Mild purification procedure and subunit structure of glucosephosphate isomerase from baker's yeast

Molecular basis for the isozymes of bovine glucose-6-phosphate isomerase

Glucose-6-phosphate isomerase exists as multiple, catalytically active isozymes which can be resolved by polyacrylamide gel electrophoresis, isoelectric focusing, and ion-exchange chromatography. GPI from bovine heart was purified to homogeneity and each of the isozymes resolved. Four of the five isozymes were characterized with regard to their physical, chemical, and catalytic properties in order to establish their possible physiological significance and to ascertain their molecular basis. The isozymes exhibited identical native (118,000) and subunit (59,000) molecular weights but had different apparent pI values of 7.2, 7.0, 6.8, and 6.6. Kinetic constants, such as turnover number, Km and Ki values, were identical for all isozymes in either reaction direction. Structural analyses showed that the amino termini were blocked and the carboxyl terminal sequences were -Glu-Ala-Ser-Gly for all four isozymes. The most basic isozyme was more stable than the more acidic isozymes at pH extremes, at high ionic strength, in the presence of denaturants, or upon exposure to proteases. When the most basic isozyme was incubated in vitro under mild alkaline conditions, there was a spontaneous generation of the more acidic isozymes with electrophoretic properties identical to those found in vivo. The simultaneous release of ammonia along with the spontaneous shift to more acidic isozymes indicates deamidation as the molecular basis for the formation of the acidic isozymes both in vivo and in vitro. The change in the peptide fragmentation patterns following cleavage by hydroxylamine further suggests that deamidation of specific Asn-Gly bonds accounts for the structural basis of the isozymes.

Subunit hybridization and immunological studies of duplicated phosphoglucose isomerase isozymes

We examined structural and functional properties of the recently duplicated cytosolic isozymes of phosphoglucose isomerase (PGI) (EC 5.3.1.9) of the wild plant Clarkia xantiana and related species. A new purification protocol yielded PGIs with high specific activities. The duplicated PGI isozymes showed similar substrate affinities (apparent Km) in both catalytic directions. A newly devised competitive enzyme-linked immunosorbent assay (ELISA), using polyclonal antibodies, distinguished sodium dodecyl sulfate (SDS)-denatured PGIs coded by the duplicated loci but did not discriminate between allelic products of one of the loci. In vitro dissociation and reassociation experiments revealed that the duplicated subunits differed in their efficiency of reassociation. The difference was closely correlated with differences between the isozymes in their in vivo accumulation. In contrast, allelic subunits in species with or without the duplication were able to reassociate with similar efficiency. The duplicated PGI isozymes have diverged more in structural properties than kinetic ones. The total evidence suggests that mechanisms have evolved which reduce the potential consequences of the duplication.