The structural change of heme oxygenase upon heme binding

Abstract

The incorporation of heme into apo-forms of hemoproteins is an indispensable process for them to acquire biological functions. Heme oxygenase (HO) catalyzes the oxidative cleavage of protoheme to biliverdin utilizing dioxygens and reducing equivalents from cytochrome P450 reductase. HO is not a hemoprotein by nature, but once it binds heme, it behaves like a hemoprotein. Because apo-states of most hemeproteins are not native forms and unstable, HO (apoHO) could be a useful model to investigate the formation of holohemoproteins from their apo-states. Rat apoHO was crystallized in hemihedral twin with three molecules in an asymmetric unit and its crystal structure was determined at 2.55 Å resolution. Although the structure of apoHO is similar to the structure of its complex with heme (HO-heme), the structure around the hemepocket shows distinct differences. In apoHO, the proximal helix (A-helix) is disordered and the following helix (B-helix), a portion of which builds the hemepocket, is shifted toward the hemepocket. Notably, Gln38 in the B-helix is shifted toward a position where the heme is present in HO-heme. In HO-heme, Gln38 is hydrogen-bonded to Glu29 that is located at the C-terminal side of the A-helix in HO-heme. This finding suggests that this hydrogen bond restrains the angle between the A-and B-helices in HO-heme. In addition, the secondary structure of distal helix in apoHO changes to random coil. These structural features of apoHO imply that the orientation of the proximal helix and the position of His25 are fixed upon heme binding.