Plural of haem; iron-containing compounds that are the active part of hemoglobin and other proteins that carry oxygen and electrons in the body.
From Greek 'haima' (blood). Haem (or heme in American spelling) is the iron-containing porphyrin ring that makes blood red and enables it to carry oxygen.
Haems are so important that they appear in multiple proteins—hemoglobin carries oxygen in blood, myoglobin stores it in muscles, and cytochrome proteins use them in cells' energy factories. They're among the most crucial chemical structures in life.
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