Lactoferrin-melanin interaction and its possible implications in melanin polymerization: Crystal structure of the complex formed between mare lactoferrin and melanin monomers at 2.7-angstrom resolution

The concentration of melanin determines the intensity of colors of the skin and hair of animals. Melanin pigments are tyrosine-based polymers formed i n melanocytes within specialized organelles called melanosomes. In order to understand the mechanism of melanin polymerization, lactoferrin, a basic p rotein with a pl value of 9.0, has been used to produce melanin. Lactoferri n is a monomeric iron-binding protein with a molecular weight of 80 kDa. Th e crystals of lactoferrin were soaked in a solution containing dihydroxyphe nylalanine (DOPA) and tyrosinase enzyme. These crystals were used for X-ray intensity data collection. The intensity data were collected to 2.7-Angstr om resolution to an overall completeness of 91% with an R-sym of 0.071. The crystals belong to orthorhombic space group P2(1)2(1)2(1) with cell dimens ions: a = 85.0 Angstrom, b = 99.8 Angstrom, c = 103.4 Angstrom. The structu re was determined by molecular replacement method, using the model of difer ric mare lactoferrin, and refined to an R-factor 0.215 (R-free = 0.287) for all the data to 2.7-Angstrom resolution. The final model comprises 5,281 p rotein atoms from 689 amino acids, 2Fe(3+), 2CO(3)(2-) ions, 2 indole-5,6-q uinone molecules (IQ), and 73 water molecules. Two IQ molecules, one in eac h lobe, bind to lactoferrin. In the C-lobe, the IQ binds in the iron-bindin g cleft, whereas in the N-lobe, it is located in the side pocket between tw o a-helices, filled with solvent molecules in the native iron-saturated mar e lactoferrin. The IQ molecules interact with protein molecule mainly throu gh glutamic acid in both lobes, without significant perturbation to the pro tein structure. The orientation of N- and C-lobes in the present structure is similar to that observed in the native iron-saturated protein. However, as a result of the binding of IQ molecules, the orientations of the domains N1, N2 and C1, C2 in the two cases differ slightly. (C) 2001 Wiley-Liss,In c.