Structures of copper-bound Mbns. In all structures, residues that are sometimes absent are denoted in gray. Additional C-terminal residues appear to be lost in all Methylocystis Mbns (20). A, Ms. trichosporium OB3b CuMbn crystal structure and chemical structure. Both heterocycles are oxazolones (labeled OxaA and OxaB). B, Ms. sp. LW4 CuMbn chemical structure. As with Ms. trichosporium OB3b CuMbn, both heterocycles are oxazolones (labeled OxaA and OxaB). C, Mc. sp. SB2 CuMbn chemical structure. Heterocycle A has been described as an imidazolone (labeled ImiA), whereas heterocycle B is an oxazolone (labeled OxaB). D, Mc. hirsuta CSC1 CuMbn crystal structure and chemical structure. Heterocycle A has been depicted as a pyrazinediol (labeled PyrA), whereas heterocycle B is an oxazolone (labeled OxaB). E, Mc. rosea SV97 CuMbn chemical structure. Heterocycle A has been depicted as a pyrazinediol (labeled PyrA), whereas heterocycle B is an oxazolone (labeled OxaB). F, Mc. sp. M CuMbn crystal structure and chemical structure. Heterocycle A has been depicted as a pyrazinediol (labeled PyrA), whereas heterocycle B is an oxazolone (labeled OxaB). G, possible identities for heterocycle A in Methylocystis Mbns. Hydroxypyrazinone and pyrazinedione tautomers are potentially consistent with the observed crystal structures and NMR data.