Wuerges Jochen, Geremia Silvano, Fedosov Sergey N, Randaccio Lucio
Centre of Excellence in Biocrystallography, Department of Chemical Sciences, University of Trieste, Trieste, Italy.
IUBMB Life. 2007 Nov;59(11):722-9. doi: 10.1080/15216540701673413.
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 H2O at the upper (or beta) axial side of H2O-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 SO(3)(-2)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 SO(3)(-2)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 H2O and regarding the role of beta-conjugated Cbl-analogues recently proposed for targeted delivery of imaging agents.
钴胺素（Cbl，维生素B12）是一种仅由细菌合成的必需微量营养素。哺乳动物已形成一套复杂的系统，用于从饮食中摄取这种维生素。三种结合蛋白（运钴胺蛋白、内因子、转钴胺素（TC））和几种特定的细胞表面受体参与肠道吸收、血浆运输和细胞摄取过程。本文简要综述了有关这些结合蛋白的最新文献。提出了一项结构研究，探讨了三种蛋白中TC的一个独特特征，即H2O-Cbl上（或β）轴向侧的弱Co(III)-配体H2O被组氨酸侧链取代。我们通过在氰化物或亚硫酸盐存在下结晶牛全转钴胺素来晶体学研究与TC结合的钴胺素上的β-配体交换。所得电子密度图显示，基于它们对Co的更高亲和力和相对于组氨酸的过量浓度，组氨酸侧链被外源性配体CN(-)或SO(3)(-2)取代的程度低于预期。这可能反映了CN(-)和SO(3)(-2)的亲和力降低，或者是在竞争与TC结合的钴胺素的β位点时，蛋白质整合的His残基具有有利的结合。在His-Cbl配位键断裂后，容纳组氨酸残基的环似乎更具柔韧性，但在全转钴胺素的整体结构中未观察到其他差异。结合组氨酸取代H2O的可能生理作用以及最近提出的β-共轭钴胺素类似物在成像剂靶向递送中的作用，对这些结构结果进行了讨论。
