3BUL:
reference: A disulfide-stabilized conformer of methionine synthase reveals an unexpected role for the histidine ligand of the cobalamin cofactor., Datta S, Koutmos M, Pattridge KA, Ludwig ML, Matthews RG, Proc Natl Acad Sci U S A. 2008 Mar 18;105(11):4115-20. Epub 2008 Mar 10. PMID: 18332423

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