hemeihin

Hemeihin is the Finnish term used to refer to heme groups, the iron-containing prosthetic groups found in a wide range of biological proteins. A heme consists of a porphyrin ring, known as protoporphyrin IX, coordinated to a central iron atom. In many proteins, the iron can reversibly bind small molecules such as oxygen, carbon monoxide, or nitric oxide, enabling essential biological functions.

Structure and chemistry. The iron atom sits at the center of the planar porphyrin ring and is coordinated to four nitrogen atoms. Depending on the protein, additional axial ligands complete the iron’s coordination sphere. Different oxidation states (Fe2+ and Fe3+) allow electron transfer and catalytic activities. Several heme types exist, including heme b, heme a, and heme c; heme c is covalently attached to the protein through thioether bonds to cysteine residues, which distinguishes it from other hemes.

Biosynthesis and degradation. In most organisms, heme biosynthesis occurs partly in the mitochondria and partly in the cytosol, starting from precursors such as glycine and succinyl-CoA, with eight enzymatic steps. The process is tightly regulated, and the insertion of iron is the final step carried out by ferrochelatase. Heme degradation is primarily mediated by heme oxygenase, producing biliverdin, free iron, and carbon monoxide.

Biological roles. Heme groups are central to oxygen transport (as in hemoglobin and myoglobin), electron transfer (cytochromes in respiration and photosynthesis), and catalysis in various enzymes. Different heme types enable specialization for diverse cellular tasks.

Clinical and research relevance. Abnormal heme synthesis or degradation can lead to disorders such as porphyrias. Lead exposure and certain drugs can disrupt heme metabolism. Heme-containing proteins remain important targets in biomedical research and biotechnological applications.