Three-dimensional structure of mare diferric lactoferrin at 2.6 angstrom resolution

Lactoferrin is a monomeric glycoprotein with a molecular mass of approximat ely 80 kDa. The three-dimensional structure of mare diferric lactoferrin (m lf) has been determined at 2.6 Angstrom resolution. The protein crystallize s in the spate group P2(1)2(1)2(1) with alpha = 85.2 Angstrom, b = 99.5 Ang strom, c = 103.1 Angstrom with a solvent content of 55% (v/v). The structur e was solved by the molecular replacement method using human diferric lacto ferrin as the model. The structure has been refined using XPLOR to a final R-factor of 0.194 for all data in the 15-2.6 Angstrom resolution range. The amino acid sequence of mlf was determined using a cDNA method. The final r efined model comprises 5281 protein atoms, 2 Fe3+, 2 CO32- and 112 water mo lecules. The overall folding of mlf is similar to that of other proteins of the transferrin family. The protein folds into two globular lobes, N and C . The lobes are further divided into two domains, N1 and N2, and C1 and C2. The iron-binding cleft is situated between the domains in each lobe. The N lobe appears to be well ordered and is more stable than the C lobe in mlf unlike in other lactoferrins, where the C lobe is the more stable. The open ing of the binding cleft in the N lobe of mlf is narrower than those in oth er proteins of the transferrin family. This is very unusual and is found on ly in mare lactoferrin. Apart from certain hydrophobic interactions at the mouth of the cleft, one salt-bridge (Lys301... ...Glu216) crosses between t he two walls of the cleft. The two lobes are connected covalently by a thre e-turn alpha-helix involving residues 334-344. The N lobe displays a highly ordered structure with appreciably low temperature factors. The iron coord ination is more symmetrical in the N lobe than in the C lobe. There are onl y 16 intermolecular hydrogen bonds in the structure of mlf. (C) 1999 Academ ic Press.