Magnesium porphyrin-globin and -amino acid complexes

Abstract

Complexes of magnesium mesoporphyrin (MgMP) and magnesium protoporphyrin (MgPP) with apomyoglobin and apohemoglobin, and amino acid species of the type MgMP/PP (amino acid) 2 have been studied by electronic, circular dichroism (CD) and optical rotatory dispersion (ORD) spectroscopy [1−3]. In the case of the protein complexes, specific spectral differences were observed for the myoglobin (Mb) and hemoglobin (Hb) products. These have been interpreted in terms of the formation of six-coordinate magnesium aquo species, Mg(porphyrin)(-histidine)(H 2O), for MgMPMb and MgPPMb and five-coordinate, Mg(porphyrin)(-histidine), species for MgMPHb and MgPPHb [1]. These results highlight the importance of relatively small differences in the protein environment of the heme group in myoglobin and hemoglobin, on binding at the sixth coordination site. It is possible that a favourable hydrogen bonding interaction with the distal imidazole group (of the type recently described for oxy Mb [4] and CO heme proteins [5]) may stabilize the binding of a water molecule to Mg in the Mb species. ▪ Aqueous solutions of the magnesium porphyrins containing the chiral amino acids, L-histidine, L-serine, L-threonine and L-proline, produce prominent induced Cotton effects [1, 2]. However the CD/ORD spectra of these species differ from those of the magnesium porphyrin Mb and Hb protein complexes. Electronic band positions indicate the complexes producing the Cotton effects to be six-coordinate Mg(porphyrin)(amino acid) 2 entities, containing two amino acids bound to the metal rather than only one amino acid residue as for the protein species. The electronic spectra also indicate the presence of five-coordinate (Mg(porphyrin)(amino acid) species in the solutions but these do not produce Cotton effects. Induced Cotton effects of the magnitude observed for the Mg(porphyrin)(amino acid) 2complexes presumably require reasonable firm stereochemical location of the coordinated amino acids. For L-histidine we suggest the ligands are localized primarily by ππ bonding interactions between the ligand aromatic π system and the d π orbitals of Mg [2]. For the similar amino acids, L-serine and L-threonine, molecular models indicate that hydrogen bonding between the ligand −OH groups and the porphyrin COO − side chains fix these ligands in well-defined positions. L-proline is a special case in that binding through the ring nitrogen atom produces a new chiral centre at that atom. Interestingly, steric interactions between proline and the porphyrin ring determine the chirality of nitrogen to be opposite to that at the asymmetric carbon atom (as illustrated in Fig. 1). Inversion of this type occurs for a similar reason in the case of proline complexes Cu 2+ [6].