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OBJECTIVES
After studying this chapter, you should be able to:
Write the structural formulas of the two amphibolic intermediates whose condensation initiates heme biosynthesis.
Identify the enzyme that catalyzes the key regulated enzyme of hepatic heme biosynthesis.
Explain why, although porphyrinogens and porphyrins both are tetrapyrroles, porphyrins are colored whereas porphyrinogens are colorless.
Specify the intracellular locations of the enzymes and metabolites of heme biosynthesis.
Outline the causes and clinical presentations of various porphyrias.
Identify the roles of heme oxygenase and of UDP-glucosyl transferase in heme catabolism.
Define jaundice, name some of its causes, and suggest how to determine its biochemical basis.
Specify the biochemical basis of the clinical laboratory terms “direct bilirubin” and “indirect bilirubin.”
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BIOMEDICAL IMPORTANCE
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The biochemistry of the porphyrins and of the bile pigments are closely related topics. Heme is synthesized from porphyrins and iron, and the products of degradation of heme are the bile pigments and iron. The biochemistry of the porphyrins and of heme is basic to understanding the varied functions of hemoproteins, and the porphyrias, a group of diseases caused by abnormalities in the pathway of porphyrin biosynthesis. A much more common clinical condition is jaundice, a consequence of an elevated level of plasma bilirubin, due either to overproduction of bilirubin or to failure of its excretion. Jaundice occurs in numerous diseases ranging from hemolytic anemias to viral hepatitis and to cancer of the pancreas.
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Porphyrins are cyclic compounds formed by the linkage of four pyrrole rings through methyne (
HC—) bridges (Figure 31–1). Various side chains can replace the eight numbered hydrogen atoms of the pyrrole rings.
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Porphyrins can form complexes with metal ions that form coordinate bonds to the nitrogen atom of each of the four pyrrole rings. Examples include iron porphyrins such as the heme of hemoglobin and the magnesium-containing porphyrin chlorophyll, the photosynthetic pigment of plants. Heme proteins are ubiquitous in biology and serve diverse functions including, but not limited to, oxygen transport and storage (eg, hemoglobin and myoglobin) and electron transport (eg, cytochrome c and cytochrome P450). Hemes are tetrapyrroles, of which two types, heme b and heme c, predominate (Figure 31–2). In heme c the vinyl groups of heme b are replaced by covalent thioether links to an apoprotein, typically via cysteinyl residues. Unlike heme b, heme c thus does not readily dissociate from its apoprotein.
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