Arginine anion-binding

The coupling between iron and anion binding is a critical component of transferrin function. In the absence of anion, transferrins bind Fe " weakly and nonspecifically. The putative anion site identified crystal-lographically is in an electrostatically positive region, adjacent to the side chain of an arginine and the amino terminus of an a helix. The anion site does not bridge the two domains, and so does not function directly as a latch that closes the cleft around the iron. It is possible that the anion may partially compensate for the presence of several basic residues in the cleft that enhance iron binding. 

Figure 4.3 The amino acid arginine and the arginine fork binding mode with phosphate anion residues.

Figure 19. A model for the anion- and iron-binding sites of transferrin depicted assuming an interlocking-site hypothesis. The protein furnishes five ligands to the metal in the iron binding site three tyrosines and two histidines. The carbonate ion binds to an arginine in the anion-binding site and functions as a sixth ligand to the metal center. The carbonate forms a bridge between the metal- and the anion-binding sites in the active center (36).

The X-ray studies of LADH have shown (107) that the anion Pt(CN)4 binds at the same site on the enzyme as the pyrophosphate group of the coenzyme. Arginine-47 is involved in this general anion binding site. The relation between the anion binding site, Arg-47, the catal5d ic zinc atom and the reactive Cys-46 is shown in Fig. 16. Early kinetic studies (1) and optical rotatory experiments (232,233) showed that LADH was inhibited by anions like halides, perchlorate, and thiosulfate and that this inhibition was coenzyme-competitive. Recent kinetic studies... 

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