N. Bakrim et al., Metabolite control of Sorghum C-4 phosphoenolpyruvate carboxylase catalytic activity and phosphorylation state, PHOTOSYN R, 58(2), 1998, pp. 153-162
Kinetic analyses were performed on the nonphosphorylated and in vitro phosp
horylated forms of recombinant Sorghum C-4 phosphoeno/pyruvate carboxylase
(C-4 PEPC). The native enzyme was purified by immunoaffinity chromatography
and its integrity demonstrated by Western blot analyses using anti N- and
C-terminus antibodies. At suboptimal pH (7.1 to 7.3) and PEP concentration
(2.5 mM), phosphorylation, positive metabolite effectors e.g., glucosc-6-ph
osphate, glycine and dihydroxyacetone-phosphate, or an increase in pH stron
gly activated the enzyme and lowered the inhibitory effect of L-malate. C-4
PEPC phosphorylation strengthened the effect of the positive effecters the
reby decreasing further the enzyme's sensitivity to this inhibitor. L-malat
e also decreased the phosphorylation rate of C-4 PEPC, a process antagonize
d by positive metabolite effecters. This was shown both in vitro, in a reco
nstituted phosphorylation assay containing the catalytic subunit of a cAMP-
dependent protein kinase or the Sorghum leaf PEPC-PK and in situ, during in
duction of C-4 PEPC phosphorylation in mesophyll cell protoplasts.