Bilirubin and Its Carbohydrate Conjugates

In mammals, the open tetrapyrrole bilirubin (structure 2, Fig. 1 ) is the principal degradation product of iron-protoporphyrin-IX (heme). The latter molecule is a tetrapyrrolic macrocycle and plays a critical role in aerobic metabolism by reversibly binding oxygen in hemoglobin and myoglobin, and by serving as the active site in oxidation reactions catalyzed by hemoprotein enzymes. Important cyclic tetrapyrroles in nature related to heme are chlorophylls, which contain magnesium and are derived from protoporphyrin-IX, and vitamin B₁₂ , a corrinoid derived from uroporphyrinogen-III.