Two Forms of Phosphoenolpyruvate Carboxylase in Chlorella kessleri

FPLC o f crude cell extracts o f Chlorella kessleri reveals tw o protein fractions w ith phos­ phoenolpyruvate carboxylase activity. Their m ole m asses, 955 kD a and 543 kD a, suggest that they are dimer and m onom er o f the same enzym e. Further data, how ever, indicate that they are m ore likely tw o different iso-form s o f phosphoenolpyruvate carboxylase: A ttem pts to in­ terconvert both proteins in vitro, such as through treatm ent with N aC l, m etabolites and thiolor histidine-group effecting reagents, were unsuccessful. The substrate affinity o f the large pro­ tein was slightly higher than that o f the small protein (A-̂ 1.13 and 0.76 m M , respectively); and the sensitivity to enhanced temperature was m ore pronounced in the large than in the sm aller protein (ha lf lifes = 23 min and 5 5 -6 0 m in, respectively). Som e properties o f both fractions, how ever, proved identical: 1. pH optim a at pH 8 .5 9 , 2. Hill coefficients approx. 1, 3. no significant regulatory effect o f glutam ine, glutam ate, aspartate and m alate, 4. increase in Km and in Hill coefficient by citrate, and 5. identical behaviour in ion exchange chrom atogra­ phy. Functions and localization o f the assum ed tw o iso-form s o f phosphoenolpyruvate car­ boxylase remain to be clarified. N o specific effects o f red or blue light during autotrophic growth on either protein with PEP C o activity could be found.