Surface-exposed binding sites

Electrostatic interaction with the proteoglycans, however, is not the only possibility for interaction between a DNA-carrying transfection agent and a cell surface. Many membrane proteins expose binding sites (receptors) for certain biochemical messenger molecules (ligands). In general, such receptor proteins control the specific uptake of molecules and make the cell... 

Further applications of cyclen-derived chelates include imaging of bone cracks with [Eu(L25)] " (Scheme 4.9) after hydrolysis in Me0H/H20. A sample of bovine tibia is immersed in a solution containing the Eu chelate for 24 h and imaged by confocal luminescence microscopy a healthy bone surface does not show emission while a scratched one is highly luminescent, with an intensity ratio of 1 28. The explanation is that scratched surfaces expose Ca sites which bind to the Eu chelate [151]. 

Competitive immunoassays may also be used to determine small chemical substances [10, 11]. An electrochemical immunosensor based on a competitive immunoassay for the small molecule estradiol has recently been reported [11]. A schematic diagram of this immunoassay is depicted in Fig. 5.3. In this system, anti-mouse IgG was physisorbed onto the surface of an SPCE. This was used to bind monoclonal mouse anti-estradiol antibody. The antibody coated SPCE was then exposed to a standard solution of estradiol (E2), followed by a solution of AP-labeled estradiol (AP-E2). The E2 and AP-E2 competed for a limited number of antigen binding sites of the immobilized anti-estradiol antibody. Quantitative analysis was based on differential pulse voltammetry of 1-naphthol, which is produced from the enzymatic hydrolysis of the enzyme substrate 1-naphthyl phosphate by AP-E2. The analytical range of this sensor was between 25 and 500pg ml. 1 of E2. 

The first consideration was the speciation and distribution of the metal in the sediment and water. Benthic organisms are exposed to surface water, pore water and sediment via the epidermis and/or the alimentary tract. Common binding sites for the metals in the sediment are iron and manganese oxides, clays, silica often with a coating of organic carbon that usually accounts for ca. 2% w/w. In a reducing environment contaminant metals will be precipitated as their sulfides. There is not necessarily a direct relationship between bioavailability and bioaccumulation, as digestion affects the availability and transport of the metals in animals, in ways that differ from those in plants. 

CR3 comprises two polypeptides, an a-subunit of 185 kDa and a -sub-unit of 95 kDa, which are covalently linked into an a fl structure. Both chains are exposed at the cell surface, whilst the a-chain forms the major portion of the ligand-binding site. The /J-subunit is common to the other adhesins, LFA-1 and pi50,95. These have an antigenically-distinct a-chain but a common /J-chain (see Fig. 3.6). Of these three receptors with common j3-chains, only CR3 binds significant amounts of C3bi. 

Fig. 6. A highly idealized model for the plasma albumin molecule to account for the N-F transformation and its relationship to the titration anomaly, the cooperative detergent binding, and the altered solubility behavior of the low pH form. The model contains four folded amphipathic subunits, the hydrophobic surfaces being buried in the N form and exposed in the F form. Holes around the periphery of the molecule represent the 10 to 12 strong binding sites for detergent ions which are destroyed, upon isomerization, with the exposure of a large number of weaker sites. Reprinted with permission from Foster (1960). Copyright by Academic Press, Inc.

Ma, B. Elkayam, T. Wolfson, H. Nussinov, R. Protein-protein interactions structurally conserved residues distinguish between binding sites and exposed protein surfaces. Proc. Natl Acad. Sci. USA 2003, 100, 5772-5777. 

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