Plants homocysteine synthesis

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. 

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. 

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. 

SMM synthesis is mediated by the enzyme methionine S-methyltransferase (MMT) through the essentially irreversible, AdoMet-mediated methylation of methionine.48"5 Both MMT and SMM are unique to plants 48,50 The opposite reaction, in which SMM is used to methylate homocysteine to yield two molecules of methionine, is catalyzed by the enzyme homocysteine S-methyltransferase (HMT).48 Unlike MMT, HMTs also occur in bacteria, yeast, and mammals, enabling them to catabolize SMM of plant origin, and providing an alternative to the methionine synthase reaction as a means to methylate homocysteine. Plant MMT and HMT reactions, together with those catalyzed by AdoMet synthetase and AdoHcy hydrolase, constitute the SMM cycle (Fig. 2.4).4... 

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. 

Animals - Mammals require methionine (Met) in their diets (i.e.. Met is an essential amino acid) and Cys can be made from Met, as shown in Figure 2L7. Thus, Cys is nonessential as long as sufficient Met is present in the diet. Mammals make Met from homocysteine, as shown in the reaction here. Figure 2L8 shows the pathway from Met to Cys and reveals that it is quite similar to the reverse of the methionine synthesis pathway in bacteria shown in Figure 21.5. Plants and bacteria also use the pathway shown in Figure 2L8 so they can synthesize one from the other, depending on their immediate needs. Methionine can also be made by conversion of choline, as shown here. 

The synthesis of methionine in mammals is more complex than in plants and requires cobalamin, a coenzyme form of vitamin B,2. Because methionine is an essential amino acid, it must be supplied in the diet methionine that is used for methylation is converted to homocysteine, and this is remethylated to regenerate methionine. These reactions merely recycle methionine and do not constitute a means of net synthesis. 

Most of the inorganic sulfate assimilated and reduced by plants appears ultimately in cysteine and methionine. These amino acids contain about 90% of the total sulfur in most plants (Allaway and Thompson, 1966). Nearly all of the cysteine and methionine is in protein. The typical dominance of protein cysteine and protein methionine in the total organic sulfur is illustrated in Table I by analyses of the sulfur components of a lower plant (Chlorella) and a higher plant (Lemna). Thede novo synthesis of cysteine and methionine is one of the key reactions in biology, comparable in importance to the reduction of carbon in photosynthesis (Allaway, 1970). This is so because all nonruminant animals studied require a dietary source of methionine or its precursor, homocysteine. Animals metabolize methionine via cysteine to inorganic sulfate. Plants complete the cycle of sulfur by reduction of inorganic sulfate back to cysteine and methionine, and are thus the ultimate source of the methionine in most animal diets (Siegel, 1975). 

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. 

Plants, in common with microorganisms and animals, require methionine chiefly for three roles, (a) As a component of protein, a role which accounts for most of the methionine in the cell, (b) As a component of methionyl tRNA (in eukaryotes) and formylmethionyl tRNA (in chloroplasts, mitochondria, and prokaryotes), factors required for initiation of protein synthesis. (c) As a component of AdoMet, the chief biological methyl donor, the obligatory precursor of spermidine and spermine, and an effector of certain enzymes. In addition to these chief roles, a major pathway for the metabolism of methionine in certain plant tissues is its conversion to ethylene (see Yang and Adams, this series, Vol. 4, Chapter 6). Only plants and microorganisms can synthesize the homocysteine moiety of methionine novo, and the importance of this synthesis in the sulfur cycle has been noted in the introduction. 

Plants contain the key enzymes and substrates required for synthesis of homocysteine by both the pathways in Fig. 4 (see below). Which of the two pathways is of physiological significance in plants Unfortunately methionine auxotrophs, which have helped provide an answer to the analogous question in bacteria and fungi, are not available in green plants. An alternative approach was adopted, based upon determination of the patterns of assimilation of [ S]04 into key sulfur amino acids. Chlorella cells were... 

Synthesis by plants of homocysteine, the immediate precursor of methionine, is a key reaction in biology (Allaway, 1970). 

Plants were shown to contain the two enz3rmes required for synthesis of homocysteine by transsulfuration. The first enzyme (Giovanelli and Mudd, 1966) catalyzes the reaction ... 

During these early studies, an interesting finding was made in our laboratory with plants, and independently in other laboratories with microorganisms enzymic synthesis of homocysteine occurs by a direct reaction of sulfide with a homoserine ester (Giovanelli and Mudd, 1967) ... 

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