Hemerythrin:

Question

Hemerythrin:

Accepted Answer

The correct option is 2:
Hemerythrin is fundamentally defined by its unique oxygen-binding site and is accurately classified as a **non-heme iron protein**. The term 'heme' refers to an auxiliary group—a complex porphyrin ring with a single iron atom at its center—that is characteristic of proteins like Hemoglobin, Myoglobin, and Cytochromes. In contrast, Hemerythrin's oxygen-binding core, known as a binuclear or **diiron center**, contains two iron atoms that are directly coordinated by the protein's polypeptide chain. These two iron atoms are held in place by the side chains of seven amino acid residues: five histidine residues, one glutamate, and one aspartate. The crucial point is the absence of the porphyrin ring, meaning there is **no haem** group. Instead, the iron atoms are bound directly to the nitrogen (from histidine) and oxygen (from glutamate/aspartate) atoms of the protein, which is precisely what the correct option states. This structural difference is a major evolutionary distinction from the globins, highlighting the diversity of oxygen transport systems in the animal kingdom, particularly among marine invertebrates.

The incorrect option is 1:
This statement is a perfect and well-known description of a different respiratory protein called **Hemocyanin**. Hemocyanin is the oxygen-carrying molecule responsible for the respiratory function in the hemolymph of most molluscs (such as snails and octopuses) and certain arthropods (like the horseshoe crab and some crustaceans). Crucially, the active site of Hemocyanin uses a pair of **copper atoms** to reversibly bind a single oxygen molecule, rather than the iron atoms used by Hemerythrin. Therefore, equating Hemerythrin with a copper-binding molecule fundamentally confuses two distinct classes of metalloproteins.

The incorrect option is 3:
Hemerythrin functions as an oxygen-transport and storage protein primarily in specific marine invertebrate phyla. Historically, it was thought to be restricted to the coelomic fluid and muscle tissue of four groups: Sipunculida (peanut worms), Priapulida, Brachiopoda (lamp shells), and a single annelid worm (Magelona). While modern genomics has revealed that various bacteria and even some human proteins contain 'hemerythrin-like domains' (often used for sensing oxygen or redox status), its primary role as a respiratory pigment is **not** associated with the phylum **Arthropoda**. The dominant respiratory pigment in most large arthropods is Hemocyanin, making this option an incorrect generalization of Hemerythrin's biological distribution.

The incorrect option is 4:
The color change described in this option is incorrect for Hemerythrin, and involves a mix-up with other pigments. When Hemerythrin is deoxygenated (deoxyhemerythrin), both iron atoms are in the ferrous (Fe²⁺) state, and the protein is visually **colorless** or pale yellow in solution. Upon binding oxygen to form oxyhemerythrin, the complex turns a distinct **violet-pink** or reddish-violet due to the iron atoms oxidizing to the ferric (Fe³⁺) state and forming an oxo/hydroperoxo bridge. The description of being "green when oxygenated" actually corresponds to a different pigment, **Chlorocruorin**, found in certain marine annelids, thus making this color description an error.