Heme biosynthesis

The enzymatic process that produces heme is properly called porphyrin synthesis, as all the intermediates are tetrapyrroles that are chemically classified are porphyrins. The process is highly conserved across biology. In humans, this pathway serves almost exclusively to form heme. In other species, it also produces similar substances such as cobalamin (vitamin B12). The pathway is initiated by the synthesis of D-Aminolevulinic acid (dALA or δALA) from the amino acid glycine and succinyl-CoA from the citric acid cycle (Krebs cycle). The rate-limiting enzyme responsible for this reaction, ALA synthase, is strictly regulated by intracellular iron levels and heme concentration. A low-iron level, e.g., in iron deficiency, leads to decreased porphyrin synthesis, which prevents accumulation of the toxic intermediates. This mechanism is of therapeutic importance: infusion of heme arginate or hematin can abort attacks of porphyria in patients with an inborn error of metabolism of this process, by reducing transcription of ALA synthase. The organs mainly involved in heme synthesis are the liver and the bone marrow, although every cell requires heme to function properly. Heme is seen as an intermediate molecule in catabolism of haemoglobin in the process of bilirubin metabolism. Source: Wikipedia http://en.wikipedia.org/wiki/Heme

生成四吡咯类化合物（化学上归类为卟啉）的酶促过程，通常被称为卟啉合成，因为所有中间体均为四吡咯。这一过程在生物学领域高度保守。在人类中，此途径几乎专门用于生成血红素。在其他物种中，它还产生类似物质，如钴胺素（维生素B12）。该途径的启动由从甘氨酸和柠檬酸循环（克雷布斯循环）中的琥珀酰辅酶A合成的D-氨基乙酰丙酸（dALA或δALA）。负责此反应的限速酶ALA合酶受到细胞内铁水平和血红素浓度的严格调控。铁水平低，例如在缺铁性贫血中，会导致卟啉合成减少，从而防止有毒中间体的积累。这一机制具有重要的治疗意义：通过减少ALA合酶的转录，输注血宁酸或血蓝素可以中断具有该代谢过程先天性缺陷的患者紫质病的发作。在胆红素代谢过程中，血红素被视为血红蛋白分解代谢的中间分子。来源：维基百科 http://en.wikipedia.org/wiki/Heme