ABSTRACT
The interaction of proteins with surfaces is a phenomenon that occurs in numerous processes [1-5]. This arises from the strong affinity for the solid-liquid and liquid-gas interfaces of these biopolymers, which is a consequence of the flexibility of the polypeptide chain and, above all, of the diversity of the physicochemical properties of their constitutive monomers. The 20 amino acids have side chains that can be classified on a hydrophobicity scale as nonpolar to polar and on an electrical scale as negatively, neutrally, or positively charged. This variety is far greater than the physicochemical properties of the constitutive sugars of polysaccharides or the nucleotides of nucleic acids and explains the unique properties of proteins with regard to their adsorption.These surface-active properties of proteins are important for understanding overall soil functions. Biogeochemical cycles of major elements (C, N, P, and S), rhizosphere phenomena, and some aspects of the degradation of xenobiotics are affected by protein adsorption [6-30]. Two properties of proteins are at the basis of this ecological importance. First, many of these proteins are enzymes,i.e., biological catalysts, and are involved in various chemical reactions in soils. Second, proteins have an average N content of 16% and are a main source of 171
this element when proteins are released in soil after death of the biota and lysis of membranes. Thus, in the soil nitrogen cycle, proteins can be both catalysts, such as proteases and amidases, and N-rich substrates.The enzymes found in soil have several origins and mechanisms of release. The most important sources are microorganisms, both bacteria and fungi. Root exudates make lesser contributions and soil fauna even less. The specialization of microorganisms in the secretion of extracellular enzymes is well evidenced in mycorrhizal associations where the fungal symbionts secrete phosphatases [31] and proteases [32] to mobilize P and N from soil organic matter. As concerns the mechanism of release, the enzymes are either essential for the survival and growth of the microorganism and are actively secreted, or they are released in soil after the death of the organism and the lysis of membranes.The main functions of extracellular enzymes are to transform insoluble substrates into soluble products and to decrease the structural complexity of the original substrates. Most of the chemical compounds released in soil after the death of organisms are in a polymeric form and are frequently either insoluble (e.g., cellulose, starch, chitin) or adsorbed on soil surfaces. As microorganisms have limited mobility in the pore system of soil, they need to secrete enzymes to reach these polymers and to hydrolyze them into compounds with lower molecular weight that can be desorbed and made soluble. These products of the enzyme reaction can diffuse in the water-filled part of the pore system, and a fraction can reach the microorganism producing the enzyme. The interception of the diffusing products is more efficient if a spatially scattered population of microorganisms is considered rather than a single isolated microorganism. The second effect is linked to the specificity of the membrane transport systems of organisms. Compared with the relatively small number of constitutive monomers of biopolymers (20 amino acids, approximately 10 relatively abundant monosaccharides, five nucleotide bases) or chemical groups important in nutrition that can be released from these polymers (e.g., orthophosphate, sulfate), the number of molecular conformations increases exponentially with the number of monomers present in the polymer. To avoid this exponential requirement for different specific membrane transport systems, the enzymatic hydrolysis of biopolymers into monomers or low-molecular-weight oligomers is a favored mechanism. As the biogeochemical cycles of C, N, P, and S always imply a transformation from a polymeric form to small simple chemical groups, it can be assumed that all biogeochemical cycles in soil depend at some stage on the action of extracellular enzymatic systems (cellulases, amylases, xylanases, proteases, amidases, nucleases, lipases, phosphatases, arylsulfatases, etc.).From a historical point of view, soil scientists led the research on the interaction between proteins and colloidal surfaces from the 1930s to the 1960s [3336]. In particular, McLaren and co-workers, as early as the 1950s [37-39], demonstrated two essential aspects that explained most of the behavior of enzymes adsorbed on electrically solid surfaces, i.e., the shift in pH for the optimal activity
of an adsorbed enzyme and the maximum in the adsorption of proteins at the isoelectric point (pi). However, they failed to reach the appropriate mechanistic explanation for these two observations, for reasons that will be explained later and that were related to a lack of understanding of the stability of protein structure in the scientific community at that time and to a misinterpretation of a thermodynamic parameter.In the last 30 years, several new topics involving the interaction of proteins with surfaces have emerged, and the human and financial forces engaged in understanding this phenomenon have now shifted largely toward biotechnological processes and problems of human health. These new topics include immobilized enzyme reactors, solid-phase immunoassays, enzyme electrodes, biocompatibility of medical implants to avoid induction of thrombosis, and fouling of contact lenses and filtration membranes for water treatment. This list will, undoubtedly, become larger with time.The aim of this chapter is to show that if the enzyme activity in soil is analyzed at a molecular level as the consequence of physicochemical interactions, a vast scientific literature becomes available to soil scientists. To achieve this, analogies must be drawn between various systems, and, therefore it is necessary to understand the nature of the forces involved. The new biotechnological and medical studies do not focus on surfaces or coating polymers that are relevant to soil science, such as clays, oxyhydroxides, or humic substances. A noticeable exception is the growing use of muscovite surfaces as a support for the study of polymers by techniques such as surface force apparatus or atomic force microscopy where atomically smooth surfaces are required. Because muscovite, a mica, is a phyllosilicate, it shares many surface properties with clay minerals.Nevertheless, the long-term consequences of interactions with solid surfaces regarding genetic evolution toward enzyme structures better adapted for catalysis in the adsorbed state (i.e., not unfolding on surfaces) will remain specific to the biology of natural ecosystems, especially soils. In contrast, all the biotechnological and medical devices involving adsorption of proteins are man-made situations where the proteins used will never be in contact with the surface of interest under natural conditions, thereby allowing no expression of evolutionary phenomena. In the future, the topic of evolutionary adaptation of enzymes that are naturally in the presence of solid surfaces will probably attract a growing interest of scientists working in other fields than soil for its possible biotechnolog ical applications.
