Examples from the Chemical Biology of Metal Nitrosyl Complexes

Examples from the Chemical Biology of Metal Nitrosyl Complexes 

NO concentrations generated for bioregulatory purposes are low, and sub-micromolar values have been reported in endothelium cells for 

Measurements of the proximal histidine-iron stretching frequency by Resonance Raman spectroscopy revealed that this bond is very weak in relation to other heme protein systems (vFe.His = 204 cm-1) (130). Formation of the sGC-NO complex labilizes this ligand resulting in the formation of a 5-coordinate high spin iron(II) complex, and the conformational change responsible for the several hundred-fold increase in catalytic activity (126,129,130). 

Bohle and co-workers (133) have demonstrated that varying the electronic and stereochemical properties of porphyrin substituents can strongly influence the rates of NO labilization (Eq. (11)). For example, the displacement of NO from Fe(TPP)(NO) by pyridine is many orders of magnitude slower than from Fe(OBTPP)(NO) (OBTPP = octabromo-tetraphenylporphyrin). An analysis of the kinetics of the latter reaction indicated saturation in [L], and the mechanism was suggested to involve reversible formation of Fe(OBTPP)(L)(NO) followed by NO dissociation (Eq. (50)). Clearly changes in porphyrin properties can lead to enhanced reactivity toward NO loss. 

Myeloperoxidase (MPO) is a heme containing protein that catalyzes the two electron oxidation of halides (Cl-, Br- and I-) and pseudohalide (SCN-) to the corresponding hypohalous acid (Eq. (51)) in a process that is dependent upon [X-], [H202] and [H+] (135). MPO is found in high concentrations in neutrophiles and plays important roles in immune response and inflammation (136). 

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