Heat stability of the potato tuber ADP-glucose pyrophosphorylase: Role of cys residue 12 in the small subunit

Most of the ADP-glucose pyrophosphorylases from different sources are stabl e to a heat treatment. We found that in the potato (Solanum tuberosum L.) t uber enzyme, the intermolecular disulfide bridge located between Cys(12) of the small subunits is responsible for the stability at 60 degrees C. When this unique disulfide bond is cleaved the enzyme is stable up to 40 degrees C. Mutation of Cys(12) in the small subunit into either Ala or Ser yielded enzymes with stability similar to the reduced form of the wild type. Concu rrently, the enzyme with a truncated small subunit on the N-terminal was st able only up to 40 degrees C. Thus, the N-terminal is important for the sta bility of the enzyme because of the presence of a disulfide bond. (C) 1999 Academic Press.