Biological Chemistry - Berezov, T. T., & Korovkin, B. F. 1998

Metabolism of Complex Proteins
Chromoprotein Metabolism

The problems of Synthesis and degradation of Chromoproteins have long captured the interest of both researchers and clinicians for two main reasons. First, they perform a remarkably diverse array of biologically vital Functions, as seen in Hemoglobin, chlorophyll, and Cytochromes. At The Heart of all these molecules lies a porphyrin ring capable of coordinating with Metal Ions (see Chapter 2). Second, any disruption in the synthesis or breakdown of Porphyrins and their Structure/178.html">Protein Complexes inevitably leads to impaired vital functions and the onset of various diseases in humans and animals.

This section explores contemporary insights into the synthesis and degradation of iron porphyrins, with a particular focus on hemoglobin—the most extensively studied chromoprotein to date.

The Human Body contains approximately 4.5–5.0 g of iron. Assuming the total body iron to be 100%, Blood hemoglobin accounts for 60–70%, Myoglobin for 3–5%, ferritin for 20% (ranging from 17 to 23%), transferrin for about 0.18%, and functional tissue iron for up to 5%. Body iron levels are regulated primarily by The rate of intestinal absorption of dietary iron, with any excess remaining unabsorbed. The body's requirement for iron increases dramatically in various types of anemia. Iron is absorbed in the intestine as inorganic divalent Fe²⁺ ions after being released from protein complexes. Within the intestinal mucosal Cells, iron is converted to the trivalent Fe³⁺ form and binds to the protein apoferritin to form a stable ferritin complex. Further transport of iron to hematopoietic sites is carried out in a complex with serum ß₁-globulins (known as transferrin), or the iron binds to tissue apoferritin, where it is stored as ferritin. In certain pathological conditions, such as hemochromatosis, excess iron accumulates in the Cells of the macrophage system as hemosiderin, a metabolically inert iron-protein complex.

The primary sources of iron for synthetic processes include dietary products, as well as the iron released during the continuous breakdown of erythrocytes in the cells of The Liver and Spleen (approximately 25 mg daily). The prosthetic groups of dietary chromoproteins (such as hemoglobin and myoglobin), including chlorophyll-Proteins, are not utilized for the Synthesis of the body's iron proteins. Following Digestion, their non-protein heme component is oxidized to hematin, which, much like chlorophyll, is not absorbed in the intestine. Typically, these pigments are excreted in the feces either unchanged or as breakdown products resulting from the action of intestinal bacterial Enzymes. Consequently, dietary heme-containing compounds are not used as a source of the porphyrin ring; instead, the complex pyrrole structure is synthesized de novo from low-molecular-weight precursors within the body.