MHB is a coiled-coil molecule that binds free haem in mycobacterial cytoplasm to deliver it to membrane proteins for shuttling through the membrane [1].

1: Discovery and characterization of a unique mycobacterial heme acquisition system. Tullius MV, Harmston CA, Owens CP, Chim N, Morse RP, McMath LM, Iniguez A, Kimmey JM, Sawaya MR, Whitelegge JP, Horwitz MA, Goulding CW; Proc Natl Acad Sci U S A. 2011;108:5051-5056. PMID:21383189