Acta Cryst. (1999). D55, 1799±1804 Sharma & Singh  Lactoferrin±metal interactions 1799 research papers Acta Crystallographica Section D Biological Crystallography ISSN 0907-4449 Lactoferrin±metal interactions: ®rst crystal structure of a complex of lactoferrin with a lanthanide ion (Sm 3+ ) at 3.4 A Ê resolution Ashwani K. Sharma and Tej P. Singh* Department of Biophysics, All India Institute of Medical Sciences, New Delhi 110 029, India Correspondence e-mail: tps@medinst.ernet.in # 1999 International Union of Crystallography Printed in Denmark ± all rights reserved Lactoferrin is an important member of the transferrin family. A characteristic property of transferrins is their ability to bind very tightly (K app ' 10 20 ) but reversibly two Fe 3+ ions. The structural consequences of binding a metal other than Fe 3+ have been examined by crystallographic analysis at 3.4 A Ê resolution of mare samarium±lactoferrin (Sm 2 Lf). The struc- ture was re®ned to an R factor of 0.219 for 8776 re¯ections in the resolution range 17.0±3.4 A Ê . The samarium geometry (distorted octahedral coordination) is similar in both lobes. However, the anion interactions are quite different in the two lobes. In the N lobe, the anion is able to form only two hydrogen bonds instead of the four observed in the C lobe of Sm 2 Lf and the six observed in Fe 2 Lf. This is because Arg121, Thr117 and Gly124 have moved away from the anion as a consequence of the binding of the Sm 3+ ion. The protein ligands in the binding cleft of Sm 2 Lf show large displacements, but the overall protein structure remains the same. The binding of Sm 3+ by lactoferrin shows that the protein is capable of sequestering ions of different sizes and charges, though with reduced af®nity. This conclusion should be true of other transferrins also. Received 13 March 1999 Accepted 19 July 1999 PDB Reference: lactoferrin± Sm 3+ complex, 1qjm. 1. Introduction Proteins of the transferrin family, which include serum trans- ferrin and lactoferrin, control the levels of free iron in animals through their very tight (K ' 10 20 ) but reversible binding of iron. These proteins fold into two lobes which are connected by a 10±12 residue long peptide. Each lobe is further divided into two domains, forming a deep binding cleft between them. Each binding cleft can house one Fe 3+ and the synergistic CO 2 3 ion. In addition to this very high af®nity for Fe 3+ and CO 2 3 , these proteins can also bind a wide variety of other cations and anions at the same speci®c sites. Metal ions of varying sizes and oxidation states ranging from +2 to +5 can be accommodated, including Cu 2+ , Co 3+ , Mn 3+ (Ainscough et al., 1979), VO 2+ (Chasteen et al., 1977), VO  2 (Harris & Carrano, 1984), lanthanide cations (Luk, 1971; Zak & Aisen, 1988; O'Hara & Bersohn, 1982) and Th 4+ (Harris et al. , 1981) Crystallographic analysis of rabbit serum transferrin (Bailey et al., 1988), diferric human lactoferrin (Anderson et al., 1989; Haridas et al. , 1995), hen ovotransferrin (Kurokawa et al., 1995), duck ovotransferrin (Rawas et al. , 1996), bovine lacto- ferrin (Moore et al. , 1997), mare lactoferrin (Sharma, Para- masivam et al., 1999) and buffalo lactoferrin (Karthikeyan et al., 1999a,b) have de®ned the location and nature of the iron sites. All these proteins have essentially the same bilobal structure, with one iron site in each lobe, deep in a cleft between two domains. The iron ligands (two Tyr, one His, one