Marques H M, Perry C B
Department of Chemistry, University of the Witwatersrand, Johannesburg, South Africa.
J Inorg Biochem. 1999 Jul 15;75(4):281-91. doi: 10.1016/s0162-0134(99)00100-2.
The acetylation of the hemeundecapeptide prepared by proteolysis of cytochrome c yields a species di(N-acetyl)-microperoxidase-11, NAcMP11, that is monomeric in aqueous solution at least for concentrations below 20 microM, in contrast to MP11 itself, which aggregates because of intermolecular coordination of Fe(III) by the N-terminal amino group or the amino group of the side chain of Lys-13. The present report complements a report by Peterson and co-workers on the preparation and properties of NAcMP11 (Inorg. Chem. 35 (1996) 6885). We show that NAcMP11 has six spectroscopically observable pH-dependent transitions at 1.90 +/- 0.03, 3.37 +/- 0.01, 4.6 +/- 0.1, 5.4 +/- 0.03, 9.56 +/- 0.01 and 12.4 +/- 0.03. The first is probably due to displacement of one of two H2O molecules from the coordination sphere of Fe(III) by the C-terminal Glu-21 carboxylate (giving the axial ligand combination RCOO-/H2O); as the pH is raised, His-18 is deprotonated and coordinates the metal (His/H2O). The next two transitions are due to ionization of heme propionic acid groups; the penultimate is caused by the ionization of Fe(III)-bound H2O (His/OH-); and the final transition is from ionization of His-18 to form a histidinate (His-/OH-). The EPR spectrum of NAcMP11 at pH 0.7 is consistent with a mixture of a di(aqua) and a mono(aqua) species. Both the aqua complex of NAcMP11 (at pH 7.6) and the hydroxo complex (at pH 11.0) are in equilibrium between a quantum-mechanically admixed spin state (S = 3/2, 5/2) and a low-spin state (S = 1/2). The crystal field parameters of the two complexes (which are similar) as derived from the EPR spectrum are reported. The EPR spectrum at pH 13.8 shows that the hydroxo-histidinate complex of NAcMP11 undergoes a slow reaction, possibly to form a di(hydroxo) complex with displacement of the histidinate ligand, or a dimerization with the histidinate acting as bridging ligand. The coordinated H2O molecule in NAcMP11 is readily replaced by an exogenous ligand, and binding constants for coordination of cyanide, imidazole, azide and chloride are reported. NAcMP-11 is shown to display similar physical and chemical properties to the analogous octapeptide, NAcMP-8, but is easier to prepare; this makes NAcMP-11 a useful alternative model for the hemoproteins.
通过细胞色素c蛋白水解制备的血红素十一肽的乙酰化产生了一种二（N-乙酰基）-微过氧化物酶-11（NAcMP11），与MP11本身不同，它在水溶液中至少在浓度低于20μM时是单体，MP11本身由于N端氨基或Lys-13侧链氨基对Fe(III)的分子间配位而聚集。本报告补充了Peterson及其同事关于NAcMP11的制备和性质的报告（《无机化学》35 (1996) 6885）。我们表明，NAcMP11在1.90±0.03、3.37±0.01、4.
