Files
Abstract
Aminolevulinic acid synthase 2 (ALAS2) is the rate-limiting enzyme of heme synthesis in the mammalian erythroid compartment and controls the production of approximately 400 billion hemoglobin molecules per second in adult human bone marrow. The ALAS2 gene is induced late in erythropoiesis when numerous developing erythroblasts are intimately associated with a central macrophage in microniches known as erythroblastic islands. Based largely on work on hemoglobin quantitation in whole cells, structure-function and kinetic studies on recombinant human ALAS2, and metabolomic analysis of mammalian erythroblast model cultures, we have proposed explanations here for the molecular-level impacts of ALAS2 catalysis in various blood disorders, including X-linked protoporphyria (XLP), X-linked sideroblastic anemia (XLSA), and anemia of chronic disease (inflammation). Notably, we have found that the eukaryotic-specific C-terminal extension of the human ALAS2 enzyme provides an autoinhibitory form of that is disrupted in XLP and enhanced in XLSA, resulting in gain- and loss-of function enzymes, respectively. We have also elucidated the metabolic fate of the immunoactivated macrophage compound itaconic acid in erythroid progenitors and have developed a new model for terminal erythropoiesis in the erythroblastic island during an inflammatory response. More specifically, we have shown that the itaconate derivative itaconyl-CoA is produced by red cell precursors after uptake of itaconate and that itaconyl-CoA competitively inhibits ALAS2 catalysis. We have therefore implicated macrophage-derived itaconate as key factor in states of inflammatory anemia. Finally and perhaps most importantly, we have used the knowledge gained from these studies to identify small-molecule antagonists of ALAS2, including itaconate, that may one day be used to treat XLP or other conditions involving porphyrin accumulation.