Thierry Lonhienne, Mario D. Garcia, Gregory Pierens, Mehdi Mobli, Amanda Nouwens and Luke W. Guddat

PNAS 2018 February 27, 115 (9) E1945-E1954

Significance

Herbicide-resistant weeds are a major threat to the world’s food security and result in the loss of billions of dollars of income to crop producers. Penoxsulam, a member of the triazolopyrimidine family of commercial herbicides, has become a center of focus due to an increase in the number of weeds that have developed resistance to this compound. Thus, understanding its mode of action will assist in managing this problem. Here, our crystallographic data capture “in action” the molecular mechanisms that underpin how this herbicide operates. As well as having an effective binding affinity for acetohydroxyacid synthase, it is able to induce and enhance the production of peracetate, a highly reactive oxidant that induces the accumulative inhibition of its target.

Abstract

Acetohydroxyacid synthase (AHAS), the first enzyme in the branched amino acid biosynthesis pathway, is present only in plants and microorganisms, and it is the target of >50 commercial herbicides. Penoxsulam (PS), which is a highly effective broad-spectrum AHAS-inhibiting herbicide, is used extensively to control weed growth in rice crops. However, the molecular basis for its inhibition of AHAS is poorly understood. This is despite the availability of structural data for all other classes of AHAS-inhibiting herbicides. Here, crystallographic data for Saccharomyces cerevisiae AHAS (2.3 Å) and Arabidopsis thaliana AHAS (2.5 Å) in complex with PS reveal the extraordinary molecular mechanisms that underpin its inhibitory activity. The structures show that inhibition of AHAS by PS triggers expulsion of two molecules of oxygen bound in the active site, releasing them as substrates for an oxygenase side reaction of the enzyme. The structures also show that PS either stabilizes the thiamin diphosphate (ThDP)-peracetate adduct, a product of this oxygenase reaction, or traps within the active site an intact molecule of peracetate in the presence of a degraded form of ThDP: thiamine aminoethenethiol diphosphate. Kinetic analysis shows that PS inhibits AHAS by a combination of events involving FAD oxidation and chemical alteration of ThDP. With the emergence of increasing levels of resistance toward front-line herbicides and the need to optimize the use of arable land, these data suggest strategies for next generation herbicide design.

See: http://www.pnas.org/content/115/9/E1945

Figure 2: Stereo images of the CC and herbicide binding regions of ScAHAS:PS. (A) The fold of a subunit of the ScAHAS:PS complex in the herbicide binding region. In this subunit, the ThDP-peracetate adduct is present. The polypeptide is in green, the FAD is in yellow, and ThDP-peracetate has light brown carbon atoms. PS has magenta carbon atoms and is in ball and stick representation. (B) A superposition of the subunit in A with the CC_FAD_bent subunit of ScAHAS:pyr. The two pyruvate molecules (pyr1 and pyr2) and ThDP have thin brown bonds that involve carbon atoms. In both images, the Connolly surface is overlaid and color-coded according to electrostatic charge.