Human neuroglobin (Ngb) contains a heme group and three Cys residues (Cys46, Cys55 and Cys120) in the polypeptide chain. By introducing an additional Cys at position 15, the X-ray structure of A15C Ngb mutant was solved at a high resolution of 1.35 Å, which reveals the formation of both the native (C46-C55) and the engineered (C15-C120) disulfide bonds, likely playing a functional and structural role, respectively, according to the geometry analysis. Unexpectedly, 1,4-dioxane from the crystallization reagents was bound not only to the protein surface but also to the heme distal pocket, providing insights into protein-ligand interactions for the globin and guiding the design of functional heme enzymes.

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