Proposed Structure of Heme d, a Prosthetic Group of Bacterial Terminal Oxidases

Heme d has been isolated from the purified terminal oxidase complex of Escherichia coli strain MR43L/F152. After conversion to the metal-free methyl ester form, termed chlorin d, its structure was investigated. It was determined to be a chlorin, that is, a porphyrin with one pyrrole ring saturated so that the βcarbons have sp³ hybridization. Substituents were identified by means of visible, infrared, mass spectrometric, and ¹H NMR spectroscopy. The configuration of substituents was assigned by means of nuclear Overhauser enhancements between nearest neighbors and by the differential effects of a lanthanide shift reagent. The proposed structure of chlorin d contains the substituents 1-methyl, 2-vinyl, 3-methyl, 4-vinyl, 5-methyl, 5-hydroxyl, 6-(γ-lactone), 7-propionyl methyl ester, and 8-methyl. Ring C is the saturated pyrrole, and the 6-substitutent is an unusual spiro-γ-lactone.