Overview

Haem (British spelling haem, American spelling heme) is a common biological cofactor formed by an iron ion held in the center of a porphyrin ring. As a prosthetic group it appears in a family of proteins collectively called haemoproteins. These cofactors enable diverse chemical functions, most notably reversible oxygen binding in blood and electrons transfer in respiratory chains.

Structure and chemistry

At its core is an iron atom (commonly Fe2+ or Fe3+) coordinated within a tetrapyrrole macrocycle called a porphyrin (organic ring). The iron can form additional bonds above and below the ring plane, permitting ligation of small molecules such as O2, CO or NO. Many haem groups are derivatives of protoporphyrin IX and differ by side chains or covalent attachments to the host protein.

Biological roles

Haem participates in several fundamental processes. In animals, haem in haemoglobin and myoglobin binds and releases oxygen, giving blood its red colour (blood). In mitochondria and bacteria, haems in cytochromes shuttle electrons during respiration. Other haem enzymes catalyse peroxide decomposition, oxygenation and redox reactions.

Examples and functions

  • Oxygen transport and storage: haemoglobin, myoglobin.
  • Electron transfer: cytochrome complexes in respiratory and photosynthetic chains.
  • Catalysis: peroxidases and catalases that break down hydrogen peroxide.
  • Sensing and signalling: haem-containing sensors detect gases and regulate proteins.

Biosynthesis, metabolism and medical relevance

Haem biosynthesis is a conserved multistep pathway that assembles the porphyrin ring and inserts iron. Pathway intermediates and defects in enzymes cause clinical conditions (porphyrias) and alter pigment production. Degradation of haem from aged red blood cells yields biliverdin and bilirubin, compounds important in physiology and diagnostics.

Types, variations and distinctions

Several named haem types exist (commonly called haem a, b, c) that differ by modifications to the porphyrin or how the haem is attached to the protein. Not all porphyrins bind iron and not all metalloproteins contain haem; the term haem specifically denotes the iron–porphyrin prosthetic group (co-factor), while the metal aspect may be referenced separately as an iron center. Research continues to explore haem trafficking inside cells and how proteins tune haem's reactivity.

For general background and further reading see introductory resources and reviews on porphyrins and haem biology (British spelling overview, American spelling overview, and specialized articles on haemoproteins and metabolism).