title: 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-A resolution.
abstract: The concentration of melanin determines the intensity of colors of the skin and hair of animals. Melanin pigments are tyrosine-based polymers formed in melanocytes within specialized organelles called melanosomes. In order to understand the mechanism of melanin polymerization, lactoferrin, a basic protein with a pI value of 9.0, has been used to produce melanin. Lactoferrin is a monomeric iron-binding protein with a molecular weight of 80 kDa. The crystals of lactoferrin were soaked in a solution containing dihydroxyphenylalanine (DOPA) and tyrosinase enzyme. These crystals were used for X-ray intensity data collection. The intensity data were collected to 2.7-A 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 dimensions: a = 85.0 A, b = 99.8 A, c = 103.4 A. The structure was determined by molecular replacement method, using the model of diferric mare lactoferrin, and refined to an R-factor 0.215 (R(free) = 0.287) for all the data to 2.7-A resolution. The final model comprises 5,281 protein atoms from 689 amino acids, 2Fe(3+), 2CO(2-)(3) ions, 2 indole-5,6-quinone molecules (IQ), and 73 water molecules. Two IQ molecules, one in each lobe, bind to lactoferrin. In the C-lobe, the IQ binds in the iron-binding cleft, whereas in the N-lobe, it is located in the side pocket between two alpha-helices, filled with solvent molecules in the native iron-saturated mare lactoferrin. The IQ molecules interact with protein molecule mainly through glutamic acid in both lobes, without significant perturbation to the protein 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.
authors.affiliation: Department of Biophysics, All India Institute of Medical Sciences, New Delhi, India.
authors.forename: A K
authors.initials: AK
authors.lastname: Sharma
authors.forename: S
authors.initials: S
authors.lastname: Kumar
authors.forename: V
authors.initials: V
authors.lastname: Sharma
authors.forename: A
authors.initials: A
authors.lastname: Nagpal
authors.forename: N
authors.initials: N
authors.lastname: Singh
authors.forename: I
authors.initials: I
authors.lastname: Tamboli
authors.forename: I
authors.initials: I
authors.lastname: Mani
authors.forename: G
authors.initials: G
authors.lastname: Raman
authors.forename: T P
authors.initials: TP
authors.lastname: Singh
mesh_terms: D000818:Animals; D000838:Anions / Q000737:chemistry; D001665:Binding Sites; D018360:Crystallography, X-Ray / Q000379:methods*; D005260:Female; D006736:Horses; D007211:Indoles / Q000737:chemistry; D007501:Iron / Q000737:chemistry; D007781:Lactoferrin / Q000737:chemistry*; D008543:Melanins / Q000737:chemistry*; D008958:Models, Molecular; D011108:Polymers / Q000737:chemistry*; D011487:Protein Conformation; D025941:Protein Interaction Mapping / Q000379:methods; D011809:Quinones / Q000737:chemistry
chemical_list: D000838:Anions; D007211:Indoles; D008543:Melanins; D011108:Polymers; D011809:Quinones; D007501:Iron; D007781:Lactoferrin