Cystathionine formation

Homocysteine also accumulates in the blood if a mutation is present in the enzyme that converts N, methylene FH4 to N -methyl FH4. When this occurs, the levels of N -methyl FH4 are too low to allow homocysteine to be converted to methionine. The loss of this pathway, coupled with the feedback inhibition by cysteine on cystathionine formation, will also lead to elevated homocysteine levels in the blood. 

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. 

Figure 22.7 Homocysteine formation from methionine and formation of thiolactone from homocysteine. The homocysteine concentration depends upon a balance between the activities of homocysteine methyltransferase (methionine synthase) and cystathionine p-synthase. Both these enzymes require vitamin B12, so a deficiency can lead to an increase in the plasma level of homocysteine. (For details of these reactions, see Chapter 15.) Homocysteine oxidises spontaneously to form thiolactone, which can damage cell membrane.

Figure 9-5. Pathway for formation of cysteine from methionine. Only the enzymes involved in known diseases of this pathway are shown. Cystathionase is deficient in cysthioninuria, which leads to accumulation of cystathionine without producing frank symptoms. Cystathionine p-synthase deficiency causes homocystinuria.

Cyclohexanedione, reaction with guanidinium groups, 126 Cyclophilin 488 human 488s D-Cycloserine 739s Cyclosporin 488, 488s p Cylinders 65, 66, 686 Cystathionine, 746s formation 746 Cystathionine p lyase 742 Cystathionine p-synthase 744 Cystathionine y-synthase 743, 746 Cystatins 622, 629... 

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. 

Radioactive L-cystathionine (10) (765 mg.) containing 6.85 x 106 counts per minute of S35 was fed to the human cystinuric patient who had served previously as the subject in an experiment demonstrating the formation of cystine from sulfur-labeled methionine (11). The same precautions were followed with regard to human experimentation involving radioactive material as in the latter experiment. After the feeding of the cystathionine, 24-hour urine specimens were collected for 3 days and sulfur distributions were determined by the titrimetric method of Fiske (6). Cystine determinations were carried out by the procedure of Sullivan, Hess, and Howard (12). 

Our findings in this study are in harmony with the concept that L-cystathionine is an intermediate in the formation of cystine from methionine in man. Direct evidence for the existence of cystathionine in man was provided by the demonstration by Tallan, Moore, and Stein (13) of the occurrence of L-cystathionine in extracts of human brain. Moreover, cases of human cystathioninuria have been reported by Harris, Penrose, and Thomas (9) and by Frimpter, Haymovitz, and Horwith (8). The latter authors have also stated that an increased renal clearance of cystathionine is not observed in cystinuria. It is of considerable interest, however, that the mixed disulfide of L-cysteine and L-homo-... 

Methionine, homocysteine, and cysteine are linked by the methylation cycle and transsulfuratlon pathway (Figure 55-9). Conversion of methionine into homocysteine proceeds via the formation of S-adenosyl intermediates including S-adenosylmethionine, die methyl group donor in a wide range of transmethylation reactions. Homocysteine is further condensed with serine by cystathionine 3-synthase to form cystathionine. 

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. 

Several inherited disorders of methionine metabolism (Chapter 17) give rise to exeessive production of homocysteine, HS-CH2-CH2CH(NH3 )COO , and its excretion in urine. The most common form of homocystinuria is due to a deficiency of cystathionine synthase (Chapter 17). A major clinical manifestation of homocystinuria is connective tissue abnormalities that are probably due to the accumulation of homocysteine, which either inactivates the reactive aldehyde groups or impedes the formation of polyfunctional cross-links. 

Methionine. Methionine degradation begins with the formation of S-adeno-sylmethionine, which is followed by a demethylation reaction, as described (Figure 14.16). S-Adenosylhomocysteine, the product of the latter reaction, is hydrolyzed to adenosine and homocysteine. Homocysteine then combines with serine to yield cystathionine. Cysteine, a-ketobutyrate, and NH4 result from... 

The mechanism of 7-elimination reaction involves abstraction of the Cq, and C protons and removal of the 7-leaving group of the substrate side chain, with formation of a-ketobutyrate and ammonia. Cystathionine 7-lyase (CGL) and methionine 7-lyase (MGL) are representative enzymes of this class. 

Several PLP-dependent enzymes catalyze elimination and replacement reactions at the y-carbon of substrates, an unusual process which provides novel routes for mechanism-based inactivation. An example of this class of enzymes is cystathionine y-synthase [0-succinylhomoserine (thiol)-lyase], which converts (7-succinyl-L-homoserine and L-cysteine to cystathionine and succinate as part of the bacterial methionine biosynthetic pathway (Walsh, 1979, p. 823). Formation of a PLP-stabilized o-carbanion intermediate activates the )8-hydrogen for abstraction, yielding j8-carbanion equivalents and allowing elimination of the y-substituent. The resulting j8,y-unsaturated intermediate serves as an electrophilic acceptor for the replacement nucleophile. Suitable manipulation of the j8-carbanion intermediate allows strategies for the design of inactivators which do not affect enzymes which abstract only the a-hydrogen. 

Cystathionine y-synthase is the best studied enzyme catalyzing both y-replacement and /3,y-elimination reactions. The enzyme is found in plants and bacteria and normally functions to catalyze the formation of cystathionine from 0-acylhomoserine and cysteine during the biosynthesis of methionine (66) [Eq. (57)] ... 

Cystathionine-y-synthase isolated from Salmonella typhimurium is a tetramer (molecular weight 160000) and catalyses, in vivo, the y-replacement of O-suc-cinylhomoserine with cysteine [79] to yield cystathionine. The latter, by way of homocysteine, is involved in the biosynthesis of methionine. In other species of bacteria and plants the succinyl moiety may be replaced by acetyl, phosphoryl, or malonyl moieties [80]. In the absence of cysteine the enzyme catalyses an abnormal reaction resulting in the formation of a-oxobutyrate. The latter reaction has been utilised for mechanistic investigations pertinent to the y-eUmination-deamination process (vide infra). 

While the studies of Dougall (1965) are suggestive, it has not yet been firmly established whether or not methionine also controls de novo synthesis of the carbon moieties of methionine. Of course such regulation of the four-carbon portion would be expected if the effect of exogenous methionine is exerted upon the cystathionine synthesis step, since it is at this step that both the sulfur and the four-carbon moieties become committed to methionine formation. 

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