Branched-Chain Amino Acid Biosynthesis

J. Stetter (ed.), Herbicides Inhibiting Branched Chain Amino Acids Biosynthesis. Recent Development [Chemistry of Plant Protection, Vol. 10], Springer-Verlag, Berlin, 1994. 

Branched Chain Amino Acid Biosynthesis. The branched chain amino acids, leucine, isoleucine and valine, are produced by similar biosynthetic pathways (Figure 2.11). In one pathway, acetolactate is produced from pyruvate and in the other acetohydroxybutyrate is produced from threonine. Both reactions are catalysed by the same enzyme that is known as both acetolactate synthase (ALS) and acetohy-droxy acid synthase (AHAS). 

Figure 2.12 Compounds that inhibit branched chain amino acid biosynthesis...

Herbicide resistance is becoming a problem in many situations. The rush of compounds that interfere with branched chain amino acid biosynthesis has meant that a very high area of land has been treated with compounds that have the same mode of action. The application... 

Three other classes of compounds, although quite different from each other, are all inhibitors of acetohydroxy acid synthase (an enzyme required for branched-chain amino acid biosynthesis (see fig. 21.10). These three classes are sulfonylureas, imidazolinones, and triazolpyrimidines, which are the active ingredients in, respectively, Oust, Sceptor, and a third commercial herbicide still under development (fig. 21.11). 

They are absorbed through the roots and foliage and translocated to the growing or meristematic regions of the plant. Several members of this group have been shown to inhibit branched-chain amino acid biosynthesis. 

The mode of their action consists in the acetolactate synthase (ALS) inhibition and, as a result, in the inhibition of the branched-chain amino acid biosynthesis from acetolactate (90MI1, 92MI7, 98MI4). Results of QSAR studies were published (03MI3). Another approach was used to analyze a model of inhibition of photosystem II (96MI1). 

Branched chain amino acid synthesis has been extensively studied (, Figure 3). In branched chain amino acid biosynthesis ALS Is the second enzyme of Isoleuclne biosynthetic pathway and the first enzyme of valine synthesis. Commonly the first enzyme of a synthetic pathway is Inhibited by the end-product thus preventing waste of carbon skeletons and energy. Two ALS Isozymes exist In... 

Inhibitors of Acetolactate Synthase (ALS/AHAS) The enzyme acetolactate synthase (ALS) plays in plants an essential role in branched-chain amino acid biosynthesis. In the pathway leading to valine and leucine, ALS catalyzes the formation of 2-acetolactate from two pyruvate molecules, and in the pathway to isoleucine the formation of 2-acetohydroxybutyrate from 2-ketobutyrate and pyruvate. Due to this double function the enzyme is also called with a more general term aceto-hydroxyacid synthase. ALS is inhibited by several groups of herbicides, mainly the sulfonylureas (SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinylthiobenzoates(PTBs) and sulfonylaminocarbonyltriazolinone (SCTs) (see Chapter 2.1, M. E. Thompson). 

J. Stetter (volume editor) Herbicides inhibiting branched-chain amino acid biosynthesis. Springer, Berlin Heidelberg, Volume 10, 47-81,... 

S. K. Gee, J. V. Hay, Recent developments in the chemistry of sulfonylurea herbicides, in W. Ebing (editor-in-chief) Chemistry of Plant Protection, J. Stetter (volume editor) Herbicides inhibiting branched-chain amino acid biosynthesis. Springer, Berlin Heidelberg, Volume 10, 15—46, (1994). 

Kleschick WA, Triazolopyrimidine Sulfonanilides and Related Compounds. In Herbicides Inhibiting Branch Chain Amino Acid Biosynthesis, Stetter, J. Ed., Spinger-Verlag, Germany, 1994, Vol. 10, pp 119-143. 

Acetyl-CoA synthetase (ACS) catalyzes the conversion of acetate to acetyl-CoA. In photosynthetic tissue, this enzyme is localized in the chloroplast (l) where it potentially provides a key source of acetyl-CoA for fatty acid, isoprenoid, and branch-chain amino acid biosynthesis. Acetyl-CoA synthetase s contribution to chloroplast acetyl-CoA is presently controversial because of the identification of alternative sources of acetyl-CoA (i. e. pyruvate dehydrogenase complex (2,3), and carnitine acyltransferase (4)), in the chloroplast. To further elucidate the role of ACS in chloroplast acetyl-CoA metabolism we have partially purified and characterized ACS from mature spinach leaves. 

Imazaquin, an example of the imidazolinone class of herbicides, is particularly selective to soybeans. These compounds have a similar mode of action to that of the sulfonylurea herbicides, which will be discussed later. They block branched chain amino acid biosynthesis by inhibition of the enzyme acetolactate synthase (ALS). 

INHIBITORS OF BRANCHED-CHAIN AMINO ACID BIOSYNTHESIS... 

---