Heme b

Heme b

Heme b, also known as protoheme IX, is a type of heme, an iron-containing porphyrin, that serves as a prosthetic group in various proteins. It is the most common form of heme and is found in hemoglobin, myoglobin, and many cytochromes. Heme b plays a crucial role in oxygen transport, electron transfer, and catalysis in biological systems.

Structure

Heme b consists of an iron ion (Fe) coordinated to a porphyrin ring, which is a large, heterocyclic organic ring composed of four pyrrole subunits interconnected via methine bridges. The iron ion in heme b can exist in either the ferrous (Fe²⁺) or ferric (Fe³⁺) state, allowing it to participate in redox reactions.

Function

Heme b is essential for the function of several proteins:

• Hemoglobin and Myoglobin: In these oxygen-binding proteins, heme b binds oxygen molecules, facilitating their transport and storage in tissues.
• Cytochromes: In cytochromes, heme b is involved in electron transfer processes within the electron transport chain, crucial for cellular respiration.
• Enzymes: Heme b is a cofactor in various enzymes, such as catalases and peroxidases, where it participates in the breakdown of hydrogen peroxide and other substrates.

Biosynthesis

The biosynthesis of heme b involves several enzymatic steps starting from simple precursors like glycine and succinyl-CoA. The pathway includes the formation of porphobilinogen, uroporphyrinogen, and protoporphyrin IX, which is finally converted to heme b by the insertion of iron.

Clinical Significance

Disorders in heme b metabolism can lead to various medical conditions:

• Porphyrias: These are a group of disorders caused by defects in the heme biosynthesis pathway, leading to the accumulation of porphyrin precursors.
• Anemias: Deficiencies in heme production can result in anemias, characterized by reduced oxygen-carrying capacity of the blood.

Also see

• Hemoglobin
• Myoglobin
• Cytochrome
• Porphyrin
• Electron transport chain
• Anemia
• Porphyria