Trans cobalamin

Cobalamine can only be resorbed in the small intestine when the gastric mucosa secretes what is known as intrinsic factor—a glycoprotein that binds cobalamine (the extrinsic factor) and thereby protects it from degradation. In the blood, the vitamin is bound to a special protein known as trans-cobalamin. The liver is able to store vitamin Bi2 in amounts suf cient to last for several months. Vitamin B12 deficiency is usually due to an absence of intrinsic factor and the resulting resorption disturbance. This leads to a disturbance in blood formation known as pernicious anemia. 

Jiang, W., Sequeira, J. M., Nakayama, Y, Lai, S. C., Quadros, E. V. 2010. Characterization of the promoter region of TCblR/CD320 gene, the receptor for cellular uptake of trans-cobalamin-bound cobalamin. Gene, 466, 49-55. 

Nex0e, E., and Andersen, J. (1977). Unsaturated and cobalamin saturated trans-cobalamin I and II in normal human plasma. Scand. J. Clin. Lab. Invest. 37 723-728. 

Once in the blood, hydroxocobalamin, the precursor vitamin, is transported by two different serum globulins, transcobalamins (TC) I and II. Trans-cobalamin II is a /3-globulin and is concerned with the transport of newly absorbed vitamin while transcobalamin I, an a-globulin, carries the majority of the vitamin found in plasma and is also concerned with storage (Rosenberg, 1978). Hydroxocobalamin is transported into the cell bound to transcobalamin II via a specific cell-surface calcium-dependent receptor-mediated endocytic... 

Fukui et /. have found that acetylcobinamide reacts faster than the cobalamin with hydroxide (t,/2 = 4 and 30 min, respectively, in 0.05 N KOH) (67). It is not known whether the cobinamide is five- or six-coordinate, but these relative rates establish the labilizing effect of the trans-axial ligand for heterolysis to give Co(I) as HjO or none > 5,6-dimethylbenziminazole (see also Section B,3). 

Only a few other cobalt complexes of the type covered in this review (and therefore excluding, for example, the cobalt carbonyls) have been reported to act as catalysts for homogeneous hydrogenation. The complex Co(DMG)2 will catalyze the hydrogenation of benzil (PhCOCOPh) to benzoin (PhCHOHCOPh). When this reaction is carried out in the presence of quinine, the product shows optical activity. The degree of optical purity varies with the nature of the solvent and reaches a maximum of 61.5% in benzene. It was concluded that asymmetric synthesis occurred via the formation of an organocobalt complex in which quinine was coordinated in the trans position (133). Both Co(DMG)2 and cobalamin-cobalt(II) in methanol will catalyze the following reductive methylations ... 

Insertion of the C=C bond into an M-H bond precedes frequently the C-X bond activation in halogenated alkenes. Such a pathway has been suggested for the cobalamin-mediated dechlorinations of cis- and trans-DCE, as well as VC with Ti(III) citrate as a reducing agent [141]. 

Glod, G., U. Brodmann, W. Angst, C. Holliger, and R. P. Schwarzenbach, Cobalamin-mediated reduction of cis- and trans-dichlorethene, 1,1-dichloroethene, and vinyl chloride in homogeneous aqueous solution Reaction kinetics and mechanistic considerations , Environ. Sci. Technol., 31, 3154-3160 (1997b). 

Giese and coworkers determined on the basis of experimental results obtained by Scheffold et al. [268, 299] that addition reactions of alkyl bromides 249 to a,p-unsaturated nitriles or esters 248 catalyzed by cobalamine 247 are free radical reactions (Fig. 61) [300], This conclusion was based on the similar cis/trans-selectivities in addition reactions of the 4-tert-butylcyclohexyl radical to different electron-poor alkenes 248 using 247 as a catalyst on one hand and classical tributyltin hydride conditions on the other. The kinetics of the radical addition was determined. 

A perspective report emphasised the key role of the application of pressure in kinetic studies in bringing clarity to understanding the mechanism of substitution reactions of cobalamins.193 The effect of various alkyl substituents in the trans position on the kinetic, thermodynamic and ground-state properties has been studied. Cobalamins featuring in these studies were cyanocobalamin (vitamin Bi2), aquacobalamin and the complex formed when the cyano or water ligand is replaced... 

Irradiation of trans-[P(C2H5)3]2PtClH and CF3— C=CH produced cis-[P(C2H5)3]2PtCl-C(CF3)=CHa 213b Elimination of C2H4 from —Co—C2H5 in ethyl-cobalamine and the (intermediate) formation of —Co—H can be looked upon as the reversal of this type of reaction 402>. 

Thusius has studied the kinetics of cobalamin-ligand substitution reactions trans to the pendant 5,6-dimethylbenzimidazole ligand (Eqn. 59) for a variety of inorganic ligands including thiocyanate, azide, iodide and bromide ions [103]. 

Andruniow, T., Kuta, J., Zgierski, M.Z., and Kozlowski, P.M., 2005. Molecular orbital analysis of anomalous trans effect in cobalamins. Chemical Physics Letters. 410 410 16. 

Fedosov, S. N., Berglund, L., Fedosova, N. U., Nexo, E., Petersen, T. E. 2002. Comparative analysis of cobalamin binding kinetics and Ugand protection for intrinsic factor, trans-cobdamin, and haptocorrin. J Biol Chem, 111, 9989-96. 

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