Serine acetyltransferase

A control protein, CAT, is used to show data reliability. [Reprinted, with permission, from V. J. Hindson, P. C. E. Moody, A. J. Rowe, and W. V. Shaw, The Journal of Biological Chemistry 275, No. 1, 2000, 461-466. Serine Acetyltransferase from Escherichia coli Is a Dimer of Trimers . Copyright 2000 by The American Society for Biochemistry and Molecular Biology, Inc.]... 

Equation (1) is catalyzed by serine acetyltransferase (E.C. 2.3.1.30) and Eq. (2) by cysteine synthase (E.C. 4.2.99.8). Evidence that these reactions represent the major pathway for cysteine biosynthesis is as follows (a) Serine acetyltransferase has been demonstrated in a number of plants (see Section II,B,1), and OAS is a natural constituent of cultured tobacco cells, being present at a concentration of at least 120 nmoles/g fresh weight (Smith, 1977). (b) Cysteine synthase has been demonstrated in a wide variety of plants. All such enzyme preparations show activities with OAS far in excess of those with serine (Section II,B,2). (c) The physiological role of the two enzymes is well established in microorganisms (Siegel, 1975), lending credence to their role in plants. 

This enzyme, catalyzing Eq. (1), has been demonstrated in a number of plants (Smith and Thompson, 1%9 Ngo and Shargool, 1974 Ascano and Nicholas, 1977). The 54-fold purified enzyme from kidney bean seedlings (Smith and Thompson, 1971) had values of 6 x lO IW for serine and 2 x Qr M for acetyl-CoA. The enzyme does not catalyze the acetylation of homoserine or threonine. Activities with other acyl-CoA derivatives were not tested. During purification, this enzyme was separated fix>m cysteine synthase, in contrast to the serine acetyltransferase from SalmoneUa typhimurium which has been isolated predominantly complexed with cysteine synthase (Kredich et al., 1969). 

Properties were determined for the free enzyme, which is believed to be identical with that present as a complex with serine acetyltransferase. 

Two separate pathways converge in the reaction catalyzed by cysteine synthase the reductive assimilation of sulfate to sulfide, and the synthesis of OAS. All the reactions of reductive sulfate assimilation are present in chloroplasts, but the quantitative significance of these organelles in providing the sulfur precursor for cysteine synthesis is not clear (see Anderson, this volume. Chapter 5). A recent review (Givan and Harwood, 1976) indicates that serine is formed in chloroplasts fairly directly from intermediates of the carbon reduction pathway, but that this synthesis also requires extrachloro-plastic factors yet to be defined. Serine acetyltransferase has been reported in a fraction consisting mainly of mitochondria (Smith and Thompson, 1%9 ... 

Smith, 1972) and in uncharacterized particles obtained by centrifuging a 10,000 X g supernatant firaction at 144,000 x g for 1 h (Ascaho and Nicholas, 1977). In none of these studies were intact chloroplasts assayed for serine acetyltransferase. Acetyl-CoA, the substrate for the enzyme, may be generated in both chloroplasts and mitochondria (Givan and Harwood, 1976). 

In summary, synthesis of serine from CO requires the cooperation of chloroplasts and extrachloroplastic components. It is not known whether serine acetyltransferase is chloroplastic, but at least some of this enzyme activity is present in particles that are probably mitochondria. Acetyl-CoA may be generated in both chloroplasts and mitochondria. Reductive assimilation of sulfate to sulfide, and the sulfhydration of OAS occur in chloroplasts, but the quantitative significance of these reactions in chloroplasts remains to be assessed. 

Apparent Lack of Feedback Control of Serine Acetyltransferase and... 

In bacteria, cysteine regulates its own synthesis by feedback inhibition of serine acetyltransferase and by repression of the synthesis of cysteine synthase (Umbarger, 1978). In plants, neither ofthese effects have been observed. To attain a 44% inhibition of serine acetyltransferase from kidney bean seedlings, 0.5 mM L-cysteine was required (Smith and Thompson, 1971), a concentration 500 times greater than that required to inhibit the bacterial enzyme by 85% (Kredich and Tomkins, 1966). The activities of serine acetyltransferase in the roots of kidney beans growing either under conditions of sulfur deficiency or with adequate sulfate nutrition were not significantly... 

Francois, J.A. et al. (2006) Structural basis for interaction of O-acetylserine sulfhydrylase and serine acetyltransferase in ibc Arabidopsis cysteine synthase complex. Plant Cell 18,3647 3655... 

Kumaran, S. and Jez, J.M. (2007) Thermodynamics of the interaction between O-acetylserine sulfhydrylase and the C-terminus of serine acetyltransferase. Biochemistry (Mosc). 46, 5586-5594... 

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