Reactions between iron porphyrins and tetrahydropterins

Abstract

Data from the last few years have revealed a novel biological role of the tetrahydrobiopterin ( H4B ) cofactor, in one-electron transfers to the heme of the active site of NO-synthases (NOSs) with intermediate formation of a H4B -derived radical. These electron transfers play a key role in the catalytic cycles of the two steps catalyzed by NOS, the Nω-hydroxylation of L-arginine, and the three-electron oxidation of Nω-hydroxyarginine to L-citrulline and NO. Recent experiments performed between various tetrahydropterins and iron porphyrins have shown that the one-electron transfer from tetrahydropterins, such as the natural cofactors H4B and tetrahydrofolate or the synthetic 6,7-dimethyltetrahydropterin (diMeH4P), to Fe(III) porphyrins of sufficiently high redox potentials (> about -100 mV versus NHE for the Fe(III)/Fe(II) couple) is a very general reaction that occurs with formation of a tetrahydropterin-derived radical. Reaction of diMeH4P with a stable porphyrin Fe(II)-O2complex leads to a diMeH4P-derived radical and a transient Fe(III)-OOH complex, mimicking the reaction between H4B and heme Fe(II)-O2in the NOS catalytic cycle. Tetrahydropterins such as diMeH4P also reduce hemeproteins Fe(III) of sufficiently high redox potentials, such as cytochromes c and b5or metmyoglobin, to the corresponding hemeproteins Fe(II) .