Proteins main-chain anion binding sites

Main Chain Anion Binding Sites in Proteins Nests... 

Watson, J. D., Milner-White, E. J., A novel main-chain anion-binding site in proteins the nest. A particular combination of ( > and ((/ values in successive residues gives rise to anion-binding sites that occur commonly and are found often at functionally important regions , J. Mol. Biol. 2002, 315, 171-182. 

In the pH range close to the protein s lEP an interesting phenomenon of non-uniform redistribution of protein molecules among polysaccharide chains occurs (Tolstoguzov et al. 1985). The reason is that in the vicinity of the protein lEP the hydrophobic protein-protein and electrostatic protein-polysaccharide interactions can be energetically comparable with each other. Protein-protein association on the anionic polysaccharide matrix (or self-association of proteins), which is mainly due to hydrophobic interactions, is usually enhanced when the pH approaches the protein lEP. Accordingly, under conditions of a relatively weak protein-polysaccharide interaction, each free site situated near the site on the polysaccharide chain already occupied by a protein molecule becomes thermodynamically preferable for further binding of protein molecules. This leads to cooperative protein adsorption on an anionic polysaccharide. Some parts of polysaccharide chains tend to be completely covered by protein molecules (as in a virus) while other parts are completely free of protein. 

The main proteins present in blood plasma are albumin (Alb, M 70 kDa, c 600 xm) and transferrin (Tf, M 80 kDa, c 35 xm). Apo-Tf is a bilobal single-chain protein and efficient transporter for Ee + and other tri- and divalent cations, including V + and VO +. Anions, e.g. [H2V04], are also taken up, but possibly reconstructed and bound in the form of VOj. A maximum of two metal ions can be coordinated, one in each binding site... 

Further studies of ET between the protein and inorganic complexes have defined three sites for binding. Site I is positioned at the top left as pictured in Fig. 10 (17, 175), and includes residues Met-65, and Lys-89, -5, -86, -87 (92). This site is preferred by physiological protein partners (92). Site II includes Val-11, Ala-15, Thr-19, and Lys-7, -25, -27. Site III, to the left of the heme edge, may include Ile-81, Phe-82, Ala-83, and Lys-13,-72, -86 sites I and III have high affinities for anionic reactants. Cationic oxidants such as [Co(phen)3] interact mainly at site II, possibly a result of the presence of Glu-21 and fewer positively charged side chains in this region (16, 35). 

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