Cobalamin redox chemistry — The NADH/flavoprotein system reduces a disulfide (S–S) to a dithiol (SH, SH) that then converts vitamin B12 from which state to which state?

Introduction / Context:
Vitamin B12 (cobalamin) functions through multiple oxidation states of cobalt: Co(III), Co(II), and Co(I). In certain B12-dependent reactions, a cellular reducing system (NADH via a flavoprotein and a disulfide/dithiol protein) must generate the highly nucleophilic cob(I)alamin state.

Given Data / Assumptions:

• NADH donates electrons through a flavoprotein.
• A specific protein disulfide is reduced to a dithiol (SH, SH).
• This reduced protein further reduces cobalamin.

Concept / Approach:
Cobalamin interconverts among Co(III) (oxidized), Co(II) (one-electron reduced), and Co(I) (two-electron reduced, supernucleophilic). The NADH/flavoprotein system ultimately yields cob(I)alamin. Thus, the physiological step relevant here is reduction of B12 from Co(II) to Co(I) mediated by a dithiol protein.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical studies of methionine synthase activation show requirement for cob(I)alamin regeneration via NADPH-dependent reductive systems, consistent with Co(II) → Co(I) conversion.

Why Other Options Are Wrong:

• Co2+ → Co (neutral) is not the conventional notation/state used for cobalamin in biochemistry.
• Co → Co2+ or Co+ → Co2+ represent oxidation, not the required reduction to active Co(I).

Common Pitfalls:
Confusing cobalamin oxidation states and symbols; in biochemical notation Co(III) = Co3+, Co(II) = Co2+, Co(I) = Co+.

Final Answer:
B12(Co2+) to B12(Co+)