Dioxygen Transport

In the human body approximately 65% of Fe is a component of hemoglobin. The protein molecule consists of four heme subunits. The 'resting state' contain high-spin Fe²⁺ but coordination of the strong π acceptor ligand O₂ alters it to the lowspin form. It is significant for the action of hemoglobin, like the uptake of O₂ through one heme sub-unit raises the O₂ affinity of another, the cooperative influence making the uptake and release more proficient. High-spin Fe²⁺ is little too large to fit comfortably within the heme ring, although the low-spin ion is very smaller and so O₂ coordination basis the Fe to shift into the ring center. The 'proximal' histidine within the Fe coordination sphere as well moves and acts like a means of communicating among sub-units. Other feature of hemoglobin is designed to lessen its affinity for another ligands. The position of the 'distal' histidine displayed in diagram 1b forces coordination in a nonlinear geometry. This is agreeable for O₂ but not for strongly competing species like CO and CN^-; even though these are still extremely toxic they would be even more so otherwise.

Fig. 1. Fe and Co in biomolecules: (a) heme; (b) O₂ binding in hemoglobin (see text); (c) [4Fe-4S] center in iron-sulfur proteins; (d) cobalamin (B₁₂) structure.