Shown here is the crystal structure of leghemoglobin derived from root nodules of yellow lupine (Lupinus luteus L.). Leghemoglobin is a red-colored enzyme active in the environment of N2-fixing nodules; leghemoglobin mediates the high O2 requirements of the N2-fixing bacteroids and the necessity of the O2-limited environment (10-40 nM) for the proper function of nitrogenase in reducing N2 to ammonia.

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Show heme group with associated histidine and dioxygen (O2)
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Show globin as strands with heme and dioxygen

Central to the activity of leghemoglobin is the heme group, in which four 5-membered pyrrole rings are joined to form a giant macrocycle in which a central Fe atom resides coordinated with four equatorial nitrogens. The central Fe is further coordinated with an additional N from a nearby histidine residue and, at the other pole, dioxygen (O2). Heme groups are also found in cytochromes, catalases, and peroxidases, as well as hemoglobin, an oxygen-carrying enzyme familiar in the animal kingdom. The tetrapyrrole macrocycle appears also in chlorophyll, with Mg as the central atom.

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TN Safonova, AV Teplyakov, GV Obmolova, AN Popov, IP Kuranova, EG Harutyunyan. 1991.Crystal structure of ferric complexes of the yellow lupin leghemoglobin with isoquinoline at 1.8 angstroms resolution (Russian). Bioorg. Khim. 17:1605.