THE ROLE OF PEA CHLOROPLAST ALPHA-GLUCOSIDASE IN TRANSITORY STARCH DEGRADATION

Pea chloroplastic alpha-glucosidase (EC 3.2.1.20) involved in transito ry starch degradation was purified to apparent homogeneity by ion exch ange, reactive dye, hydroxylapatite, hydrophobic interaction, and gel filtration column chromatography. The native molecular mass and the su bunit molecular mass were about 49.1 and 24.4 kD, respectively, sugges ting that the enzyme is a homodimer. The enzyme had a K-m of 7.18 mM f or maltose. The enzyme's maximal activity at pH 7.0 and stability at p H 6.5 are compatible with the diurnal oscillations of the chloroplasti c stromal pH and transitory starch accumulation. This pH modulation of the alpha-glucosidase's activity and stability is the only mechanism known to regulate starch degradative enzymes in leaves. Although the e nzyme was specific for the alpha-D-glucose in the nonreducing end as t he glycon, the aglycon moieties could be composed of a variety of grou ps. However, the hydrolysis rate was greatly influenced by the aglycon residues. Also, the enzyme could hydrolyze glucans in which carbon 1 of the glycon was linked to different carbon positions of the penultim ate glucose residue. The ability of the alpha-glucosidase to hydrolyze alpha-1,2- and alpha-1,3-glucosidic bonds may be vital if these bonds exist in starch granules because they would be barriers to other star ch degradative enzymes. This purified pea chloroplastic alpha-glucosid ase was demonstrated to initiate attacks on native transitory chloropl astic starch granules.