Gaseous O-2, NO, and CO in Signal Transduction: Structure and Function Relationships of Heme-Based Gas Sensors and Heme-Redox Sensors
Abstract
The heme iron complex is one of the most important cofactors in biological systems (Figure 1). The major functions of the heme iron complex are to serve as the O2 binding site for hemoglobin (O2 carrier protein), myoglobin (O2 storage protein), and cytochrome P450 (O2 activating enzyme); the peroxide activation site for peroxidase; and the electron transfer site for cytochromes.1MINUS SIGN 8 The heme iron complex in well-known heme proteins acts as the functional active center by virtue of its binding to an O2 molecule forming a heme ironMINUS SIGN O2 complex, which carries the O2 molecule to peripheral tissues as hemoglobin, stores the O2 molecule in muscle as myoglobin, and helps activate the O2 molecule by cleaving the OMINUS SIGN O bond in cytochrome P450 monooxygenase.
Importantly, the O2 molecule binds only to heme Fe(II) complexes and not to heme Fe(III) complexes. For cytochromes, electrons are transferred to/from the partner protein via the heme iron complex, whereas for peroxidases, the active heme iron species required for function, compound I and compound II, are formed by hydrogen peroxide, reinforcing the idea that the heme iron complex is the important active center for these heme proteins.6