Acetyl grasses

Most pyruvate carboxylases of animal and of yeast are allosterically activated by acetyl-CoA, but those of bacteria are usually not. The enzyme from chicken liver has almost no activity in the absence of acetyl-CoA, which appears to increase greatly the rate of formation of carboxyphosphate and to slow the side reaction by which carboxyphosphate is hydrolyzed to bicarbonate and phosphate.70 The acetyl-CoA carboxylases of rat or chicken liver aggregate in the presence of citrate to form 8000-kDa rods. Citrate is an allosteric activator for this enzyme but it acts only on a phosphorylated form and the primary control mechanism.7172 This enzyme in plants is a target for a group of herbicides that are selectively toxic to grasses.73... 

Previously, we determined that the only known mode of action that is selective for grasses/monocots is acetyl CoA carboxylase inhibition.6 This enzyme is the first of two enzymes involved in de novo fatty acid biosynthesis. This mode of action prevents the synthesis of many essential wax compounds. In order to screen our compounds for this mode of action, we used resistant oat (Avena sp.) seeds. The emodin analogues caused dose-dependent bleaching (Fig. 1.7) and a severe decrease in germination for both resistant and nonresistant grasses. 

Figure 4.46 Xylan from grasses. The major hemicellulose of hardwoods lacks the arabinofuranosyl groups and the 4-O-methyl-a-glucuronosyl residues are attached only to 02 of xylose. The minor hemicellulose of softwoods lacks the acetyl groups, but again the 4-O-methyl-a-glucuronosyl residues are attached only to 02 of xylose.

Hemicelluloses are non-cellulosic low molecular weight polysaccharides that are found together with cellulose in plant tissues 1311. In the cell walls of land plants, xylan is the most common hemicellulosic polysaccharide, representing more than 30% of the dry weight 1321. Most xylans are heteropolysaccharides which are composed of 1,4-linked (3-D-xylopyranosyl residues 133, 134, 135). This backbone chain is substituted with acetyl, arabinosyl, and glucuronosyl residues 133. Homoxylans, on the other hand, consist of xylosyl residues exclusively and have been isolated from esparto grass 136, tobacco stalks 137, and guar seed husk 138. ... 

Acetyl-CoA carboxylase (ACC) catalyzes the first step in fatty acid biosynthesis. Owing to its role it has been exploited as an important herbicide target. Two chemical classes, the aryl-oxy-phenoxy-propionate (AOPP or fop) and the cyclo-hexanedione (CHD or dim) herbicides are widely used to control a broad selection of grass weeds in dicot crops and some of them even in cereals or in rice [1, 2]. Their frequent use has resulted in the development of resistance in several grass species [3],... 

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