Heteronuclear NMR studies of cobalamins. 31P NMR observations of cobalamins bound to a haptocorrin from chicken serum

Heteronuclear NMR studies of cobalt corrinoids. 20. 31P chemical shift anisotropy of aquacobalamin and its complex with a haptocorrin from chicken serum

Static light scattering measurements have been used to determine the molecular mass (65.3 kDa) and second virial coefficient (3.66 x 10(-4) mol mL g-2) for the complex between chicken serum haptocorrin (HC) and aquacobalamin (H2OCbl). Dynamic light scattering measurements have also been used to determine the translational diffusion coefficient of the H2OCbl-HC complex as a function of concentration. From the diffusion coefficient at infinite dilution (4.71 x 10(-7) cm2 s-1), the hydrodynamic radius (45.5 A) and rotational correlation time (85.4 ns) have been calculated. Using the latter, and measured values of the 31P NMR linewidths of the H2OCbl-HC complex at several field strengths, a detailed analysis of the 31P nuclear relaxation is possible. The chemical shift anisotropy term from the transverse relaxation equation is determined to be 95.7 ppm, and the average phosphorus-proton internuclear distance is 2.05 A. For comparison to protein-free H2OCbl, the chemical shift anisotropy of the phosphorus atom was studied by solid state NMR spectroscopy and the 31P relaxation by solution T1 measurements. These studies give values of 110.3 ppm for the chemical shift anisotropy term and 2.45 A for the average phosphorus-proton internuclear distance. The results are consistent with a significant change in the conformation of the H2OCbl phosphodiester upon binding to haptocorrin which could be due to a shortening of the axial Co-N bond.

Thermolysis of Neopentylcobalamin Analogs Complexed to Haptocorrin: Side Chain Entropy and Activation of Organocobalamins for Carbon-Cobalt Bond Homolysis

The kinetics of the thermal Co-C bond homolysis of the complexes of a vitamin B(12) binding protein (haptocorrin) with a series of analogs of neopentylcobalamin modified in side chain structure have been studied. The analogs include the C13 epimer in which the e propionamide side chain adopts an "upwardly" axial conformation and a series of c side chain-modified analogs, including the c-monocarboxylate, the c-N-methylamide, the c-N,N-dimethylamide, and the c-N-isopropylamide. Activation parameters for the thermal homolysis of these complexes show that the previously observed stabilization of alkylcobalamins by haptocorrin is due to both enthalpic and entropic factors. With the exception of that for the analog having the bulkiest c side chain substituent, neopentylcobalamin-c-N-isopropylamide, the enthalpies of activation are independent of analog structure, but the entropies of activation increase with the steric bulk of the c side chain and with the number of "upwardly" projecting side chains, as previously observed for protein-free neopentylcobalamin and its analogs. The results are discussed in terms of the solvent cage effect on Co-C bond homolysis and the importance of corrin ring side chain thermal motions to the entropy of activation for this reaction.

Human cobalophilin: the structure of bound methylcobalamin and a functional role in protecting methylcobalamin from photolysis

The interactions of methylcobalamin with cobalophilin from human serum were analyzed using extended X-ray absorption fine structure (EXAFS) spectroscopy, photolysis of the cobalt-carbon bond of methylcobalamin, and a pKa determination of the protonation of the coordinated nitrogen of 5,6-dimethylbenzimidazole (DMB). These results are consistent with the idea that the DMB nitrogen is still coordinated when protein is bound; however, the ability of a methyl radical (generated by photolysis) to escape the geminate cage of the protein is considerably reduced. For methylcobalamin in solution, the DMB nitrogen ligand is at a distance of 2.20 +/- 0.03 A from cobalt [Sagi, I., & Chance, M. R. (1992) J. Am. Chem. Soc. 114, 8061-8066]. This distance to the lower axial ligand does not change when protein binds (2.20 +/- 0.04 A), nor do the optical spectra exhibit any base-off character. The average of the distance from cobalt to the four equatorial nitrogens of the corrin plane is also unchanged. The pKa for the conversion of the "base-on" to the "base-off" form of methylcobalamin, where the above DMB nitrogen becomes protonated and the Co-N axial bond is cleaved, does not deviate from the free cobalamin value of 2.7 when methylcobalamin is bound to cobalophilin. These results indicate that replacement of the DMB ligand with a ligand from the protein is unlikely. Although the background-subtracted EXAFS data sets for free methylcobalamin and for the protein complex are extremely similar, more accurate data with explicit higher shell analysis would be required to entirely rule out ligand replacement. The chemical and electronic nature of the ligand changes little.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of the cobalt-N coordination on the cobamide recognition by the human vitamin B12 binding proteins intrinsic factor, transcobalamin and haptocorrin

The binding of several corrinoids to the binding site of human intrinsic factor, transcobalamin or haptocorrin was investigated, p-Cresolyl cobamide and 2-amino-vitamin B12 are complete corrinoids, whose nucleotide at the lower face of the corrin ring is not coordinated to the cobalt. These corrinoids were greater than or equal to 10(3) times less efficiently recognized by intrinsic factor or transcobalamin than vitamin B12, which contains a Co-coordinated nucleotide. Pseudovitamin B12, with a weak Co-N coordination bond, revealed only moderate affinity to intrinsic factor. From these findings it is concluded that the cobamide binding to intrinsic factor and transcobalamin is strongly affected by the Co-N coordination bonds of their lower cobalt nucleotide ligands. We suggest that the Co-N coordination bond positions the nucleotide at a critical distance to the corrin ring, which is recognized by the binding proteins. Human haptocorrin, however, disclosed to distinctive selectivity regarding the different corrinoid structures. The protein bound all corrinoids with similar efficiency, independent of the strength of their Co-N coordinations, or the structures of their lower Co alpha ligands. Hence, the corrin ring, rather than a structural feature induced by the Co-N coordination, has to be considered responsible for the corrinoid binding to haptocorrin.