Structural properties of peroxidases.

Peroxidases are heme proteins which are able to catalyze the oxidation of a large variety of substrates through the reaction with hydrogen peroxide. The specific biological function, the reduction potential of the iron and the nature of the substrates which can be oxidized, are strongly determined by the structural features of the protein matrix around the prosthetic group. In particular, two main features are considered to be responsible of the specificity of the biological function: the strong anionic character of the fifth, proximal ligand to the iron, which is able to stabilize high oxidation states, and the hydrophilic nature of the residues in the distal pocket. Beside the correct reduction potential for the oxidation reaction, the specificity towards different substrates also depends on the protein structural arrangement which can determine specific binding sites for substrates and mediators. Particularly, in the case of MnP,the Mn2+ binding site has been individuated in the X-ray structure. NMR studies were previously reported which provided an iron-manganese distance consistent with that from the X-ray structure. This information can help in defining the possible pathway for the electron transfer from the Mn2+ ion to the iron. On the contrary, in the case of LiP no information is available on the possible binding site of veratryl alcohol as well as of other aromatic substrates. This article reviews these structural properties of peroxidases with particular emphasis to their implications in the catalytic process. Finally, the calcium ions have been located in the structure of LiP and the MnP: their structural relevance will be discussed on the light of the possible role in determining the optimal arrangement of residues in the distal cavity for the enzymatic reaction.