Efficient and Flexible Preparation of Biosynthetic Microperoxidases

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 °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 k_on, k_off, and K (= k_off/k_on) 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.

Influence of heme c attachment on heme conformation and potential

Heme c is characterized by its covalent attachment to a polypeptide. The attachment is typically to a CXXCH motif in which the two Cys form thioether bonds with the heme, "X" can be any amino acid other than Cys, and the His serves as a heme axial ligand. Some cytochromes c, however, contain heme attachment motifs with three or four intervening residues in a CX₃CH or CX₄CH motif. Here, the impacts of these variations in the heme attachment motif on heme ruffling and electronic structure are investigated by spectroscopically characterizing CX₃CH and CX₄CH variants of Hydrogenobacter thermophilus cytochrome c₅₅₂. In addition, a novel CXCH variant is studied. ¹H and ¹³C NMR, EPR, and resonance Raman spectra of the protein variants are analyzed to deduce the extent of ruffling using previously reported relationships between these spectral data and heme ruffling. In addition, the reduction potentials of these protein variants are measured using protein film voltammetry. The CXCH and CX₄CH variants are found to have enhanced heme ruffling and lower reduction potentials. Implications of these results for the use of these noncanonical motifs in nature, and for the engineering of novel heme peptide structures, are discussed.

Totally synthetic microperoxidase-11

A totally synthetic microperoxidase-11 (MP-11) is reported. Accordingly, the undecapeptide (VQKCAQCHTVE) was synthesized by solid-phase peptide synthesis followed by the thiol-ene click reaction with haemin for reconstitution. High-speed atomic force microscopy measurement conducted in water confirmed the protein reconstitution by visualizing the morphological differences as animated molecular images. The synthetic MP-11 showed a considerable magnitude of catalytic activity (27%) against the natural MP-11 in the oxidation of 3,3',5,5'-tetramethylbenzidine by hydrogen peroxide, whereas it showed very low (2.7%) activity of a synthetic variant with a point mutation (VQKCAQC M TVE, H8M). Slab waveguide spectroscopic measurements revealed that the ferrous/ferric redox reaction occurred by the direct electron transfer with specific spectral changes. Indeed, if hydrogen peroxide existed in the solution phase, the peroxidase-modified electrode showed catalytic current-voltage behaviour regardless of whether it was prepared using natural MP-11 or the synthetic MP-11. If a substrate recycling reaction was assumed, computer simulation well reproduced the experimental curves to give a global set of electrocatalytic reaction parameters. In any of the experiments, the synthetic MP-11 and natural MP-11 gave almost identical results. Our approach will be a convenient means of preparing MP-11, as well as its mutants, that does not rely on nature.

Structure and Catalysis of Fe(III) and Cu(II) Microperoxidase-11 Interacting with the Positively Charged Interfaces of Lipids

Numerous applications have been described for microperoxidases (MPs) such as in photoreceptors, sensing, drugs, and hydrogen evolution. The last application was obtained by replacing Fe(III), the native central metal, by cobalt ion and inspired part of the present study. Here, the Fe(III) of MP-11 was replaced by Cu(II) that is also a stable redox state in aerated medium, and the structure and activity of both MPs were modulated by the interaction with the positively charged interfaces of lipids. Comparative spectroscopic characterization of Fe(III) and Cu(II)MP-11 in the studied media demonstrated the presence of high and low spin species with axial distortion. The association of the Fe(III)MP-11 with CTAB and Cu(II)MP-11 with DODAB affected the colloidal stability of the surfactants that was recovered by heating. This result is consistent with hydrophobic interactions of MPs with DODAB vesicles and CTAB micelles. The hydrophobic interactions decreased the heme accessibility to substrates and the Fe(III) MP-11catalytic efficiency. Cu(II)MP-11 challenged by peroxides exhibited a cyclic Cu(II)/Cu(I) interconversion mechanism that is suggestive of a mimetic Cu/ZnSOD (superoxide dismutase) activity against peroxides. Hydrogen peroxide-activated Cu(II)MP-11 converted Amplex Red^® to dihydroresofurin. This study opens more possibilities for technological applications of MPs.