FIG 3 .

Crystal structure of SCHIC domain and His²⁸⁴ as the heme coordinating residue. (A) Crystal structure of SCHIC domain contains two subunits (subunit A shown in orange, and subunit shown B in light blue). (B) SCHIC domain (subunit A [orange]) shares similar overall conformation with E. coli MetH B₁₂-binding domain (green). The His loop in MetH is highlighted in magenta, and the vitamin B₁₂ molecule is shown as purple sticks. Subunit A of SCHIC domain is used for the structure comparison. (C) His²⁸⁴ in SCHIC (orange) and His⁷⁵⁹ in MetH (green) are located on the His loop region. Note the differences in their positions and orientations. (D) UV-visible spectrum of AppA-heme and AppA_H284A-heme under aerobic conditions. The spectrum of AppA/AppA_H284A is subtracted. Heme (5 µM) was incubated with 10 µM AppA/AppA_H284A for at least 20 min before the spectrum was taken. Normalized absorbance is shown on both the right and left y axes.