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(a) The sperm whale Mb skeleton, with side chains Tyr-103, -146, and -151, Trp-7 and -14, Met-55 and -131, His-64 and -93, and Xe1, Xe2, Xe3, and Xe4 from ref. 15. (b) Cartoon of Mb as NO is reacting with a bound O2. The heme cavity is on the distal (Upper), the Xe1 cavity on the proximal (Lower) side of the heme. A dioxygen molecule is shown bound covalently to the iron atom in the heme cavity. A NO molecule can move from the Xe1 cavity via the path shown to react with the O2.
