Acetyl-coenzyme A carboxylase: Quaternary structure and inhibition by graminicidal herbicides

Aryloxyphenoxypropionates (AOPPs) and cyclohexanediones (CHDs) are two classes of postemergence herbicides that are used for the control of annual and perennial grasses in broadleaf crops. The herbicidal properties of these distinct chemistries were discovered in the early 1970s, and since that time there has been considerable effort spent to elucidate the mode of action and their respective target sites. Both cyclohexanediones and aryloxyphenoxypropionates show similar species specificity profiles and symptoms in susceptible plants. Based on these similarities, it has been postulated that both classes of herbicides target the same system. It has been proposed (1, 2) that this target site is the biotin-containing enzyme acetyl-coenzyme A carboxylase (E.C. 6.4.1.2). Acetyl-coenzyme A carboxylase (ACCase) is the first dedicated enzyme in the de novo fatty acid biosynthetic pathway. ACCase catalyzes the ATP-dependent conversion of acetyl-coenzyme A to malonyl-coenzyme A. First discovered in 1959, ACCase has since been investigated in a variety of mammalian, prokaryotic, and plant species. In all species studied, ACCase plays the same role in fatty acid biosynthesis, despite vast differences in its regulation and protein structure. The implication that ACCase is the target site for the AOPP and CHD classes of herbicides has been supported by the work of many researchers; however, there are discrepancies in the literature with respect to the quaternary structure of ACCase and its role as the only site of action for these herbicides. Similarly, it is commonly believed that resistance to these herbicides is due to an altered form of this enzyme. While it is true that the evidence supporting these hypotheses are convincing, alternative modes of action and mechanisms of resistance have been proposed and cannot be discredited without proper examination. The purpose of this paper is to review the literature on (i) the ACCase quaternary structure in plants compared to prokaryotic and mammalian enzymes, (ii) the regulation of mammalian, prokaryotic, and plant ACCase, (iii) the modes of action of CHDs and AOPPs and their target sites, and (iv) the mechanisms by which plants are rendered susceptible to these classes of graminaceous herbicides. (C) 1997 Academic Press