X-ray-determined structure of left Mb at pH 5 (Protein DataBank file
1spe, from ref. 38), and right Mb at pH 7 (Protein Data Bank file 1a6g,
from Ref. 23). His-64 (Upper Left) has moved outside the
heme pocket in A0, and smaller changes are
visible throughout the protein. The B-site is labeled B, and the xenon
cavities 1, 2, 3, and 4 are labeled with numbers.