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Possible advantages of coordinate repression in branched metabolic pathways
ARCHIVES
OF
BIOCHEMISTRY
AND
BIOPHYSICS
Letter Possible
Advantages in Branched
of Coordinate Metabolic
415-416 (1965)
109,
to the Editor
Repression
Pathways
The biosynthesis of lysine, threonine, and methionine from aspartate in Escherichia coli proceeds through the common intermediates, aspartyl phosphate and aspartate semialdehyde, but thereafter it diverges to each amino acid (see, for example, Ref. 1). In 1961, Stadtman and coworkers (1) described the occurrence in E. coli of two separate aspartate kinases (EC 2.7.2.4) which catalyzed the synthesis of the first intermediate. One of these enzymes was specifically and noncompetitively inhibited by L-lysine, and its formation was repressed by growth on n-lysine. The other was specifically and competitively inhibited by L-threonine. Cells were thus afforded the potential capacity to regulate the synthesis of either lysine or threonine through specific feedback control. As these workers pointed out, this situation was advantageous over one in which a single aspartate kinase was regulated by lysine or threonine, or both, because feedback control by one amino acid would not result in a marked decrease in synthesis of the other. The relevance of this view depended upon whether the aspartokinases fed a common pool of aspartyl phosphate. Indeed, recent evidence discussed by Stadtman (2) suggests that this is the case. The possibility that repression or feedback inhibition of the enzymes involved beyond the branch point might then direct the ultimate fate of asparbyl phosphate was also discussed by Stadtman and co-workers. We wish to propose that coordinate regulation (3) of enzymes beyond the branch point and of one of the multiple enzymes catalyzing a common step could be advantageous. A general model is used to develop the reasons. In the following model metabolites C and D regulate the synthesis of enzymes c and d, respectively, by a repressive mechanism:
It should be emphasized that, if enzymes c and cl feed a common pool of B, either end-metabolite could cont,rol the concentration of B. Thus, the
rates of synthesis of C and D would become interdependent if the concentration of B were diminished by one metabolite to a level which limited the rate of production of the other. The consequences of coordinate regulation, however, prove to be quite interesting. Let us assume that (a) [B] is in a steady state, i.e., d[B]/dt = 0; (b) d[B]/dt, = -d[Bl/dt~+~ (where d[B]/dt, is the rate of synthesis of B catalyzed by c and -d[B]/ dts+c is the rate of disappearance of B through the B ---) C segment); and (c) the reaction involving enzyme c and all reactions in the B + C segment occur in a first-order range wit.h respect to dependence on enzyme concentration. It is clear, then, that if c and all enzymes in the B + C segment were coordinately repressed by C, d[B]/dt would remain zero and the B -+ D limb would be unaffected, if it is postulated that changes in [A] would have no effect upon [B]. An organism endowed with this type of control would have an advantage over one with control exclusively two or more steps beyond the branch point in that “useless” intermediates might not be accumulated and paths from B to C and from B to D would be truly independent. Although a number of ramifications of the model are possible, one is of particular interest. This would obtain where d[B]/dt, >> d[B]/dtd or d[B]/d& << d[B]/dta in a system for which enzyme c and enzymes along B ---j C are coordinately repressible but B + D is only feedback inhibited. In these situations, [B] and the fate of B would depend upon the relative degrees of repression and feedback inhibition, but, in general, the paths B -+ C and B --) D should remain more independent in the former ease, i.e., where dIB]/dt, > d[B]/dtd. It is enlightening to consider the model in terms of the limited, available experimental data. In the case of threonine biosynthesis there is essentially no repression of the corresponding aspartate kinase by threonine (1). Hence, the direct effect of threonine upon the aspartyl phosphate pool would be exerted through feedback inhibition, and its magnitude would depend critically upon the threonine concentration. Lysine may be involved in the coordinate repression described. In this connection, coordinate repression by lysine of lysine-sensitive aspartate kinase and of aspartate semialdehyde dehydrogenase (EC 1.2.1.11), i.e., enzymes before the branch point,
415
416
LETTER
TO THE
has been reported (4). It would be of interest to know whether there are multiple dehydrogenases, although recent work renders this doubtful (4). In conclusion, it will be interesting to note whether the suggested coordinate regulation occurs in branched metabolic sequences. The occurrence of coordinate repression for at least portions of two major biosynthetic routes is well supported (3, 5). Branched metabolic sequences are common, but, unfortunately, there is a paucity of information about their regulation and the existence of multiple catalysts of common steps. ACKNOWLEDGMENTS We wish to thank Dr. R. J. Foster for his suggestions. This work was supported in part by research grant GM-09039 and by a Research Career Development Award (l-K3-AI-5, 268) from the National Institutes of Health, U. S. Public Health Service.
EDITORS REFERENCES
1. STADTMAN, E. R., COHEN, G. N., LEBRAS, G.. AND DE ROBICHON-SZIJL~UJSTER, H., J. Viol. Chem. 236,2033 (1961). 2. STADTM.IN, E. R., Bacleriol. Xec. 27, 170 (1963). 3. AMES, B. N., AND GARRY, B., Proc. Satl. Acad. sci. u. s. 46, 1453 (1959). 4. COHEN, G. N., PATTE, J., AND BOEZI, J., Compt. Rend. 266, 2939 (1963). 5. BECKWJTH, J. R., PBRDEE, A. B., AUSTRIAN, R., AND JACOB, F., J. Mol. Biol. 6, 618 (1962). B. A. MCFADDEN W. I-. HOWES’ Department of Chemistry Washington State University Pullman, Washington Received December 11, 1964 i Present address: Department of Biology, Illinois Institute of Technology, Chicago, Illinois.
OF
BIOCHEMISTRY
AND
BIOPHYSICS
Letter Possible
Advantages in Branched
of Coordinate Metabolic
415-416 (1965)
109,
to the Editor
Repression
Pathways
The biosynthesis of lysine, threonine, and methionine from aspartate in Escherichia coli proceeds through the common intermediates, aspartyl phosphate and aspartate semialdehyde, but thereafter it diverges to each amino acid (see, for example, Ref. 1). In 1961, Stadtman and coworkers (1) described the occurrence in E. coli of two separate aspartate kinases (EC 2.7.2.4) which catalyzed the synthesis of the first intermediate. One of these enzymes was specifically and noncompetitively inhibited by L-lysine, and its formation was repressed by growth on n-lysine. The other was specifically and competitively inhibited by L-threonine. Cells were thus afforded the potential capacity to regulate the synthesis of either lysine or threonine through specific feedback control. As these workers pointed out, this situation was advantageous over one in which a single aspartate kinase was regulated by lysine or threonine, or both, because feedback control by one amino acid would not result in a marked decrease in synthesis of the other. The relevance of this view depended upon whether the aspartokinases fed a common pool of aspartyl phosphate. Indeed, recent evidence discussed by Stadtman (2) suggests that this is the case. The possibility that repression or feedback inhibition of the enzymes involved beyond the branch point might then direct the ultimate fate of asparbyl phosphate was also discussed by Stadtman and co-workers. We wish to propose that coordinate regulation (3) of enzymes beyond the branch point and of one of the multiple enzymes catalyzing a common step could be advantageous. A general model is used to develop the reasons. In the following model metabolites C and D regulate the synthesis of enzymes c and d, respectively, by a repressive mechanism:
It should be emphasized that, if enzymes c and cl feed a common pool of B, either end-metabolite could cont,rol the concentration of B. Thus, the
rates of synthesis of C and D would become interdependent if the concentration of B were diminished by one metabolite to a level which limited the rate of production of the other. The consequences of coordinate regulation, however, prove to be quite interesting. Let us assume that (a) [B] is in a steady state, i.e., d[B]/dt = 0; (b) d[B]/dt, = -d[Bl/dt~+~ (where d[B]/dt, is the rate of synthesis of B catalyzed by c and -d[B]/ dts+c is the rate of disappearance of B through the B ---) C segment); and (c) the reaction involving enzyme c and all reactions in the B + C segment occur in a first-order range wit.h respect to dependence on enzyme concentration. It is clear, then, that if c and all enzymes in the B + C segment were coordinately repressed by C, d[B]/dt would remain zero and the B -+ D limb would be unaffected, if it is postulated that changes in [A] would have no effect upon [B]. An organism endowed with this type of control would have an advantage over one with control exclusively two or more steps beyond the branch point in that “useless” intermediates might not be accumulated and paths from B to C and from B to D would be truly independent. Although a number of ramifications of the model are possible, one is of particular interest. This would obtain where d[B]/dt, >> d[B]/dtd or d[B]/d& << d[B]/dta in a system for which enzyme c and enzymes along B ---j C are coordinately repressible but B + D is only feedback inhibited. In these situations, [B] and the fate of B would depend upon the relative degrees of repression and feedback inhibition, but, in general, the paths B -+ C and B --) D should remain more independent in the former ease, i.e., where dIB]/dt, > d[B]/dtd. It is enlightening to consider the model in terms of the limited, available experimental data. In the case of threonine biosynthesis there is essentially no repression of the corresponding aspartate kinase by threonine (1). Hence, the direct effect of threonine upon the aspartyl phosphate pool would be exerted through feedback inhibition, and its magnitude would depend critically upon the threonine concentration. Lysine may be involved in the coordinate repression described. In this connection, coordinate repression by lysine of lysine-sensitive aspartate kinase and of aspartate semialdehyde dehydrogenase (EC 1.2.1.11), i.e., enzymes before the branch point,
415
416
LETTER
TO THE
has been reported (4). It would be of interest to know whether there are multiple dehydrogenases, although recent work renders this doubtful (4). In conclusion, it will be interesting to note whether the suggested coordinate regulation occurs in branched metabolic sequences. The occurrence of coordinate repression for at least portions of two major biosynthetic routes is well supported (3, 5). Branched metabolic sequences are common, but, unfortunately, there is a paucity of information about their regulation and the existence of multiple catalysts of common steps. ACKNOWLEDGMENTS We wish to thank Dr. R. J. Foster for his suggestions. This work was supported in part by research grant GM-09039 and by a Research Career Development Award (l-K3-AI-5, 268) from the National Institutes of Health, U. S. Public Health Service.
EDITORS REFERENCES
1. STADTMAN, E. R., COHEN, G. N., LEBRAS, G.. AND DE ROBICHON-SZIJL~UJSTER, H., J. Viol. Chem. 236,2033 (1961). 2. STADTM.IN, E. R., Bacleriol. Xec. 27, 170 (1963). 3. AMES, B. N., AND GARRY, B., Proc. Satl. Acad. sci. u. s. 46, 1453 (1959). 4. COHEN, G. N., PATTE, J., AND BOEZI, J., Compt. Rend. 266, 2939 (1963). 5. BECKWJTH, J. R., PBRDEE, A. B., AUSTRIAN, R., AND JACOB, F., J. Mol. Biol. 6, 618 (1962). B. A. MCFADDEN W. I-. HOWES’ Department of Chemistry Washington State University Pullman, Washington Received December 11, 1964 i Present address: Department of Biology, Illinois Institute of Technology, Chicago, Illinois.