Heme is a complex of iron with protoporphyrin IX that is essential for the function of all aerobic cells. Heme serves as the prosthetic group of numerous hemoproteins (eg, hemoglobin, myoglobin, cytochromes, guanylate cyclase, and nitric oxide synthase) and plays an important role in controlling protein synthesis and cell differentiation.
Synthase is the committed step of the heme synthesis pathway, and is usually rate-limiting for the overall pathway.
Heme synthesis begins in mitochondria with condensation of glycine & succinyl-CoA, with decarboxylation, to form delta-aminolevulinic acid (ALA). Pyridoxal phosphate (PLP) serves as coenzyme for delta-Aminolevulinate Synthase (ALA Synthase). Condensation with succinyl-CoA takes place while the amino group of glycine is in Schiff base linkage to the aldehyde of Coenzyme A and the carboxyl of glycine are lost following the condensation reaction.
How does it work? synthase removes the carboxil group from glycine and the CoA from the succinyl-CoA by means of its prosthetic group pyridoxal phosphate (a vitamin B6 derivative), forming δ-aminolevulinic acid (dALA), so called because the amino group is on the fourth carbon atom in the molecule. Glycine is initially deprotonated by a highly conserved active site lysine, leading to condensation with succinyl-CoA and loss of CoA. Protonation of the carbonyl group of the intermediate by an active site histidine leads to loss of the carboxyl group. The last intermediate is finally reprotonated to produce Dissociation of from the enzyme is the rate limiting step of the enzymatic reaction and was shown to be depended upon a slow conformational change of the enzyme. The function of pyridoxal phosphate is to facilitate the removal of hydrogen, by utilizing the electrophilic pyridinium ring as an electron sink.
Hemin is an iron-containing porphyrin. More specifically, it is Protoporphyrin IX containing a ferric iron ion (Heme B) with a chloride ligand.
Low concentrations of exogenous hemin (30-35 microM) inhibited the biosynthetic labelling of mature erythroid synthase. Parallel experiments using antibodies directed against human H-chain ferritin confirmed the specificity of the effects of hemin on translation of the e-ALA synthase mRNA and its degradation. Although the mechanism of hemin action is unknown, it is apparently independent of 5'-iron-response elements and influences the translational activity of erythroid synthase mRNA.
Hepatocyte heme production must be controlled to respond to changing metabolic requirements. As we said, hepatocytes express -1 and increasing heme levels create a negative feedback that downregulate transcription of -1 and inhibit its import into the mitochondrial matrix. -1 transcription is upregulated by peroxisome proliferator-activated receptor coactivator 1 (PGC-1). Transcription of -1 is regulated by glucose levels. Hypoglycemia induces -1 production, increasing -1 and heme synthesis. This promotes the clinical appearance of the acute porphyrias. Moreover, -1 increases mitochondrial genes expressions, such as oxidative respiration genes, it increases fatty acid oxidation and glucose uptake by 4.