Functions of the D-ribosyl moiety and the lower axial ligand of the nucleotide loop of coenzyme B(12) in diol dehydratase and ethanolamine ammonia-lyase reactions.

Publisher: Oxford University Press Country of Publication: England NLM ID: 0376600 Publication Model: Print Cited Medium: Print ISSN: 0021-924X (Print) Linking ISSN: 0021924X NLM ISO Abbreviation: J Biochem Subsets: MEDLINE

Publication: : Abingdon, UK : Oxford University Press
Original Publication: Tokyo : Japanese Biochemical Society

Ligands* ; Nucleic Acid Conformation*; Cobamides/*chemistry ; Ethanolamine Ammonia-Lyase/*metabolism ; Propanediol Dehydratase/*metabolism; Apoenzymes/chemistry ; Apoenzymes/metabolism ; Bacterial Proteins/chemistry ; Bacterial Proteins/metabolism ; Cobamides/metabolism ; Ethanolamine Ammonia-Lyase/chemistry ; Molecular Structure ; Propanediol Dehydratase/chemistry ; Spectrum Analysis

The roles of the D-ribosyl moiety and the bulky axial ligand of the nucleotide loop of adenosylcobalamin in coenzymic function have been investigated using two series of coenzyme analogs bearing various artificial bases. The 2-methylbenzimidazolyl trimethylene analog that exists exclusively in the base-off form was a totally inactive coenzyme for diol dehydratase and served as a competitive inhibitor. The benzimidazolyl trimethylene analog and the benzimidazolylcobamide coenzyme were highly active for diol dehydratase and ethanolamine ammonia-lyase. The imidazolylcobamide coenzyme was 59 and 9% as active as the normal coenzyme for diol dehydratase and ethanolamine ammonia-lyase, respectively. The latter analog served as an effective suicide coenzyme for both enzymes, although the partition ratio (k(cat)/k(inact)) of 630 for ethanolamine ammonia-lyase is much lower than that for diol dehydratase. Suicide inactivation was accompanied by the accumulation of a cob(II)amide species, indicating irreversible cleavage of the coenzyme Co-C bond during the inactivation. It was thus concluded that the bulkiness of a Co-coordinating base of the nucleotide loop is essential for both the initial activity and continuous catalytic turnovers. Since the k(cat)/k(inact) value for the imidazolylcobamide in diol dehydratase was 27-times higher than that for the imidazolyl trimethylene analog, it is clear that the ribosyl moiety protects the reaction intermediates from suicide inactivation. Stopped-flow measurements indicated that the rate of Co-C bond homolysis is essentially unaffected by the bulkiness of the Co-coordinating base for diol dehydratase. Thus, it seems unlikely that the Co-C bond is labilized through a ground state mechanochemical triggering mechanism in diol dehydratase.

0 (Apoenzymes)
0 (Bacterial Proteins)
0 (Cobamides)
0 (Ligands)
EC 4.2.1.28 (Propanediol Dehydratase)
EC 4.3.1.7 (Ethanolamine Ammonia-Lyase)
F0R1QK73KB (cobamamide)

Date Created: 20021211 Date Completed: 20040219 Latest Revision: 20190513

20250114

10.1093/oxfordjournals.jbchem.a003307

12473196