Oxygen transport by hemerythrin

Hemerythrin is a respiratory protein found in some marine invertebrates. It is involved in the transport and storage of oxygen, similar to hemoglobin in vertebrates. Hemerythrin contains iron ions that bind to oxygen molecules, allowing for efficient oxygen transport.

Binding of oxygen to hemerythrin

The binding of oxygen to hemerythrin occurs through the coordination of oxygen molecules to the iron ions present in the protein. Each hemerythrin molecule contains two iron ions, typically in the +2 and +3 oxidation states. Oxygen can bind to either the +2 or +3 iron ion, resulting in different forms of oxygen-bound hemerythrin.

Options for oxygen binding

a) O2 to one Fe(III) only
- This option suggests that oxygen molecules bind to the +3 iron ion only. However, this is not the case as hemerythrin can bind oxygen to both the +2 and +3 iron ions.

b) HO2 to one Fe(III) only
- This option suggests that oxygen binds as HO2 to the +3 iron ion. HO2 refers to a hydroperoxy group (-OOH) attached to the iron ion. This is the correct option as it accurately describes the binding of oxygen to hemerythrin.

c) O22 to one Fe(II) and one Fe(III)
- This option suggests that oxygen molecules bind to both the +2 and +3 iron ions. However, oxygen binding typically occurs to either the +2 or +3 iron ion, not both simultaneously.

d) O22 to two Fe(II)
- This option suggests that oxygen molecules bind to two +2 iron ions. However, oxygen binding typically occurs to either the +2 or +3 iron ion, not two +2 ions.

Conclusion

The correct answer is option 'b' - HO2 binds to one Fe(III) only. This accurately describes the binding of oxygen to hemerythrin, where oxygen molecules coordinate to the +3 iron ion in the form of a hydroperoxy group (-OOH). This binding allows for efficient oxygen transport and release in the organism.