Coordination Chemistry Controls Coenzyme B₁₂ Synthesis by Human Adenosine Triphosphate:Cob(I)alamin Adenosyltransferase

This study reports the first experimentaldeterminationof the redox potential of four-coordinate cob(II)alamin (-325 & PLUSMN; 9 mV), which was achieved by stabilizing cob(I)alamin on theR190H clinical variant of human adenosine triphosphate (ATP):cob(I)alaminadenosyltransferase (MMAB), which decreases the rate of the adenosylationreaction. The study also reports that mitochondrial adrenodoxin isan electron donor to MMAB. Loss of coordination control in selectpatient variants of MMAB results in their inability to synthesize5 & PRIME;-deoxyadenosylcobalamin. Cobalamin(or vitamin B-12)-dependent enzymes and traffickingchaperones exploit redox-linked coordination chemistry to controlthe cofactor reactivity during catalysis and translocation. As thecobalt oxidation state decreases from 3+ to 1+, the preferred cobalamingeometry changes from six- to four-coordinate (4-c). In this study,we reveal the sizable thermodynamic gain that accrues for human adenosinetriphosphate (ATP):cob(I)alamin adenosyltransferase (or MMAB) by enforcingan unfavorable 4-c cob(II)alamin geometry. MMAB-bound cob(II)alaminis reduced to the supernucleophilic cob(I)alamin intermediate duringthe synthesis of 5 & PRIME;-deoxyadenosylcobalamin. Herein, we reportthe first experimentally determined reduction potential for 4-c cob(II)alamin(-325 & PLUSMN; 9 mV), which is 180 mV more positive than forthe five-coordinate (5-c) water-liganded species. The redox potentialof MMAB-bound cob(II)alamin is within the range of adrenodoxin, whichwe demonstrate functions as an electron donor. We also show that stabilizationof 5-c cob(II)alamin by a subset of MMAB patient variants compromisesthe reduction by adrenodoxin, explaining the underlying pathogenicmechanism.