Saturday, October 30, 2010

Applications of the HSAB Principle.

Ligand selections in metalloproteins and enzymes.

Ions of many of the 3d transition metals are essential for life in trace amounts. A number of proteins and enzymes incorporate these metal ions specifically into their structures, forming adducts with the metal ion using donor atoms on the side chains of their amino acids. Side chains containing oxygen, nitrogen, and sulfur donors are usually involved in adduct formation. It is interesting to apply HSAB ideas to attempt to understand why certain amino acids are frequently found with their side chain donors bound to particular metal ions. As a particular example, consider the Fe2+ ion, perhaps the most-used metal ion in life processes. This ion is found in species such as myoglobin and hemoglobin, in which is function is to accept an electron pair from an O2 molecule to form an adduct, carry the O2 to the muscle tissues of the body, and release the O2 unaltered where it is needed. It is found in electron transfer proteins such as cytochrome c, where its function is to mediate the movement of an electron between other proteins. It is found in the ironsulfur proteins, where its function is again redox related. In all of these varied proteins, with their different functions, the Fe2+ ion is invariably associated with N and S donor atoms. This is sensible in terms of HSAB theory, because Fe2+ is a borderline acid. Its association with the borderline donor, N, and the soft donor, S, is therefore understandable. Similarly, the Zn2+ and Cu2+ ions are attached to proteins and enzymes via N and O donors. Fe3+, on the other hand, is a hard acid and is invariably found bound by O donors.

1 comment:

Paty said...

Ojalá puedan entrar al link de donde obtuve la información; vienen explicados de manera sencilla otros 4 ejemplos más de aplicaciones así como definiciones y ejemplos de la teoría de HSAB :)