Magnetic Studies on the Ferri‐haem Undecapeptide of Cytochrome c

Abstract

The pH-dependence of the magnetic moment of a ferri-haem undecapeptide, produced by peptic digestion of cytochrome c, has been measured in aqueous solution using a nuclear magnetic resonance method.Below pH 3 the magnetic moment is consistent with the p.resence of hydroxo-bridged dimers of high-spin iron(II1). Above pH 7 the iron(II1) is low-spin, the spin crossover conforming to a simple titration curve with pK 6.3 (n = 1). This transition is discussed with reference to spectrophotometric studies of ligand binding at the haem.The influence of the protein on the metal ion at the active site of a metalloenzyme is central to the theory of entasis [l]. The influence of globin on haem has been studied using the properties of intact haemoglobin [2], while iron-sulphur proteins have been approached in a complimentary manner by studying the effects of binding small peptides to their chemically synthesized iron-sulphur cores [3]. An intermediate approach, which we are beginning to investigate, is to make use of metal-containing peptide fragments resulting from the enzymic digestion of native enzymes.Of particular interest is a haem-containing undecapeptide ( Fig. 1) derived from peptic hydrolysis of cytochrome c [4]. This provides an opportunity to study the haem group in aqueous solution relatively unshielded by protein, yet soluble over an appreciable pH range. In addition, only a limited number of donor groups may be expected to interact with

Microperoxidase-11: Molecular Dynamics and Q-Band Excited Resonance Raman of the Oxidized, Reduced and Carbonyl Forms

“…This LS Fe3+ assignment at neutral pH was confirmed by other workers (14) but is inconsistent with reports concluding that MPII is present in two spin states in the neutral pH range (15,16). Finally, the helical arrangement of the polypeptide has also been inconsistently described as a random coil ( 11) and as maintaining the helical structure which it has in the parent cytochrome (12,13). We felt that resonance Raman should contribute to elucidating some of these questions.…”

“…Cys 14 and cys 17 provide the sulfurs for the thioether linkages and the imidazole ofhis18 is coordinated to the iron as a fifth ligand (10) (Figure 1). No xray structure has been determined but the undecapeptide has been characterized by visible optical spectroscopy, circular dichroism (8,9,11,12), NMR (13) and Moessbauer (14)(15)(16). These studies have shown that MPll is monomeric at acid pH and polymeric at alkaline pH, the state of aggregation being dispersed however, by coordination of the iron to a sixth ligand or by dilution to the mM range (8).…”

“…These studies have shown that MPll is monomeric at acid pH and polymeric at alkaline pH, the state of aggregation being dispersed however, by coordination of the iron to a sixth ligand or by dilution to the mM range (8). In the ferric species, the presence of 6-coordinate iron has been proposed with two possible spin states: at pH< 7, HS Fe3+ with his18 as 5th ligand and a water as 6th ligand, and at pH -7-11, LS Fe3+ with a stronger field 6th ligand, such as valll or lys13 of a second MP11 aggregated molecule (13). This LS Fe3+ assignment at neutral pH was confirmed by other workers (14) but is inconsistent with reports concluding that MPII is present in two spin states in the neutral pH range (15,16).…”

Molecular Dynamics of Microperoxidases in Aqueous and Nonaqueous Solutions

“…24 The hindrance of the polymerizing unit is more evident for MP8M. Finally, since the structure of the octapeptide is quite flexible, the R-amino group of Cys14 can be hindered or not to coordination when the main chain undergoes conformational fluctuations.…”

Fast Coordination Changes in Cytochrome c Do Not Necessarily Imply Folding

“…Since in this case the proximal His is present but other His or Met are absent, it was postulated that the sixth ligand might be provided by the terminal amino groups of the same molecule or via aggregation of different molecules (21,22). To obtain model spectra for the four-, five-and six-coordinate protoheme in the reduced state, the prosthetic group was dissolved in 5% SDS in the presence of different types of ligands (Fig.…”