Evidence for an in vivo conformational change in ribulose bisphosphate carboxylase-oxygenase from Puma rye during cold adaptation.

Ribulose bisphosphate carboxylase-oxygenase (RUBPCase) from leaves of cold-hardened and unhardened Puma rye was purified by gel filtration and ion exchange chromatography. Chemical properties that might be associated with a previously demonstrated difference in molecular charge of purified RUBPCase from cold-hardened and unhardened Puma rye were investigated. Amino acid analyses indicated no significant differences in amino acid composition or average hydrophobicity per residue. Enzymes from hardened and unhardened rye were reversibly cold inactivated at 0 degrees C. However, the former was more stable at this temperature than the latter. Titration with 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) indicated the presence of fast and slow titrating sulfhydryl groups in both enzyme preparations but there were 50% fewer SH groups titrated in the enzyme from hardened rye in 30 min that in the enzyme from unhardened rye. Activation of both enzymes by HCO-3 enhanced the reactivity of sulfhydryl groups to titration with DTNB. According to the kinetics of the slow titrating SH groups, the enzyme from hardened rye was less susceptible to denaturation by sodium dodecyl sulfate than was the same enzyme from unhardened rye. It is concluded that the tertiary structure of RUBPCase from Puma rye is affected during low-temperature adaptation.