Azurin: A Model to Study a Metal Coordination Sphere or Electron Transfer in Metalloproteins.

Azurin is a small blue copper protein that participates in redox reactions during anaerobic respiration in , and there are a significant number of studies employing this model to investigate the electron transfer (ET) processes or coordination sphere of metal ion in metalloproteins. Azurin naturally contains Cu(II/I) as a central ion and is redox-active for a single electron ET. Moreover, azurin with no central ion (apo-azurin) is capable of binding other metal cofactors-e.g., Zn(II)-forming redox-inactive Zn-form and many others impacting the redox potential and structural variation in the active site's arrangement. Also, mutations of amino acid residues in the immediate vicinity of the metal ion can influence the structure and functionality of a particular metalloprotein. Therefore, this review aims to summarize the abundant information about selected topics related to redox reactions and blue copper proteins, particularly azurin, and is structured as follows: (i) introduction to the structure, properties, and physiological role of this group of metalloproteins, (ii) the role of the equatorial and axial ligands of the central metal ions, or metal species, in the active site on the metal coordination sphere's structure and related determination of the particular azurin form's redox potentials, and (iii) the effects of the particular amino acid's moiety (Phe, Tyr and Trp residues together with acceleration employing Trp-Trp π-π stacking interactions contrary to ET distance dependence) on the preferable type of long-range ET mechanism in an azurin-mediated model biomolecule. We assume that azurin is a suitable model to study the structural functionality of a particular central metal ion or individual amino acid residues in the central ion coordination sphere for studying the redox potential and ET reactions in metalloproteins.