Properties to two high-molecular-mass forms of glyceraldehyde-3-phosphate dehydrogenase from spinach leaf, one of which also possesses latent phosphoribulokinase activity

Authors


Correspondence to R. Powls, Department of Biochemistry, University of Liverpool. P. O. Box 147, Liverpool, England L69 3BX

Abstract

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.

Abbreviations
G3PDH

glyceraldehyde-3-phosphate dehydrogenase

PRK

phosphoribulokinase

Enzymes
 

Glyceraldehyde-3-phosphate dehyddrogenase (NADP) (EC. 1.2.1.13)

 

glyceraldehyde-3-phosphate dehydrogenase (NAD) (EC 1.2.1.12)

 

phosphoribulokinase (EC 2.7.1.19)

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