STRUCTURAL INVESTIGATION OF THE COMPLEXATION PROPERTIES BETWEEN HORSESPLEEN APOFERRITIN AND METALLOPORPHYRINS

Crystallographic studies of L-chain horse spleen apoferritin (HSF) co- crystallized with Pt-hematoporphyrin IX and Sn-protoporphyrin IX have brought significant new insights into structure-function relationships in ferritins. Interactions of HSF with porphyrins are discussed. Stru ctural results show that the nestling properties into HSF are dependen t on the porphyrin moiety. (Only protoporphyrin IX significantly inter acts with the protein, whereas hematoporphyrin IX does not.) These stu dies additionally point out the L-chain HSF ability to demetalate meta lloporphyrins, a result which is of importance in looking at the iron storage properties of ferritins, In both compound investigated (whethe r the porphyrin reaches the binding site or not), the complexation app ears to be concomitant with the extraction of the metal from the porph yrin. To analyze further the previous results, a three-dimensional ali gnment of ferritin sequences based on available crystallographic coord inates, including the present structures, is given. It confirms a high degree of homology between these members of the ferritin family and t hus allows us to emphasize observed structural differences: 1) unlike L-chain HSF, H-chain human ferritin presents no preformed binding site ; and 2) despite the absence of axial ligands, and due to the demetala tion, L-chain HSF is able to host protoporphyrin at a similar location to that naturally found in bacterioferritin. (C) 1996 Wiley-Liss, Inc .