Glycine heme from

Figure 3 The synthesis of heme from glycine and sucdnyl-CoA. The enzymes are ALAS, S-aminolevulinic acid (ALA) synthase ALAD, S-aminolevulinic acid dehydratase PBGD, porphobilinogen deaminase UROIIIS, uroporphyrinogen III synthase UROD, uroporphyrinogen decarboxylase CPO, coproporphyrinogen oxidase PPO, protoporphyrinogen oxidase and FECH, ferrochelatase.

Scheme 2. Pathway for the synthesis of heme from glycine and succinyl...

Hgb consists of a protein component with two a and two p chains each chain is linked to a heme group consisting of a porphyrin ring structure with an iron atom chelated at its center, which is capable of binding oxygen. The initial step in the synthesis of heme from the substrate succinyl CoA and glycine requires the presence of pyridox-ine phosphate (vitamin Be) as a catalyst. Following its synthesis in the cytoplasmic mitochondria of the RBC, heme diffuses into the extra-mitochondrial space, combines with the completed a and p chains, and forms Hgb. 

Vitamin Be deficiency symptoms include dermatitis, a microcytic, hypochromic anemia, weakness, irritability, and, in some cases, convulsions. Xanthurenic acid (a degradation product of tryptophan) and other compounds appear in the urine because of an inability to completely metabolize amino acids. A decreased ability to synthesize heme from glycine may cause the microcytic anemia (see Chapter 44), and decreased decarboxylation of amino acids to form neurotransmitters (see Chapter 48) may explain the convulsions. 

Heme Glycine and succinyl CoA Liver, bone marrow Heme from liver is incorporated into cytochromes. Heme from bone marrow is incorporated into hemoglobin. 

Shemin, D., London, I. M., and Rittenberg, D. (1948). The in vitro synthesis of heme from glycine by the nucleated red blood cell. J. Biol. Chem. 173, 799-800. SoErao, R., Birnboim, H. C., and Darnell, J. E. (1966). Rapidly labeled HeLa cell nuclear RNA. II. Base composition and cellular localization of a heterogeneous RNA fraction. J. Mol. Biol. 19, 362-.3V2. 

B. Heme is synthesized from glycine and succinyl CoA precursors via a complex series of reactions (Figure 9-7). 

Many important biomolecules are derived from amino acids. Glycine is a precursor of porphyrins. Degradation of iron-porphyrin (heme) generates bilirubin, which is converted to bile pigments, with several physiological functions. 

Fig. 2. Pathway of the synthesis of heme and chlorophyll, (a) Synthesis of porphobilinogen from glycine and succlnyl CoA (b) synthesis of protoporphyrin IX from porphobilinogen. A = CH2COOH, M = CH3, P = CH2CH2COOH.

Additional reactions of glycine include its ability to become conjugated with bile acids to form conjugated bile salts (see Chapter 19), formation of heme (Chapter 7), formation of purine nucleotides (Chapter 10), formation of creatine (see later), and the formation of hippuric acid from benzoic acid. In the last case, an amide linkage is formed between the carboxyl group of benzoic acid and the amino group of glycine. 

This mitochondrial enzyme (ALA synthetase) catalyzes the formation of 6-aminolevulinic acid (ALA) from glycine and succinyl-CoA. This is the initial step in heme biosynthesis. 

Cytochrome synthesis was examined in the fat body of adult male B. discoidalis by measuring the synthesis of cytochrome hemes. Heme is synthesized from the condensation of succinate and glycine by aminolevulinic acid synthase to produce aminolevulinic acid (ALA), a specific heme precursor. A developmental pattern exists for the incorporation of [i CjALA into cytohemes of fat body mitochondria with a peak of synthesis between days 4 and 6 of adult age (60). CC ablation eliminates this peak of synthesis for cytohemes a and b CC extract injections return the synthesis of cytohemes a+b to normal levels in CC-ablated cockroaches but have no effects on the synthesis of the c-type hemes for cytochromes c and Cj, The synthesis of cytohemes a+b in response to CC extracts requires a latent period of 24-48 hr to obtain a maximum response and is dose-dependent over a range of 0.01 to 0.08 CC pair (61). The active factor in CC extracts is sensitive to chymotrypsin but not to trypsin. This "cytochromogenic hormone (CGH) is secreted on days 2-3 of adult age in males (62). Since maximal synthesis of cytohemes a+b occurs on day 4, CGH secretion on days 2-3 agrees with the earlier observation that CGH requires about 48 hr to produce its response (61). 

Using 14c, which had just become available, they discovered that 8 of the carbon atoms of heme in nucleated duck erythrocytes are derived from the a-carbon atom of glycine and none from the carboxyl carbon atom. The results of subsequent studies demonstrated that the other 26 carbon atoms of heme can arise from acetate. Moreover, the in methyl-labeled acetate emerged in 24 of these carbons, whereas the in carboxyl-labeled acetate appeared only in the other 2 (Figure 24.341. 

Heme is synthesized from glycine and succinyl CoA in humans (Figure 4-6). It is not derived from a vitamin, d. At the end of the electron transport chain, the electrons are transferred to 02, which is reduced to H20. 

Heme is produced from glycine and succinyl CoA via a series of porphyrins. 

The complex tetrapyrrole ring structure of heme is built up in a stepwise fashion from the very simple precursors sue-cinyl-CoA and glycine (Figure 32-2). The pathway is present in all nucleated cells. From measurements of total bilirubin production, it has been estimated that daily synthesis of heme in humans is 5 to 8mmol/kg body weight. Of this, 70% to 80% occurs in the bone marrow and is used for hemoglobin synthesis. Approximately 15% is synthesized in the liver and is used to produce cytochrome P-450, mitochondrial cytochromes, and other hemoproteins. The pathway is compartmentalized, with some steps occurring in the mitochondrion and others in the cytoplasm. Little is known about the transport of intermediates across the mitochondrial membrane, and no transport defect has yet been reported in the porphyrias. 

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