Cystathionine transsulfuration

The transsulfuration pathway is the major route for the metabolism of the sulfur-containing amino adds 674 Homocystinuria is the result of the congenital absence of cystathionine synthase, a key enzyme of the transsulfuration pathway 676 Homocystinuria can be treated in some cases by the administration of pyridoxine (vitamin B6), which is a cofactor for the cystathionine synthase reaction 676... 

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... 

Cysteine is formed in plants and in bacteria from sulfide and serine after the latter has been acetylated by transfer of an acetyl group from acetyl-CoA (Fig. 24-25, step f). This standard PLP-dependent (3 replacement (Chapter 14) is catalyzed by cysteine synthase (O-acetylserine sulfhydrase).446 447 A similar enzyme is used by some cells to introduce sulfide ion directly into homocysteine, via either O-succinyl homoserine or O-acetyl homoserine (Fig. 24-13). In E. coli cysteine can be converted to methionine, as outlined in Eq. lb-22 and as indicated on the right side of Fig. 24-13 by the green arrows. In animals the converse process, the conversion of methionine to cysteine (gray arrows in Fig. 24-13, also Fig. 24-16), is important. Animals are unable to incorporate sulfide directly into cysteine, and this amino acid must be either provided in the diet or formed from dietary methionine. The latter process is limited, and cysteine is an essential dietary constituent for infants. The formation of cysteine from methionine occurs via the same transsulfuration pathway as in methionine synthesis in autotrophic organisms. However, the latter use cystathionine y-synthase and P-lyase while cysteine synthesis in animals uses cystathionine P-synthase and y-lyase. 

In some organisms sulfur incorporation involves homocysteine as an intermediate. In such cases cysteine formation occurs by a transsulfuration reaction, with the intermediate formation of L,L-cystathionine (fig. 21.86). Cystathionine is formed in a simple condensation reaction from serine and homocysteine by cystathionine-jS synthase. 

The transsulfuration pathway involves conversion of homocysteine to cysteine by the sequential action of two pyridoxal phosphate (vitamin B6)-dependent enzymes, cystathionine- 5-synthase (CBS) and cystathionine y-lyase (Fig. 21-2). Transsulfuration of homocysteine occurs predominantly in the liver, kidney, and gastrointestinal tract. Deficiency of CBS, first described by Carson and Neill in 1962, is inherited in an autosomal recessive pattern. It causes homocystinuria accompanied by severe elevations in blood homocysteine (>100 (iM) and methionine (>60 (iM). Homocystinuria due to deficiency of CBS occurs at a frequency of about 1 in 300,000 worldwide but is more common in some populations such as Ireland, where the frequency is 1 in 65,000. Clinical features include blood clots, heart disease, skeletal deformities, mental retardation, abnormalities of the ocular lens, and fatty infiltration of the fiver. Several different genetic defects in the CBS gene have been found to account for loss of CBS activity. 

Pyridoxine (vitamin B6) Transsulfuration pathway cofactor for cystathionine-(3-synthase and cystathionine "/-lyase Folate cycle methylation of tetrahydrofolate cofactor for serine hydroxymethyltransferase... 

Homocysteine is metabolized in the liver, kidney, small intestine and pancreas also by the transsulfuration pathway [1,3,89]. It is condensed with serine to form cystathione in an irreversible reaction catalyzed by a vitamin B6-dependent enzyme, cystathionine-synthase. Cystathione is hydrolyzed to cysteine that can be incorporated into glutathione or further metabolized to sulfate and taurine [1,3,89]. The transsulfuration pathway enzymes are pyridoxal-5-phosphate dependent [3,91]. This co-enzyme is the active form of pyridoxine. So, either folates, cobalamin, and pyridoxine are essential to keep normal homocysteine metabolism. The former two are coenzymes for the methylation pathway, the last one is coenzyme for the transsulfuration pathway [ 1,3,89,91 ]. 

In the biosynthesis of cysteine, the sulfur comes from methionine by transsulfuration, and the carbon skeleton and the amino group are provided by serine (Figure 17-16). Cysteine regulates its own formation by functioning as an allosteric inhibitor of cystathionine y-lyase, a-Ketobutyrate is metabolized to succinyl-CoA by way of propionyl-CoA and methylmalonyl-CoA. 

Cystathionine is the first intermediate metabolite in transsulfuration, formed from HCY and serine by cystathionine-fi-synthase, a redox-sensitive, heme-containing enzyme (Banerjee et al., 2003), whose activity is lower in males vs. females (Vitvitsky et al., 2007). The higher levels of cystathionine in human brain reflect a strong diversion of HCY to transsulfuration (i.e., low methionine synthase activity and high cystathionine-f)-synthase activity), in conjunction with a decreased conversion of cystathionine to cysteine. As illustrated in Fig. 1, this indicates impaired transsulfuration in human brain. Low transsulfuration activity relative to other tissues has been described in rat or mouse brain (Finkelstein, 1990), although a... 

Selenomethionine metabolism to selenide and the incorporation into selenium-specific proteins may occur by two pathways metabolism to methane selenol and selenide or via selenocysteine. Evidence that the incorporation of selenium from selenomethionine into protein is by the transsulfuration pathway (methionine to cysteine) comes from studies of selenomethionine metabolism in lymphoblast cell lines deficient in cystathionine lyase and cystathionine synthetase, enzymes of the transsulfuration pathway (Beilstein and Whanger 1992). Deficiency in these enzymes greatly reduces the incorporation of selenomethionine into glutathione peroxidase. 

Kery, V., Bukovska, G., and Kraus, J.P (1994) Transsulfuration depends on heme in addition to pyridoxal 5 -phosphate. Cystathionine beta-synthase is a heme protein. J. Biol. Chem. 269, 25283-25288. 

Cystathionine y-synthase (CGS) is a rather unique PLP-enzyme that catalyzes a transsulfuration reaction important in microbial methionine biosynthesis. It is the only known enzyme whose function is the catalysis of a PLP-dependent replacement reaction at the y-carbon of the amino acid substrate the succinyl moiety of O-succinyl-L-homoserine is replaced by i-Cys to give the thioether linkage of L,/.-cystathionine (scheme II). In the absence of L-Cys, the enzyme catalyzes a net y-elimination reaction from OSHS (scheme II). Because both reactions require the elimination of succinate, the catalytic pathways must diverge from a common reaction intermediate. It was originally hypothesized that a vinylglycine quinonoidal intermediate (structure 11)... 

CGS catalyzes the 7-replacement reaction of an activated form of L-homoserine with L-cysteine, leading to cystathionine. 0-Succinyl-L-homoserine (l-OSHS), 0-acetyl-L-homoserine (OAHS), and 0-phospho-L-homoserine (OPHS) are substrates for CGS ftom bacteria, fungi, and plants, respectively. The plant enzyme is also able to convert the microbial substrates, albeit at much higher values. This reaction is the first step in the transsulfuration pathway that converts L-Cys into L-homocysteine, the immediate precursor of L-methionine. The 0-activated L-homoserine substrate is situated at a metabolic branch point between L-Met and L-Thr biosynthesis, and which substrate is used by CGS depends on the species. In analogy with TS, CGS is tightly regulated by SAM concentration in plants. ... 

Autotrophic organisms synthesize methionine from asparfafe as shovm in the lower right side of Fig. 24-13. This involves fransfer of a sulfur atom from cysfeine info homocysfeine, using the carbon skeleton of homoserine, the intermediate cystathionine, and two PLP-dependent enzymes, cystathionine y-synthase and cystathionine p-lyase. This transsulfuration sequence (Fig. 24-13, Eq. 14-33) is essentially irreversible because of the cleavage to pyruvate and NH4+ by the P-lyase. Nevertheless, this transsulfuration pathway operates in reverse in the animal body, which uses two different PLP enzymes, cystathionine P s3mthase (which also contains a bound heme) and cystathionine y-lyase (Figs. 24-13,24-16, steps h and i), in a pathway that metabolizes excess methionine. 

An increased plasma level of homocysteine is regarded as a risk factor for cardiovascular disease and the development of arteriosclerosis. Homocysteine concentrations in plasma are reduced by remethylation and transsulfuration (Komarnisky et al. 2003). The remethylation is catalyzed by methionine synthase, which in turn is influenced by vitamin B12 and folate. The transsulfura-tions depend on cystathionine 3-synthase. A dietary deficiency of vitamins B, B12 and folate, accompanied by a high protein intake, can cause hyperhomocystinemia in humans (Jacobsen 1998). Furthermore, a genetic disorder of enzymes involved in the metabolism of homocysteine leads to hypercystinuria (Mudd et al. 1989). 

From the above work, two unexpected findings emerged. First, crude extracts of spinach also catalyzed a direct reaction of HjS with O-acetylhomoserine to form homocysteine (Giovanelli and Mudd, 1%7). Thus the sulfur moiety of homocysteine could possibly be derived not only from cysteine via transsulfuration, but also directly from HjS via sulfhydra-tion. Second, a variety of O-acylhomoserine esters, but not homoserine itself, were active in the synthesis of both homocysteine and cystathionine (Giovanelli and Mudd, 1966, 1%7) the question arose as to which, if any, of these compounds is of physiological significance as the precursor of the four-carbon portion of homocysteine. 

Methionine is an essential amino acid with a unique role in the initiation of protein synthesis, hi addition, by conversion to 5 -adenosyhnethionine, it serves as the major methyl group donor involved in the formation of creatinine and choline, in the methylation of bases in RNA, and as the source of the aminopropyl group in the formation of polyamines. Finally, in relationship to classical homocystinuria, it is converted by way of homocysteine and cystathionine in a series of reactions termed as the transsulfuration pathway (Fig. 20.3). 

As noted above, cystathionine formation is the other major fate of methionine. The condensation of homocysteine with serine is catalyzed by the vitamin requiring enzyme cystathionine P-synthase. In the last step of the transsulfuration sequence, cystathionine undergoes cleavage to cysteine and a-ketobutyrate in yet another enzyme reaction that requires pyridoxal phosphate. 

Transsulfuration is facilitated by the action of two vitamin Be-dependent enzymes, cystalhionine-p-synthase (CBS), the enzyme deficient in homocystinuria, and cystathionine-Y-lyase (CTH). CBS catalyzes the condensation of homocysteine and serine to cystathionine, and CTH subsequently catalyzes the hydrolysis of cystathionine to cysteine and a-ketobutyrale. Cysteine is important in protein synthesis and taurine synthesis and is a precursor to glutathione, a strong antioxidant and essential compound in detoxification of many xenobiotics [8,10,11]. 

Figure 29.6 Pathways for the metabolism of homocysteine. Normal transsulfuration requires cystathionine P-synthase with vitamin Bg as cofactor. Reme-thylation requires 5,10-methylenetetrahydrofolate reductase and methionine synthase. The latter requires folate as cosubstrate and vitamin Bi2 (cobalamin) as cofactor. An alternative remethylation pathway also exists using the cobalamin independent betaine-homocysteine methyltransferase (Robinson 2000).

Remethylation pathway. In cells, homocysteine is either remethylated to methionine (via methionine synthase) or is transsulfurated to cysteine via cystathionine beta synthase). In remethylation, homocysteine receives a methyl group from 5-methyltetrahydrofolate or from betaine. Vitamin B12 is a necessary cofactor in the folate-dependent remethylation. 

Synthesis of Sulfur Amino Acids. Of the many oxidation states of sulfur, only sulfite has been shown to be utilized by cell-free systems in the net synthesis of compounds with carbon-sulfur bonds, although mutant studies have indicated that more reduced forms can be incorporated. The formation of cysteinesulfinic acid from sulfite has been demonstrated in extracts of acetone-dried rabbit kidney it is possible that this reaction participates in the principal mechanism of sulfur incorporation. In many organisms that require preformed sulfur amino acids, cysteine may be formed from methionine. Only the sulfur of methionine is transferred to cysteine the carbon skeleton of cysteine is derived exclusively from serine. Transsulfuration appears to require the formation of homocysteine from methionine. Homocysteine and serine condense to form a thioether, cystathionine (V). Pyridoxal phosphate has been... 

Figure 18.4 SAM cycle (black) and the interconnected pathways of tetrahydrofo-late metabolism and transsulfuration (gray). The MT reaction is marked by a dashed arrow. CpS, cystathionine-p-synthase CyS, cystathionine-y-synthase GHMT, glycine betaine-homocysteine methyltransferase ...

The transsulfuration reaction by which cysteine is formed depends upon the condensation of homocysteine, the demethylated product of methionine, with serine to form the intermediate cystathionine (see p. 152). 

The transsulfuration reaction from methionine to cysteine presupposes the formation of the compound cystathionine from methionine (or homocysteine) and serine as an intermediate. 

The evidence given above for the key role of cystathionine in the transsulfuration reaction to form cystine from methionine would be measurably strengthened by proof of the occurrence of this compound as a product of the metabolism of methionine. In Neurospora cystathionine has been isolated from the culture medium of certain methionineless mutants and shown to be a step in the conversion of cysteine to methio-... 

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