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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‐Å resolution
Author(s) -
Sharma Ashwani K.,
Kumar S.,
Sharma Vandana,
Nagpal Akanksha,
Singh Nagendra,
Tamboli Irfan,
Mani Indu,
Raman G.,
Singh Tej P.
Publication year - 2001
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/prot.1143
Subject(s) - lactoferrin , melanin , melanosome , monomer , tyrosinase , chemistry , molecule , polymerization , molecular replacement , tyrosine , orthorhombic crystal system , crystallography , stereochemistry , crystal structure , biochemistry , polymer , enzyme , organic chemistry
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‐Å 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 Å, b = 99.8 Å, c = 103.4 Å. 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‐Å resolution. The final model comprises 5,281 protein atoms from 689 amino acids, 2Fe 3+ , 2CO 2− 3ions, 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 α‐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. Proteins 2001;45:229–236. © 2001 Wiley‐Liss, Inc.