The tinker, tailor, soldier in intracellular B12 trafficking

Abstract

The recognition of eight discrete genetic complementation groups among patients with inherited cobalamin disorders provided early insights into the complexity of a cofactor-processing pathway that supports only two known B(12)-dependent enzymes in mammals. With the identification of all eight genes now completed, biochemical interrogations of their functions have started and are providing novel insights into a trafficking pathway involving porters that tinker with and tailor the active cofactor forms and editors that ensure the fidelity of the cofactor loading process. The principles of sequestration and escorted delivery of a rare and reactive organometallic cofactor that are emerging from studies on B(12) might be of general relevance to other cofactor trafficking pathways.

Figure 1

Structures of cobalamin derivatives and the various ligation states.

Figure 2

Schematic view of the intracellular B₁₂ trafficking pathway. The known and proposed functions of the gene products associated with the complementation groups cblA-G and mut are indicated. The question marks highlight some aspects of the trafficking pathway that are poorly characterized.

Figure 3

Proposed pathway for B₁₂ targeting to methionine synthase for MeCbl synthesis. Cofactor transfer from MMACHC to MS precedes conversion of inactive cob(II)alamin to MeCbl, in a process that requires MSR, NADPH, and AdoMet (S-adenosylmethionine). During turnover, B₁₂ bound to MS shuttles between the MeCbl and cob(I)alamin states. Oxidation of the latter creates the dependency of the MS reaction on reductive activation by MSR.

Figure 4

Mitochondrial components involved in AdoCbl synthesis and targeting. (A) ATR converts cob(II)alamin to AdoCbl and transfers the product directly to MCM, in a process that is gated by MMAA (or MeaB in bacteria) and triggered by ATP binding to ATR. MMAA/MeaB is expected to exist in a complex with MCM. (B) Schematic representation of how the histidine (shown in stick representation) on a disordered loop in the B₁₂-binding domain of MCM ‘picks’ AdoCbl out of the active site pocket of ATR.