Biotic receptors

Where do receptors with useful properties for sensor construction come from The answer depends on your definition of the word useful. If useful requires only selectivity, then nature provides both a wealth of biotic receptors (enzymes and other proteins) and powerful tools for discovering unnamral macromolecular receptors. For the purposes of this chapter, a sensor that utilizes a polypeptide, polysaccharide, or polynucleotide as the recognition element is referred to as a biosensor. By contrast, any sensor utilizing a different (usually synthetic) recognition element is referred to as a chemosensor. For the purposes of this article, these terms will apply to any device, molecule-sized or larger, that utilizes such compounds for its molecular recognition function. 

Nature has required the evolution of usefully selective hosts, and proteins (in the forms of enzymes, receptors, and antibodies) provide them. However, no individual protein molecule lasts in a cell for very long. All proteins are constantly anabolized and catabolized, with constant concentrations achieved via homeostasis. Nature never demanded permanence of its molecular recognition machinery. When we utilize biotic receptors for one-time, batch analytical applications, the receptors clearly meet the useful criterion. However, if a receptor must have an extended lifetime in a sensing device, then we propose that biotic receptors represent the easiest place in which to search instead of the right place to search. If a biotic receptor cannot reasonably be made stable enough to survive weeks of service, then it will not be useful for a sensing application no matter how avid or selective. 

Each of these examples of chemosensor creation either required extensive synthesis effort to discover the receptor or benefited from such an effort previously published. Indeed, it is precisely this effort that makes the use of biotic receptors so appealing. Is there hope that usefully selective chemosensors can be discovered with less time... 

Blumberg B, Sabbagh W, Jr, Juguilon H, Bo-lado J, Jr, van Meter CM, Ong ES, Evans RM (1998) SXR, a novel steroid and xeno-biotic-sensing nuclear receptor. Genes Dev 12, 3195-3205. 

This process measures the intensity, frequency, and duration of exposure to an agent. More precisely, it evaluates the potential exposure to each chemical of concern by the potential receptor populations or indicator species that are biotic components of any ecosystems in the site which may be affected. Two important components of an exposure assessment are (a) an exposure pathway analysis, and (b) the estimation of exposure point concentrations. To evaluate potential exposures, one requires concentrations of the chemicals of concern in the exposure media (e.g., surface water or sediments). 

The preceding discussion revealed that once an initial attack, whether biotic or abiotic, disturbed the cyclic nitramine (RDX, HMX, or CL-20), the subsequent degradation reactions are largely governed by the interactions of the newly formed intermediates with water to eventually produce N02, N20, NH3, HCHO, and HCOOH (Figures 2.2, 2.3, 2.5, and 2.6). Knowledge of the product distributions provides insight into the adverse effect of this family of explosives to various receptors in the environment. 

Explosive compounds may undergo extensive transformation in aquatic systems, by microbial attack or abiotic mechanisms such as hydrolysis, oxidation, phototransformation, and so forth. Therefore, aquatic receptors may be exposed not only to energetic compounds released to the environment but also to their numerous transformation products. The key biotic and abiotic transformations of major explosives are discussed in Chapter 2. 

Natural attenuation (NA) refers to the reduction of contaminant concentrations in environmental media by processes such as dilution, dispersion, sorption, volatilization, and biotic or abiotic transformations. As a passive remediation approach, NA is mostly applied in connection with organic contaminants which migrate from the source in groundwater. Naturally attenuating contaminant plumes show a decline in the dissolved contaminant mass as a function of time, and a decline in contaminant concentrations downgradient from the source. Before potential receptor exposure points are reached, natural attenuation is expected to reduce dissolved contaminant concentrations below regulatory standards (Wiedeme-ier et al. 1999). 

One approach to the fabrication of such hosts is to utilize the many such receptors available from biological sources. Selective binding of species is demonstrated by enzymes for their substrates (and cofactors), by transport proteins for their transportees, and by antibodies for their antigens. In particular, because antibodies can be raised to many antigens, the biotic creation of hosts proves a powerful method for binding analytes with selectivity. Indeed, it is fair to say that... 

The recognition and complexation of neutral (uncharged) guest molecules by abiotic receptors is a relatively recent area of chemical investigation, although this type of complexation plays a fundamental role in biotic systems e.g. the base-pairing of nucleic acids, or enzyme-substrate interactions. 

In QSARs, the biological activity is also produced by the interaction between the ligand and the biological receptor of an organism. The only difference is that the ligand in QSARs is a chemical species, and the biological receptor is the biotic ligand. 

---