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Labilization of the Cobalt‐carbon Bond in Vitamin B 12 Bound to Adenosyltransferase
Author(s) -
Campanello Gregory Cook,
Twahir Umar,
Brunold Thomas,
Warncke Kurt,
Banerjee Ruma
Publication year - 2017
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.31.1_supplement.603.16
Subject(s) - adenosylcobalamin , mutase , cobalamin , cofactor , chemistry , corrinoid , corrin , bond cleavage , stereochemistry , ligand (biochemistry) , enzyme , vitamin b12 , biochemistry , methyltransferase , catalysis , gene , receptor , methylation
Vitamin B 12 is an essential, scarce, and reactive cofactor that is acquired by mammals from the diet. Studies on patients with inborn errors of cobalamin metabolism have led to the discovery of nine genes involved in the tailoring, chaperoning, and utilization of cobalamin as a cofactor. One of these genes encodes adenosyltransferase (ATR), a bifunctional enzyme that is responsible for the synthesis of 5′‐deoxyadenosylcobalamin (AdoCbl) and for its delivery to the client enzyme, methylmalonyl CoA mutase (MCM). Mutations in ATR and MCM result in methylmalonic aciduria, a devastating metabolic disorder, which can result in infant mortality. ATR catalyzes the synthesis of AdoCbl via transfer of the adenosyl group of ATP to the supernucleophile cob(I)alamin, resulting in the release of inorganic tripolyphosphate (PPPi). PPPi binds to the ATR•AdoCbl complex with an apparent K d ≤ 1 μM. Binding of PPPi to ATR•AdoCbl under anaerobic conditions induces a unique UV‐visible spectroscopic signature with an absorption maximum at 439 nm. Exposure of this complex to O 2 results in cleavage of the cobalt‐carbon bond and formation of an ATR‐bound 4‐coordinate cob(II)alamin species with a λ max of 464 nm. The resulting spectrum is virtually identical to that of neopentylcobalamin, a synthetic B 12 derivative with a weak cobalt‐carbon bond. Under aerobic conditions, neopentylcobalamin also undergoes spontaneous decomposition to cob(II)alamin (1). We are using multiple spectroscopic approaches to characterize this unusual reaction and to place it in a functional context. We hypothesize that binding of PPPi to ATR‐bound AdoCbl results in weakening of the cobalt‐carbon leading to the formation of tightly bound cob(II)alamin, thus prioritizing sequestration of the cofactor under conditions where the acceptor MCM, might be limiting. Support or Funding Information We are very grateful for NIH grants awarded to G.C.C. (5 F32 GM113405‐02) and R.B. (DK45776) which supported this research.