Heme synthesis pathway mnemonic - USMLE Forums
Porphyrin and Heme Synthesis and Bilirubin Metabolism
AB - Biosynthesis of heme is important for both prokaryotes and eukaryotes. The enzymes for this multistep process are distributed between the cytosol and mitochondria in eukaryotes. In humans there are inherited and acquired disorders characterized by over synthesis of one or more enzymes or absence of an enzyme. This overview discusses each enzyme in the heme biosynthesis pathway.
The biochemistry of heme biosynthesis.
X-linked sideroblastic anemia is due to a deficiency of the erythroid form of the first enzyme in the heme biosynthetic pathway, 5-aminolevulinic acid synthase. Characteristics of the disease are variable, but typically include adult onset anemia, ineffective erythropoiesis with formation of ring sideroblasts, iron accumulation and pyridoxine responsiveness. Porphyrias are metabolic disorders due to deficiencies of other enzymes of this pathway, and are associated with striking accumulations and excess excretion of heme pathway intermediates and their oxidized products. Symptoms and signs of the porphyrias are almost all due to effects on the nervous system or skin. The three most common porphyrias, acute intermittent porphyria, porphyria cutanea tarda and erythropoietic protoporphyria, differ considerably from each other. The first presents with acute neurovisceral symptoms and can be aggravated by some drugs, hormones and nutritional changes, and is treated with intravenous heme and carbohydrate loading. The skin is affected in the latter two although the lesions are usually distinct and treatment is different. Porphyrias are more often manifest in adults than are most metabolic diseases. All porphyrias are inherited, with the exception of porphyria cutanea tarda, which is due to an acquired enzyme deficiency in liver, although an inherited deficiency is a predisposing factor in some cases.