for prions, toxic for Bt proteins). Extracellular enzymes can be secreted into soil solution by microorganisms. This process makes possible the degradation of the soil organic matter since polymers cannot usually pass through the membranes of the microorganisms and need to be hydrolysed into soluble low molecular weight compounds which can reach membrane permeases specific for monomers (sugars, amino acids) or occasionally oligomers. For this reason, they will play an important role in the biogeochemical cycles of C, N, P and S in soil. A consequence of the adsorption of these extracellular enzymes on mineral surfaces is a shift of the optimal pH of the catalytic activity toward more alkaline values and a general decrease of their activity. All proteins can make an important contribution as biochemical substrates in proteolysis reactions in the N cycle when released in soil after death of biota and lysis of the membranes since they have an average N content of 16%. Prion proteins are an infective agent of several transmissible spongiform encephalopathies (TSE). Epidemiological studies of ovine scrapie in Europe and, more recently, the chronic wasting disease of wild cervids (deer, elk and moose) in North America, suggest the possible horizontal transmission from a soil reservoir.