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 °C. At pH ≤ 7.7, 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 kon, koff, and K (= koff/kon) for MP11(III)(-NO) (de)nitrosylation are essentially pH independent, ranging between 5.8 × 105 M-1 s-1 and 1.6 × 106 M-1 s-1, between 1.9 s-1 and 3.7 s-1, and between 1.4 × 10-6 M and 4.6 × 10-6 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 [Formula: see text] and [Formula: see text] are 7.2 × 102 M-1 s-1 and 2.5 × 10-4 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.
Keywords: Ferric microperoxidase-11; Kinetics; Reductive nitrosylation.