2v3n
From Proteopedia
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CRYSTALLOGRAPHIC ANALYSIS OF UPPER AXIAL LIGAND SUBSTITUTIONS IN COBALAMIN BOUND TO TRANSCOBALAMIN
Overview
Cobalamin (Cbl, vitamin B12) is an essential micronutrient that is, synthesized only by bacteria. Mammals have developed a complex system for, internalization of this vitamin from the diet. Three binding proteins, (haptocorrin, intrinsic factor, transcobalamin (TC)) and several specific, cell surface receptors are involved in the process of intestinal, absorption, plasma transport and cellular uptake. The recent literature on, the binding proteins is briefly reviewed. A structural study is presented, addressing a unique feature of TC among the three proteins, i.e., the, displacement of the weak Co(III)-ligand H(2)O at the upper (or beta) axial, side of H(2)O-Cbl by a histidine side chain. We have investigated, crystallographically the beta-ligand exchange on Cbl bound to TC by, crystallization of bovine holo-TC in the presence of either cyanide or, sulfite. The resulting electron density maps show that the histidine side, chain has been displaced by an exogenous ligand CN(-) or to a lower extent, than expected based on their higher affinity for Co and excess, concentration with respect to histidine. This may reflect either reduced, affinities of CN(-) and or the advantageous binding of the, protein-integrated His-residue when competing for the beta-site of Cbl, bound to TC. The loop hosting the histidine residue appears more flexible, after disruption of the coordination bond His-Cbl but no other differences, are observed in the overall structure of holo-TC. These structural results, are discussed in relation to a possible physiological role of histidine, substitution for H(2)O and regarding the role of beta-conjugated, Cbl-analogues recently proposed for targeted delivery of imaging agents.
About this Structure
2V3N is a Single protein structure of sequence from Bos taurus with CYN, CL and B12 as ligands. Known structural/functional Sites: , , and . Full crystallographic information is available from OCA.
Reference
Vitamin B12 Transport Proteins: Crystallographic Analysis of beta-axial Ligand Substitutions in Cobalamin Bound to Transcobalamin., Wuerges J, Geremia S, Fedosov SN, Randaccio L, IUBMB Life. 2007 Oct 10;:1-8. PMID:17943552
Page seeded by OCA on Tue Dec 18 20:27:42 2007
