7.12 Heme

Hemoglobin is the compound in blood that carries oxygen to cells and ferries away carbon dioxide to be exhaled. Hemoglobin itself is actually a complex of proteins, but the complex incorporates four molecules of heme, which is where the magic happens. Muscles contain myoglobin, which also incorporates heme for holding oxygen.

Heme B.

The structure of heme is surprisingly similar to that of chlorophyll. The biggest difference is the replacement of iron (Fe) instead of magnesium (Mg). Unlike chlorophyll, heme does get part of its red color from its metal atom. Iron can change its valence between +2 and +3 (the plus signs mean when it forms ions the ions are positive), so the iron in heme can grab onto other things, like oxygen and carbon dioxide. The change in oxidation state (valence) of iron causes a change in the exact wavelengths the entire molecule absorbs.

Carbon monoxide is toxic because it binds to the iron in heme so strongly that the body cannot remove it. The hemoglobin becomes compromised and unable to carry oxygen, resulting in suffocation; other ill effects result from various other reactions between CO and the body's many chemicals.

Some invertebrates have hemocyanin instead of hemoglobin. Hemocyanin is also a protein, but it does not incorporate heme molecules; instead, it uses histidine side chains to bind copper ions. Copper can change its oxidation state between +1 and +2 in order to hold onto oxygen; deoxygenated hemocyanin is colorless while oxygenating it turns it blue.

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