Two High-Mr forms of chloroplast glyceraldehyde-3-phosphate dehydrogenase from spinach leaf can be separated by DEAE-cellulose chromatography. One form, the high-Mr glyceraldehyde-3-phosphate dehydrogenase, resembles an enzyme previously described [Yonuschot, G. R., Ortwerth, B. J. & Koeppe, O. J. (1970) J. Biol. Chem. 245 4193–4198]. The other, a glyceraldehyde-3-phosphate dehydrogenase/phosphoribulokinase complex, is characterised by possession of latent phosphoribulokinase activity, only expressed following incubation with dithiothreitol.
This complex is composed not only of subunits A (39.5 kDa) and B (41.5 kDa) characteristic of the high-Mr glyceraldehyde-3-phosphate dehydrogenase, but also of a thirt subunit, R (40.5. kDa) comigrating with that from the active phosphoribulokinase of spinach.
Incubation of the complex with dithiothreitol markdely stimulated both its phosphoribulokinase and NADPH-dependent dehydrogenase activities. This dithiothreitol-induced activation was accompained by depolymerisation to give two predominantly NADPH-linked tetrameric glyceraldehyde-3-phosphate dehydrogenases (the homotetramer, A4, and the heterotetramer, A2B2) as well as the active dimeric phosphoribulokinase. Incubation of the high-Mr glyceraldehyde-3-phosphate dehydrogenase with dithiothreitol promoted complete depolymerisation yielding only the heterotetramer (A2B2).
Possible structures suggested for the glyceraldehyde-3-phosphate dehydrogenase/phosphoribulokinase complex are (A2B2)2A4R2 or (A2B2)(A4)2R2.
Glyceraldehyde-3-phosphate dehyddrogenase (NADP) (EC. 126.96.36.199)
glyceraldehyde-3-phosphate dehydrogenase (NAD) (EC 188.8.131.52)
phosphoribulokinase (EC 184.108.40.206)