title: Biochemistry of B12-cofactors in human metabolism.
abstract: Vitamin B12, the "antipernicious anaemia factor", is a crystallisable cobalt-complex, which belongs to a group of unique "complete" corrinoids, named cobalamins (Cbl). In humans, instead of the "vitamin", two organometallic B12-forms are coenzymes in two metabolically important enzymes: Methyl-cobalamin, the cofactor of methionine synthase, and coenzyme B12 (adenosyl-cobalamin), the cofactor of methylmalonyl-CoA mutase. The cytoplasmatic methionine synthase catalyzes the transfer of a methyl group from N-methyl-tetrahydrofolate to homocysteine to yield methionine and to liberate tetrahydrofolate. In the mitochondrial methylmalonyl-CoA mutase a radical process transforms methylmalonyl-CoA (a remains e.g. from uneven numbered fatty acids) into succinyl-CoA, for further metabolic use. In addition, in the human mitochondria an adenosyl-transferase incorporates the organometallic group of coenzyme B12. In all these enzymes, the bound B12-derivatives engage (or are formed) in exceptional organometallic enzymatic reactions. This chapter recapitulates the physiological chemistry of vitamin B12, relevant in the context of the metabolic transformation of B12-derivatives into the relevant coenzyme forms and their use in B12-dependent enzymes.
authors: Institute of Organic Chemistry and Centre of Molecular Biosciences (CMBI), University of Innsbruck, Innsbruck, Austria, Bernhard.kraeutler@uibk.ac.at.
authors: Bernhard
authors: B
authors: Kräutler
mesh_terms: D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase / Q000378:metabolism; D003038:Cobamides / Q000737:chemistry / Q000378:metabolism / Q000502:physiology; D045728:Corrinoids / Q000737:chemistry / Q000502:physiology; D006801:Humans; D008765:Methylmalonyl-CoA Mutase / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D013329:Structure-Activity Relationship; D014805:Vitamin B 12 / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism / Q000502:physiology
chemical_list: D003038:Cobamides; D045728:Corrinoids; C019476:mecobalamin; D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase; D008765:Methylmalonyl-CoA Mutase; C000913:cobamamide; D014805:Vitamin B 12