Camel lactoferrin, a transferrin-cum-lactoferrin: Crystal structure of camel apolactoferrin at 2.6 angstrom resolution and structural basis of its dual role

Camel lactoferrin is the first protein from the transferrin superfamily tha t has been found to display the characteristic functions of iron binding an d release of lactoferrin as well as transferrin simultaneously. It was rema rk able to observe a wide pH demarcation in the release of iron from two lo bes. It loses 50 % iron at pH 6.5 and the remaining 50 % iron is released o nly at PH values between 4.0 and 2.0. Furthermore, proteolytically generate d N and C-lobes of camel lactoferrin showed that the C-lobe lost iron at pH 6.5, while the N-lobe lost it only at PH less than 4.0. In order to establ ish the structural basis of this striking observation, the purified camel a polactoferrin was crystallized. The crystals belong to monoclinic space gro up C2 with unit cell dimensions a=175.8 Angstrom, b=80.9 Angstrom, c = 56.4 Angstrom, beta = 92.4 degrees and Z = 4. The structure has been determined by the molecular replacement method and refined to an R-factor of 0.198 (R -free = 0.268) using all the data in the resolution range of 20.0-2.6 Angst rom. The overall structure of camel apolactoferrin folds into two lobes whi ch contain four distinct domains. Both lobes adopt open conformations indic ating wide distances between the iron binding residues in the native iron-f ree form of lactoferrin. The dispositions of various residues of the iron b inding pocket of the N-lobe of camel apolactoferrin are similar to those of the N-lobe in human apolactoferrin, while the corresponding residues in th e C-lobe show a striking similarity with those in the C-lobes of duck and h en apo-ovotransferrins. These observations indicate that the N-lobe of came l apolactoferrin is structurally very similar to the N-lobe of human apolac toferrin and the structure of the C-lobe of camel apolactoferrin matches cl osely with those of the hen and duck apo-ovotransferrins. These observation s suggest that the iron binding and releasing behaviour of the N-lobe of ca mel lactoferrin is similar to that of the N-lobe of human lactoferrin, wher eas that of the C-lobe resembles those of the C-lobes of duck and hen apo-o votransferrins. Hence, it:correlates with the observation of the N-lobe of camel lactoferrin losing iron at a low pH (4.0-2.0) as in other lactoferrin s. On the other hand, the C-lobe of camel lactoferrin loses iron at higher pH (7.0-6.0) like transferrins suggesting its functional similarity to that of transferrins. Thus, camel lactoferrin can be termed as half lactoferrin and half transferrin. (C) 2001 Academic Press.