Animal iron-binding proteins

The purpose of this chapter is to describe the competition for iron between iron-binding proteins of the animal and the siderophores of bacterial parasites. This discussion will be limited to two bacterial species—a slow-growing organism Mycobacterium tuberculosis and a fast-growing organism Escherichia coli. Both organisms produce specific siderophores which have been defined chemically and physically. Myco-bactin, the siderophore of M. tuberculosis, because of its hydrophobic nature, is associated mostly with the lipoidal cell wall of the tubercle bacillus (11) whereas enterochelin (enterobactin), the siderophore of E. coli and Salmonella typhimurium, is soluble in water and is rapidly lost by the bacterial cell into the surrounding medium (12, 13). 

Finally, animal, plant and microbial tissues have been shown to contain the iron storage protein ferritin. The animal protein has been extensively studied, but the mechanism of iron binding has not been completely resolved (29). Animal tissues contain, in addition, a type of granule comprised of iron hydroxide, polysaccharide and protein. The latter, called hemosiderin, may represent a depository of excess iron (30). Interestingly, a protein with properties parallel to those of ferritin has been found in a mold. Here the function of the molecule can be examined with the powerful tools of biochemical genetics (31). 

Figure 11.1 Schematic representation of iron uptake mechanisms, (a) The transferrin-mediated pathway in animals involves receptor-mediated endocytosis of diferric transferrin (Tf), release of iron at the lower pH of the endocytic vesicle and recycling of apoTf. (b) The mechanism in H. influenzae involves extraction of iron from Tf at outer membrane receptors and transport to the inner membrane permease system by a periplasmic ferric binding protein (Fbp). From Baker, 1997. Reproduced by permission of Nature Publishing Group.

As noted in Section 3, some pathogenic bacteria have transferrin receptors on their outer membranes to acquire diferric transferrin from their host. These outer membrane receptors extract the iron from the transferrin and transport it into the periplasm where it is picked up by the periplasmic ferric binding proteins (Fbp), which carry the iron to a transmembrane protein in the inner membrane that conveys it into the cytoplasm. A considerable amount of chemical and structural information has been gathered for Fbp, which is sometimes referred to as bacterial transferrin in recognition of its similarities with animal transferrin. ... 

RNA secondary structure plays a role in the regulation of iron metabolism in eukaryotes. Iron is an essential nutrient, required for the synthesis of hemoglobin, cytochromes, and many other proteins. However, excess iron can be quite harmful because, untamed by a suitable protein environment, iron can initiate a range of free-radical reactions that damage proteins, lipids, and nucleic acids. Animals have evolved sophisticated systems for the accumulation of iron in times of scarcity and for the safe storage of excess iron for later use. Key proteins include transferrin, a transport protein that carries iron in the serum, transferrin receptor, a membrane protein that binds iron-loaded transferrin and initiates its entry into cells, and ferritin, an impressively efficient iron-storage protein found primarily in the liver and kidneys. Twenty-four ferritin polypeptides form a nearly spherical shell that encloses as many as 2400 iron atoms, a ratio of one iron atom per amino acid (Figure 31.37). 

Lactoferrin has been isolated and identified from a wide variety of animal species. However, most of the studies on structure and iron-binding properties have involved either human or bovine proteins (2). Lactoferrin closely resembles transferrin in molecular weight of 75,000 to 90,000 and consists of a single polypeptide chain that binds two ferric ions. The pi of transferrin is 5.9 while that of lactoferrin is approximately 9.0 (8) and has an even higher association constant for iron-binding. Lactoferrin has the property of retaining its iron even in the presence of a relatively low-affir-nity iron chelator such as citrate below pH 4.0. Transferrin, on the other hand, looses its iron when the pH is lowered from 6 to 5 (7). There is extensive information in the literature concerning the physical properties of lactoferrin which will not be covered in this paper. 

Four lines of transgenic cows that harbor the rhLF were developed (van Berkel et al., 2002). The milk of these animals had 0.4,0.8,2, and 3 g/liter of the rhLF in their milk. These levels of expression remained constant throughout the lactation period of 280 days. The milk volume, cell counts, and proximate composition were not altered by the genetic transformation. The recombinant protein was structurally and functionally comparable to natural hLF and had similar iron binding and release and antibacterial activities. The authors further postulate that with such expression levels and an assumed milk yield of 8000 liters of milk per cow annually, one cow can produce about 24-kg rhLF in a year. Thus, a herd of a few hundred animals could produce enormous quantities of this biological protein in a year. 

Cytochrome c oxidase is the terminal member of the respiratory chain in all animals and plants, aerobic yeasts, and some bacteria." " This enzyme is always found associated with a membrane the inner mitochondrial membrane in higher organisms or the cell membrane in bacteria. It is a large, complex, multisubunit enzyme whose characterization has been complicated by its size, by the fact that it is membrane-bound, and by the diversity of the four redox metal sites, i.e., two copper ions and two heme iron units, each of which is found in a different type of environment within the protein. Because of the complexity of this system and the absence of detailed structural information, spectroscopic studies of this enzyme and comparisons of spectral properties with 02-binding proteins (see Chapter 4) and with model iron-porphyrin and copper complexes have been invaluable in its characterization. 

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