ABSTRACT
Acetohydroxyacid synthase (AHAS) (EC 2.2.1.6) (also known as acetolactate synthase) is the first common enzyme in the branched chain amino acid (BCAA) biosynthesis pathway. This pathway is present in microorganisms and in plants but not in animals, making it an attractive target for both drug and herbicide discovery. The function of AHAS is to catalyze the conversion of two molecules of pyruvate to 2-acetolactate or to convert one molecule of pyruvate and a molecule of 2-ketobutyrate into 2-aceto-2-hydroxybutyrate. Three cofactors are required for the activity of AHAS: thiamine diphosphate (ThDP), Mg²âº and flavin-adenine dinucleotide (FAD). AHAS is the target for several classes of commercial herbicides that include the sulfonylurea and imidazolinone families. These herbicides are potent and selective inhibitors of AHAS with Ki values that can be in the low nM range. Such compounds also exhibit low application rates as herbicides (typically ~3 g ha⻹) and have low mammalian toxicity (LD50 values typically >4g/kg), thereby highlighting their utility and effectiveness as biocidal agents. However, somewhat surprisingly given the central importance of AHAS in the metabolism of microorganisms, no inhibitors of this enzyme have been commercialized into antimicrobial agents. Here we provide an overview of the biochemical characterization of AHASs from bacterial and fungal sources, analyse the structural features of these enzymes that are criticial to catalysis andprovide the current data on AHAS inhibitors that have potential to be developed into antimicrobial therapeutics.
Subject(s)
Acetolactate Synthase/antagonists & inhibitors , Anti-Bacterial Agents/pharmacology , Antifungal Agents/pharmacology , Bacterial Proteins/antagonists & inhibitors , Drug Discovery , Enzyme Inhibitors/pharmacology , Fungal Proteins/antagonists & inhibitors , Acetolactate Synthase/chemistry , Acetolactate Synthase/genetics , Acetolactate Synthase/metabolism , Animals , Anti-Bacterial Agents/adverse effects , Anti-Bacterial Agents/chemistry , Antifungal Agents/adverse effects , Antifungal Agents/chemistry , Bacterial Proteins/chemistry , Bacterial Proteins/genetics , Bacterial Proteins/metabolism , Biocatalysis , Disinfectants/chemistry , Disinfectants/pharmacology , Disinfectants/toxicity , Drug Resistance, Multiple, Bacterial , Drug Resistance, Multiple, Fungal , Enzyme Inhibitors/adverse effects , Enzyme Inhibitors/chemistry , Fungal Proteins/chemistry , Fungal Proteins/genetics , Fungal Proteins/metabolism , Herbicides/chemistry , Herbicides/pharmacology , Herbicides/toxicity , Humans , Models, Molecular , Molecular Conformation , Mutant Proteins/antagonists & inhibitors , Mutant Proteins/chemistry , Mutant Proteins/metabolism , Plant Proteins/antagonists & inhibitors , Plant Proteins/chemistry , Plant Proteins/genetics , Plant Proteins/metabolismABSTRACT
The sulfonylurea herbicides exert their activity by inhibiting plant acetohydroxyacid synthase (AHAS), the first enzyme in the branched-chain amino acid biosynthesis pathway. It has previously been shown that if the gene for AHAS is deleted in Candida albicans , attenuation of virulence is achieved, suggesting AHAS as an antifungal drug target. Herein, we have cloned, expressed, and purified C. albicans AHAS and shown that several sulfonylureas are inhibitors of this enzyme and possess antifungal activity. The most potent of these compounds is ethyl 2-(N-((4-iodo-6-methoxypyrimidin-2-yl)carbamoyl)sulfamoyl)benzoate (10c), which has a K(i) value of 3.8 nM for C. albicans AHAS and an MIC90 of 0.7 µg/mL for this fungus in cell-based assays. For the sulfonylureas tested there was a strong correlation between inhibitory activity toward C. albicans AHAS and fungicidal activity, supporting the hypothesis that AHAS is the target for their inhibitory activity within the cell.
