Structural and magnetic characterisation of the haem core of ferricytochromes c ₆

Abstract

 NMR studies of the paramagnetic ¹³C shifts of haem substituents in ferricytochromes c ₆ from Monoraphidium braunii and Anabaena sp. PCC7119 were used to explore the unusual electronic structure of these two proteins and to describe the functional centre of the latter, for which there is still no other structural information. Even without evaluating the contribution of the dipolar shifts to the paramagnetic ¹³C shift of the haem substituents, a good description of the rhombic perturbation is obtained, which gives a measure of the spatial arrangement of the axial ligands of the iron as well as a preliminary characterisation of the magnetic properties of the unpaired electron. If complemented by EPR data, the dipolar field can be fully described and is a valuable aid for the assignment of NMR spectra, providing direct information on the solution structure in the oxidised state. In contrast to the great difference in the primary structure of the two cytochromes c ₆ studied, the results show close similarity between their haem cores, suggesting that this structural arrangement may have been selectively produced by nature to perform a specific electron transfer task, and that the variability in amino-acid sequence is determined by the requirements of recognition of the physiological partners.