After studying this chapter, you should be able to:
- Know the relationship between porphyrins and heme
- Be familiar with how heme is synthesized
- Understand the causes and general clinical pictures of the various porphyrias
- Know how bilirubin is derived from heme and how it is handled in the body
- Understand the nature of jaundice and appreciate how to approach determining its cause in a patient.
The biochemistry of the porphyrins and of the bile pigments is presented in this chapter. These topics are closely related, because heme is synthesized from porphyrins and iron, and the products of degradation of heme are the bile pigments and iron.
Knowledge of the biochemistry of the porphyrins and of heme is basic to understanding the varied functions of hemoproteins (see below) in the body. The porphyrias are a group of diseases caused by abnormalities in the pathway of biosynthesis of the various porphyrins. Although porphyrias are not very prevalent, physicians must be aware of them. A much more prevalent clinical condition is jaundice, due to elevation of bilirubin in the plasma. This elevation is due to overproduction of bilirubin or to failure of its excretion and is seen in numerous diseases ranging from hemolytic anemias to viral hepatitis and to cancer of the pancreas.
Porphyrins are cyclic compounds formed by the linkage of four pyrrole rings through methyne (=HC—) bridges (Figure 31–1). A characteristic property of porphyrins is the formation of complexes with metal ions bound to the nitrogen atom of the pyrrole rings. Examples are the iron porphyrins such as heme of hemoglobin and the magnesium-containing porphyrin chlorophyll, the photosynthetic pigment of plants.
The porphyrin molecule. Rings are labeled I, II, III, and IV. Substituent positions on the rings are labeled 1, 2, 3, 4, 5, 6, 7, and 8. The methyne bridges (=HC—) are labeled α, β, γ, and δ. The numbering system used is that of Hans Fischer.
Proteins that contain heme (hemoproteins) are widely distributed in nature. Examples of their importance in humans and animals are listed in Table 31–1.
Table 31–1 Examples of Some Important Human and Animal Hemoproteins1 |Favorite Table|Download (.pdf)
Table 31–1 Examples of Some Important Human and Animal Hemoproteins1
|Hemoglobin||Transport of oxygen in blood|
|Myoglobin||Storage of oxygen in muscle|
|Cytochrome c||Involvement in electron transport chain|
|Cytochrome P450||Hydroxylation of xenobiotics|
|Catalase||Degradation of hydrogen peroxide|
|Tryptophan pyrrolase||Oxidation of tryptophan|
Natural Porphyrins Have Substituent Side Chains on the Porphyrin Nucleus
The porphyrins found in nature are compounds in which various side chains are substituted ...