Structural characterization of the lactoferrin/ceruloplasmin complex by Small Angle X-Ray Scattering

Ceruloplasmin (ferro-O2-oxidoreductase, EC 1.16.3.1) is a copper protein found in vertebrate plasma, which belongs to the family of multicopper oxidases together with ascorbate oxidase, and laccases. In humans it accounts for 95% of plasma copper and seems to play an important role in iron metabolism and homeostasis by virtue of its capacity to oxidize Fe2+ to Fe3+, which allows the subsequent incorporation of the latter into apo-transferrin. Like transferrin of the blood plasma, lactoferrin, the iron-containing protein of the milk and of human saliva, tightly binds two Fe3+. Human ceruloplasmin and lactoferrin have been shown to interact both in vivo and in vitro forming a complex. Here we describe a structural study of this complex by Small Angle X-ray Scattering (SAXS). Our structural analysis shows that ceruloplasmin interacts with lactoferrin forming a complex via an interaction between the region containing the trinuclear copper cluster responsible for its ferroxidase activity and the region containing the iron binding sites of lactoferrin.The complex of lactoferrin with ceruloplamin we analyzed in vitro has possible physiological implications on account that such proteins are known to be involved in acute inflammation processes and in antimicrobial activities. Thus, taking into account that apo-lactoferrin is able to specifically bound Fe3+ ions preventing their utilization by pathogenic bacteria, we propose that the assistance of ceruloplasmin ferroxidase activity can play a key role for its efficient function as it is believed for the iron incorporation into plasma transferrin.[...]