The reductant, NADH, transfers the electrons via a flavo-proteins to the specific disulfide (S-S) protein to form a dithiol (SH,SH) protein which converts vitamin

• B12(Co2+)to B12(Co)
• B12(Co) to B12(Co2+)
• B12(Co2+)to B12(Co+)
• B12(Co+)to B12(Co2+)