Heme synthesis, regulation

Regulation of heme synthesis in erythroid cells occurs at enzymes catalyzing... 

The oxidative phosphorylation system contains over 80 polypeptides. Only 13 of them are encoded by mtDNA, which is contained within mitochondria, and all the other proteins that reside in the mitochondrion are nuclear gene products. Mitochondria depend on nuclear genes for the synthesis and assembly of the enzymes for mtDNA replication, transcription, translation, and repair (Tl). The proteins involved in heme synthesis, substrate oxidation by TCA cycle, degradation of fatty acids by /i-oxidalion, part of the urea cycle, and regulation of apoptosis that occurs in mitochondria are all made by the genes in nuclear DNA. 

Heme synthesis is controlled primarily by 8-aminolevulinate synthase (ALA synthase). There are two mechanisms of control, and each involves a process that affects the concentration of the enzyme. First, the half-life of ALA synthase, as shown by experiments in rat liver, is very short (60-70 min). Like many mitochondrial proteins, ALA synthase is encoded by nuclear genes, synthesized on cytoplasmic ribosomes, and the enzyme is translocated into the mitochondria. The second and main regulating factor is the inhibition of ALA synthase by hemin. Hemin differs from heme in that the Fe atom is in the Fe3+ oxidation state. Heme spontaneously oxidizes to hemin when there is no globin to form hemoglobin. Hemin serves a second function in the regulation of hemoglobin synthesis in reticulocytes. It controls the synthesis of globin. 

Ponka P. Tissue-specific regulation of iron metabolism and heme synthesis distant contact mechanisms in erythroid cells. Blood 1997 89 1-25. 

Regulation is also accomplished by compartmentaliza-tion of enzyme systems involved in anabolic and catabolic pathways into different cell organelles. For example, fatty acid synthesis occurs in the soluble fraction of the cytoplasm, whereas fatty acid oxidation takes place in mitochondria. Heme synthesis begins and is completed in mitochondria, but some of the intermediate reactions take place in the cytosol. Heme catabolism is initiated in the smooth endoplasmic reticulum. Transport of key metabolites across an organelle membrane system is also a form of regulation. 

The major regulatory step in heme synthesis is the ALA synthetase reaction. Through feedback inhibition, heme regulates the enzyme. Heme also inhibits the translation of ALA synthetase. At even higher levels, heme blocks transport of ALA synthetase to the mitochondrion - its site of action. 

We devised a screen for isolating mutants defective in iron-dependent regulation of heme biosynthesis that did not require prior knowledge of the mechanism or of the rate-limiting steps [83]. We speculated that if the pathway as a whole were regulated by iron, a mutant defective in that control would accumulate protoporphyrin under iron limitation. Mutants defective in the heme synthesis enzymes ferrochelatase [75] or protoporphyrinogen oxidase would likely have a similar phenotype, but porphyrin accumulation would likely be independent of iron in the structural gene mutants, and those strains would also be expected to be heme auxotrophs. 

Heme metabolism in hepatocytes is highly regulated (Bonkovsky, 1982). Free heme down-regulates its own synthesis (by decreased mRNA stability and mitochondrial import of ALAS) and up-regulates its degradation (by transcriptional increase in HO). If heme is liberated from hepatocyte CYP [which is the major heme pool in this cell (Bonkovsky, 1982)], this could result in modulation of the activities of these enzymes. As shown in Table V, treatment of hepatocytes in vitro with CME indeed results in a decline (80%) in ALAS and an increase (3.9-fold) in HO activities. This effect is inhibited by NMMA, demonstrating it to be a result of -NO synthesis, and... 

Heme regulates its own synthesis by mechanisms that affect the first enzyme in the pathway, 8-ALA synthase (see Fig. 44.3). Heme represses the synthesis of this enzyme, and also directly inhibits the activity of the enzyme (an allosteric modifier). Thus, heme is synthesized when heme levels fall. As heme levels rise, the rate of heme synthesis decreases. 

Heme also regulates the synthesis of hemoglobin by stimulating synthesis of the protein globin. Heme maintains the ribosomal initiation complex for globin synthesis in an active state (see Chapter 15). 

L.H. Conder, S. I. Woodard, H. A. Dailey (1991). Multiple mechanisms for the regulation of heme synthesis during erythroid cell differentiation. Possible role for coproporphyrinogen oxidase. Biochem. J., 275, 321-326. 

Another promising biochemical approach, which may be linked to the deterioration of the hemopoitic system, involves the subcellular effects of lead on mitochondria. In experiments described by Brierley (2), lead at the micromolar level was shown to have a variety of effects centering around the inhibition of respiration and breakdown of the regulation of mitochondrial membrane permeability. The uptake of lead into the mitochondrial matrix is closely related to the membrane transport mechanism(s) for calcium. Since lead is known to inhibit several enzymes of the Kreb s cycle and the electron transport system, heme synthesis may be inhibited at the mitochon-... 

Aminolaevulinic acid synthetase located in the mitochondrial matrix space (McKay et al, 1969) is the first in a sequence of enzymes involved in heme synthesis and may represent the rate limiting step in its regulation. This enzyme is not synthesized within the mitochondria (Jayaraman et al, 1971 Barnes et al, 1971). 

The study of cytochrome c synthesis also provides an opportunity to clarify the mechanism by which the prosthetic group (heme) and protein regulate each other s synthesis. It has been established in bacteria that heme synthesis is independent of the availability of the apoprotein, but that, in contrast, the availability of the heme regulates the synthesis of the protein. 

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