Acetohydroxyacid synthase from Claviceps purpurea : Partial purification and characterization

An acetohydroxyacid synthase (EC 4.1.3.18) which synthesizes α‐acetolactate from pyruvate has been isolated from two different Claviceps purpurea strains. A purification of about 142‐fold was achieved by ammonium sulfate fractionation and the use of Sepharose 6B and DEAE‐Sepharose CL‐6B columns. The purified enzyme requires thiamine pyrophosphate and a divalent metal ion (Mn 2 + or Mg 2 +) for maximum activity but no FAD. The optimum pH is about 6.0 and the optimum temperature is 40 °C. The enzyme is not inhibited by branched‐chain amino acids neither singly nor in combination. AHAS is strongly inhibited by p ‐chloromercuribenzoate and N‐ethylmaleimide. The apparent K m values for pyruvate and TPP are 1.7 × 10 −2 M and 1.2 × 10 −6 M , respectively.