Plants cystathionine synthesis

Together, these data strongly suggest that O-phosphohomoserine is the physiologically important a-aminobutyryl donor for synthesis of both cystathionine and homocysteine in plants. This finding is consistent with the in vivo experiments of Dougall and Fulton (1967), which showed that neither O-succinyl nor 0-acetylhomoserine was utilized in preference to homoserine for cystathionine synthesis by cells of Paul s Scarlet Rose. 

In those plants in which it has been determined, O-phosphohomoserine is present in concentrations of less than 25 ixM (Datko et al., 1974b, 1977). Since the for O-phosphohomoserine in threonine synthesis (in the absence of AdoMet) is approximately 2 mM (Madison and Thompson, 1976 Thoen et al., 1978), and for cystathionine synthesis approximately 7 mM (Madison and Thompson, 1976), it follows that (unless compartmentation is very extreme) each of these enzymes will be operating below its for O-phosphohomoserine, and the two enzymes will therefore compete for the available supply of this substrate. 

While plants are unique in using 0-phosphohomoserine as the physiological substrate, crude plant extracts can also use other homoserine esters. For cystathionine synthesis, in general... 

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. 

Cysteine synthesis is a primary component of sulfur metabolism. The carbon skeleton of cysteine is derived from serine (Figure 14.7). In animals the sulfhydryl group is transferred from methionine by way of the intermediate molecule homocysteine. (Plants and some bacteria obtain the sulfhydryl group by reduction of SOj to S2 as H2S. A few organisms use H2S directly from the environment.) Both enzymes involved in the conversion of serine to cysteine (cystathionine synthase and y-cystathionase) require pyridoxal phosphate. 

In the nonprotein fraction reduced glutathione, GSH, is ubiquitous, and is commonly a mqjor constituent (Table I). The soluble fraction of plants also includes a variety of other sulfur-containing compounds that are normally present in relatively small amounts (a) Intermediates on the route to protein cysteine and protein methionine, such as cysteine, cystathionine, homocysteine, and methionine, (b) Compounds involved in methyl transfer reactions and polyamine synthesis AdoMet.t AdoHcy, and, presumably, 5 -methyl-thioadenosine. The biochemistry of the compounds in both groups (a) and (b) will be discussed here, (c) Compounds clearly related metabolically to cysteine or methionine, such as 5-methylcysteine and 5-methylmethionine. Because in certain plants these derivatives comprise a major portion of the nonprotein sulfur amino acids, they will be discussed here, (d) A number of compounds of uncertain function, the biochemistry of which has often not been clarified. Discussion of such compounds (Richmond, 1973 Fowden, 1964) is beyond the scope of this chapter. 

This evidence is consistent with, but does not provide definitive proof for OAS being the physiologically important precursor of cysteine. Only a limited number of O-esters of serine have been tested for activity with cysteine synthase (Section II,B,2), and no systematic studies have been made to determine whether serine O-esters other than OAS are synthesized by plants, or to identify the physiologically important a-aminopropionyl donor for cysteine synthesis. Experiments analogous to those used to identify the a-aminobutyryl donor for cystathionine and homocysteine synthesis (Section III,A,3) and the physiological carrier in sulfate reduction in Chlorella (Section IV,D,3) should be informative in this respect. 

Phosphohomoserine is used as a substrate for synthesis of cystathionine [Reaction (4)J and homocysteine [Reaction (5)J by extracts of all plants examined (Datko et al., 1974, 1977), but not by any microorganism tested (See Giovanelli et al., in press). Plants are therefore unique among the organisms studied in using 0-phosphohomoserine for cystathionine or homocysteine synthesis. 

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