Hämoproteinen

Hemoproteins are a class of proteins that contain a heme prosthetic group, an iron-containing porphyrin ring, which provides a reactive center for binding or activating various ligands. The iron in heme usually cycles between Fe2+ and Fe3+ during function, and the surrounding protein environment tunes its redox potential and ligand affinity. In many hemoproteins the heme is bound non-covalently, but some, such as certain cytochromes c, carry heme covalently through thioether bonds to cysteine residues (the CXXCH motif in cytochrome c).

In globins such as hemoglobin and myoglobin, the heme iron is coordinated by a proximal histidine, while distal residues help regulate oxygen binding and prevent unwanted side reactions. Heme variants differ in their porphyrin substitutions and protein attachments. The most common heme, heme B, is found in hemoglobin and myoglobin; heme A occurs in some mitochondrial oxidases; heme C is covalently attached in several cytochromes. Hemoproteins also include catalases, peroxidases, cytochrome P450 enzymes, and nitric oxide synthases, all of which rely on the heme group for catalysis or electron transfer.

Biosynthesis and turnover: heme is synthesized primarily in mitochondria and cytosol from glycine and succinyl-CoA through several steps, with iron insertion forming protoheme. Degradation is mediated by heme oxygenase, yielding biliverdin (later bilirubin), free iron, and carbon monoxide. Free heme and iron are potentially toxic, so their production and breakdown are tightly regulated.

Occurrence and significance: hemoproteins are widespread across all domains of life and are essential for respiration, metabolism, oxygen transport, detoxification, and signaling. Defects in heme biosynthesis can cause porphyrias; iron imbalance can affect heme-containing pathways and related diseases.