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Coenzyme B12 (cobalamin)-dependent enzymes.

E N Marsh1

  • 1Department of Chemistry, University of Michigan, Ann Arbor 48109-1055, USA.

Essays in Biochemistry
|March 24, 2000
PubMed
Summary

Vitamin B12 coenzymes, methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl), are crucial for methyl group transfer and radical-based reactions. Their unique cobalt-carbon bond drives diverse enzymatic processes.

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Area of Science:

  • Biochemistry
  • Enzymology
  • Organic Chemistry

Background:

  • Vitamin B12 coenzymes, methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl), feature a unique cobalt-carbon bond essential for biological activity.
  • MeCbl acts as a methyl group carrier, shuttling between MeCbl and cob(I)alamin forms in enzymes like methionine synthase.
  • AdoCbl generates carbon-based free radicals, facilitating reactions involving bond cleavage and group migration, including in ribonucleotide reductases.

Purpose of the Study:

  • To elucidate the diverse roles and mechanisms of Vitamin B12 coenzymes in biological systems.
  • To investigate the structural basis of cobalamin-dependent enzyme function.
  • To understand the significance of cobalt coordination by histidine ligands in cobalamin-dependent enzymes.

Main Methods:

  • Review and synthesis of existing literature on cobalamin coenzymes and their dependent enzymes.
  • Analysis of structural data for cobalamin-dependent enzymes, including methionine synthase and methylmalonyl-CoA mutase.
  • Interpretation of spectroscopic evidence regarding coenzyme binding to proteins.

Main Results:

  • Identified MeCbl and AdoCbl as the two primary biologically active forms of cobalamin.
  • Described the distinct catalytic roles: MeCbl in methyl transfer and AdoCbl in radical generation.
  • Reported structural solutions for methionine synthase and methylmalonyl-CoA mutase, revealing histidine coordination to cobalt.
  • Noted conflicting evidence regarding the coenzyme's nucleotide tail coordination in different enzymes.

Conclusions:

  • Cobalamin coenzymes are versatile catalysts essential for fundamental biochemical transformations.
  • The cobalt-carbon bond's homolysis and heterolysis are key to their distinct reactivities.
  • The role of histidine coordination and the nucleotide tail in enzyme binding requires further investigation.

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