@article{ref1,
title="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",
journal="Journal of biochemistry",
year="2002",
author="Fukuoka, Masaki and Yamada, Seiki and Miyoshi, Shozo and Yamashita, Kenji and Yamanishi, Mamoru and Zou, Xiang and Brown, Kenneth L. and Toraya, Tetsuo",
volume="132",
number="6",
pages="935-943",
abstract="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.<p /><p>Language: en</p>",
language="en",
issn="0021-924X",
doi="10.1093/oxfordjournals.jbchem.a003307",
url="http://dx.doi.org/10.1093/oxfordjournals.jbchem.a003307"
}