This B12 binding domain is found in methionine synthase EC:2.1.1.13 Swiss:Q99707, and other shorter proteins that bind to B12. This domain is always found to the N-terminus of Pfam:PF02310. The structure of this domain is known [1], it is a 4 helix bundle. Many of the conserved residues in this domain are involved in B12 binding, such as those in the MXXVG motif.

1: How a protein binds B12: A 3.0 A X-ray structure of B12-binding domains of methionine synthase Drennan CL, Huang S, Drummond JT, Matthews RG, Lidwig ML; Science 1994;266:1669-1674. PMID:7992050