Heme puts iron at the centre of a ring and life runs on it
Blood is red because of heme, a flat ring of four pyrrole units holding a single iron atom. That tiny structure ferries oxygen, detoxifies drugs, senses gases and shuttles electrons. It may predate oxygen itself, and today a yeast-made version gives a plant burger its meaty taste.
The word comes from the Ancient Greek for blood. Chemically, heme is an iron ion gripped by a tetrapyrrole at four points, with one or two further attachments above and below. It sits inside a whole family of hemoproteins: hemoglobin and myoglobin, which handle oxygen, but also cytochromes, catalases, peroxidases and the enzyme that makes nitric oxide in blood vessel linings. The iron can accept or give up electrons, toggling between oxidation states, which is why the same ring serves for transport, catalysis and signalling.
What changes its job is the protein around it. In hemoglobin, nearby amino acids let the molecule grab oxygen in the lungs, where carbon dioxide is low and pH high, then release it in working muscle, where acidity rises. This is the Bohr effect: a histidine next to the heme picks up a positive charge under acidic conditions and nudges the oxygen free. Diatomic gases generally bind only when the iron is in its reduced ferrous form.
There are several varieties. Heme B is most common, with A and C also important; capital letters denote the isolated compound and lower case the protein-bound form, a convention formalised by Puustinen and Wikstrom. Heme S turns up in the blood of a few marine worms, and the structures of B and S were first worked out by the German chemist Hans Fischer. White blood cells use a variant bound to myeloperoxidase to turn chloride into bleach-like hypochlorite against invading microbes.
The body builds heme mostly in the liver and bone marrow, starting from glycine and succinyl-CoA. Heme itself throttles the first enzyme, ALA synthase, a feedback loop doctors exploit by infusing heme to halt attacks of acute intermittent porphyria. Researchers suspect hemoproteins first evolved to move electrons in sulfur-based photosynthesis before the air held oxygen. Impossible Foods now inserts soybean leghemoglobin genes into yeast to mass-produce heme for its vegan burgers.
Source: Heme
