Document Detail

Human cobalophilin: the structure of bound methylcobalamin and a functional role in protecting methylcobalamin from photolysis.
MedLine Citation:
PMID:  8268164     Owner:  NLM     Status:  MEDLINE    
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)
S M Frisbie; M R Chance
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Publication Detail:
Type:  Journal Article; Research Support, U.S. Gov't, Non-P.H.S.; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  Biochemistry     Volume:  32     ISSN:  0006-2960     ISO Abbreviation:  Biochemistry     Publication Date:  1993 Dec 
Date Detail:
Created Date:  1994-01-31     Completed Date:  1994-01-31     Revised Date:  2007-11-14    
Medline Journal Info:
Nlm Unique ID:  0370623     Medline TA:  Biochemistry     Country:  UNITED STATES    
Other Details:
Languages:  eng     Pagination:  13886-92     Citation Subset:  IM    
Department of Chemistry, Georgetown University, Washington, D.C. 20057.
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MeSH Terms
Electrophoresis, Polyacrylamide Gel
Spectrum Analysis
Transcobalamins / chemistry*,  metabolism
Vitamin B 12 / analogs & derivatives*,  chemistry,  metabolism
Grant Support
Reg. No./Substance:
0/Transcobalamins; 13422-55-4/mecobalamin; 68-19-9/Vitamin B 12

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