title: Adenosyltransferase: an enzyme and an escort for coenzyme B12?
abstract: Many organic cofactors are both rare and reactive. They are usually in low abundance, which poses problems for efficient collision-based targeting to dependent enzymes, whereas their reactivity is problematic for side reactions. Sequestration and escorted delivery presents one solution to this conundrum, but such porters, if they exist, are mostly unknown. In humans, the mitochondrial enzyme methylmalonyl-coenzyme A mutase uses coenzyme B(12) (adenosylcobalamin) but would be inactive if bound to the cofactor precursor that is delivered to the mitochondrion. Adenosyltransferase converts cob(II)alamin to coenzyme B(12). Based on kinetic evidence for interaction between the two enzymes, the 40-fold greater affinity for coenzyme B(12) and the higher coordination number for cobalt in the mutase, we propose that the adenosyltransferase is a dual-function protein: an enzyme that synthesizes coenzyme B(12) and a chaperone that delivers it.
authors.affiliation: Biochemistry Department, University of Nebraska, Lincoln, NE 68588-0664, USA.
authors.forename: Mamoru
authors.initials: M
authors.lastname: Yamanishi
authors.forename: Monica
authors.initials: M
authors.lastname: Vlasie
authors.forename: Ruma
authors.initials: R
authors.lastname: Banerjee
mesh_terms: D019883:Alkyl and Aryl Transferases / Q000737:chemistry / Q000378:metabolism*; D001692:Biological Transport; D003038:Cobamides / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D008765:Methylmalonyl-CoA Mutase / Q000378:metabolism; D014805:Vitamin B 12 / Q000378:metabolism*
chemical_list: D003038:Cobamides; D019883:Alkyl and Aryl Transferases; D008765:Methylmalonyl-CoA Mutase; C000913:cobamamide; D014805:Vitamin B 12