Reductive nitrosylation of ferric microperoxidase-11

Microperoxidase-11 (MP11) is an undecapeptide derived from horse heart cytochrome c, which is considered as a heme-protein model. Here, the reductive nitrosylation of ferric MP11 (MP11(III)) under anaerobic conditions has been investigated between pH 7.4 and 9.2, at T=20.0 degrees C. At pH7.7, NO binds reversibly to MP11(III) leading to the formation of the MP11(III)-NO complex. However, between pH 8.2 and 9.2, the addition of NO to MP11(III) leads to the formation of ferrous nitrosylated MP11(II) (MP11(II)-NO). In fact, the transient MP11{FeNO}(6) species is converted to ferrous deoxygenated MP11 (MP11(II)) by OH-- and H2O-based catalysis, which represents the rate-limiting step of the whole reaction. Then, MP11(II) binds NO very rapidly leading to MP11(II)-NO formation. Over the whole pH range explored, the apparent values of k(on), k(off), and K (=k(off)/k(on)) for MP11(III)(-NO) (de)nitrosylation are essentially pH independent, ranging between 5.8x10(5)M(-1)s(-1) and 1.6x10(6)M(-1)s(-1), between 1.9s(-1) and 3.7s(-1), and between 1.4x10(-6) M and 4.6x10(-6) M, respectively. Values of the apparent pseudo-first-order rate constant for the MP11{FeNO}(6) conversion to MP11(II) (i.e., h) increase linearly with pH; the apparent values hOH- and hH2O are 7.2x10(2)M(-1)s(-1) and 2.5x10(-4)s(-1), respectively. Present data confirm that MP11 is a useful molecular model to highlight the role of the protein matrix on the heme-based reactivity.