Abstract
The Acetohydroxyacid synthase (EC 2.2.1.6) or Acetolactate synthase (ALS) belongs to a family of thiamine diphosphate (TPP) dependent enzymes which catalyzes the first reaction in the biosynthesis of essential amino acids - isoleucine, leucine and valine. Acetohydroxyacid synthase (AHAS) is present in plants, algae, fungi and bacteria and is found to be a vital target of multiple herbicides. We revealed the homology model of OsAHAS protein using the structure of Arabidopsis thaliana AHAS (PDB ID: 3E9Y) as template. The resulting model structure was refined by PROCHECK, ProSA, RMSD and Verify3D that indicated the model structure is reliable with 76% amino acid sequence identity with template. RMSD (1.75A), Verify3D (86.02%), Z-score (-9.55) and Ramachandran plot analysis showed that conformations for 81.6% of amino acid residues are within the most favoured regions. The phylogenetic tree constructed revealed different clusters based on AHAS in respect of bacteria, fungi, algae and plants. The multiple sequence alignment of these AHAS protein sequences from different organisms showed conserved regions at different stretches with homology in amino acid residues. Through motif analysis, it was revealed that conserved AHAS domain are found in all AHAS proteins suggesting its possible role in cellular and metabolic functions.
Highlights
The Acetohydroxyacid synthase (EC 2.2.1.6) or Acetolactate synthase (ALS), a plastid enzyme [1] which catalyzes the first reaction in the biosynthesis of branched-chain essential amino acids - isoleucine, leucine and valine [2,3,4] is the vital target of multiple herbicides
Homology modelling and structural analysis: The Oryza sativa Acetohydroxyacid synthase (AHAS) (OsAHAS) protein sequence consist of 644 amino acid residues
The query sequence from OsAHAS protein was selected for homology based searching of the template structure by the BLAST program against the structural database of PDB [30,31]
Summary
The Acetohydroxyacid synthase (EC 2.2.1.6) or Acetolactate synthase (ALS), a plastid enzyme [1] which catalyzes the first reaction in the biosynthesis of branched-chain essential amino acids - isoleucine, leucine and valine [2,3,4] is the vital target of multiple herbicides. Available herbicides that inhibit AHAS include sulfonylureas (SU), imidazolinones (IMI), triazolopyrimidines (TP), pyrimidinyl-thiobenzoates (PTB) [ known as pyrimidinylsalicylic acids or pyrimidinyloxybenzoic acids] and sulfonyl-aminocarbonyl-triazolinones (SCT) [7,8]. Out of these the sulfonylureas and imidazolinones are the most significant, with the sulfonylureas being the leading group on an active ingredient basis. The mammals lack the pathway for branched-chain amino acids biosynthesis and the ALS-inhibiting herbicides are thought to be non-toxic to them [10] They are highly selective to plants and have a broad range of weed control activity [11,12,13]. Understanding its structural details would be a great revolution for engineering new herbicides, developing resistant crops and antimicrobial drugs
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