Abstract
A core‐glycosylated form of the dimeric enzyme invertase has been isolated from secretion mutants of Saccharomyces cerevisiae blocked in transport to the Golgi apparatus. This glycosylation variant corresponds to the form that folds and associates during biosynthesis of the protein in vivo. In the present work, its largely homogeneous subunit size and well‐defined quaternary structure were utilized to characterize the folding and association pathway of this highly glycosylated protein in comparison with the nonglycosylated cytoplasmic and the high‐mannose‐glycosylated periplasmic forms of the same enzyme encoded by the suc2 gene.
Renaturation of core‐glycosylated invertase upon dilution from guanidinium‐chloride solutions follows a unibimolecular reaction scheme with consecutive first‐order subunit folding and second‐order association reactions. The rate constant of the rate‐limiting step of subunit folding, as detected by fluorescence increase, is k1 = 1.6 + 0.4 × 10−3 s−1 at 20 °C; it is characterized by an activation enthalpy of ΔH = 65 kJ/mol. The reaction is not catalyzed by peptidyl‐prolyl cis‐trans isomerase of the cyclophilin type. Reactivation of the enzyme depends on protein concentration and coincides with subunit association, as monitored by size‐exclusion high‐pressure liquid chromatography. The association rate constant, estimated by numerical simulation of reactivation kinetics, increases from 5 × 103 M−1 s−1 to 7 × 104 M−1 s−1 between 5 and 30 °C.
Although reactivation kinetics and yields of the core‐glycosylated and high‐mannose‐glycosylated invertase are essentially identical between 5 and 40 °C, the nonglycosylated cytoplasmic form displays strongly reduced reactivation yields at the high end and significantly reduced reactivation rates at the low end of this temperature range.