StALS is an acetohydroxyacid synthase (AHAS), also known as acetolactate synthase (ALS), produced by Solanum tuberosum. It is a chloroplast-localized enzyme of 659 amino acids (~72 kDa) that catalyzes the first step in the biosynthesis of branched-chain amino acids (valine, leucine, and isoleucine). It requires cofactors thiamine diphosphate, Mg²⁺ and flavin adenine dinucleotide, and condenses either two pyruvate molecules or pyruvate with 2-oxobutanoate, releasing CO₂ to form 2-acetolactate or 2-aceto-2-hydroxybutyrate, respectively. The overall reactions are: 1) 2 pyruvate + H⁺ → 2-acetolactate + CO₂ (valine, leucine biosynthesis) 2) pyruvate + 2-oxobutanoate + H⁺ → 2-aceto-2-hydroxybutyrate + CO₂ (isoleucine biosynthesis) The StmALS protein in BG25 is a modified version of acetolactate synthase from S. tuberosum. The protein consist of 659 amino acids and its predicted size is ~65 kDa. StmALS is 99.7% identical to native StALS present in S. tuberosum, differing only by two amino acids (W563L and S642I).