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(a) The structure and nomenclature of the b heme (iron-protoporphyrin IX). The classic Fischer numbering system is used. Heme orientational isomerism involves a 180° rotation about the α–γ axis. (b) The heme PTM present in GlbN. His117, in the H helix, adds to the 2-vinyl group. Addition can also occur to the 4-vinyl group when the heme is in the flipped orientation [20, 21].
