Properties of phosphoenol pyruvate carboxylase in Sedum species in relation to Crassulacean Acid Metabolism (CAM)

Abstract

Parameters of Crassulacean Acid Metabolism (carbon isotope composition [δ13C values], diurnal acid cycles) were studied together with properties of phosphoenolpyruvate carboxylases (PEP‐C) in eight species of the genus Sedum. The δ13C values indicate a considerable variability of CAM capacity among the species investigated. The spectra of organic acids were similar in all species. Malate, citrate, and isocitrate were the main components of the acid fraction. Quinic acid occurred only during the light period. Molecular weights of PEP‐C were in the range of 160,000 and showed no correlation to CAM as indicated by the δ13C values. However, there were differences in the kinetic data of PEP‐C. Sedum species with less negative δ13C (i.e. with high CAM capacity) have PEP‐C with higher Vmax, higher Km (PEP) and higher sensitivity for malate inhibition.

Changes in properties of phosphoenolpyruvate carboxylase from the CAM plant Sedum praealtum D.C. upon dark/light transition and their stabilization by glycerol

“…Thus, the lowering of the cytoplasmic pH, which is assumed to happen with the onset of light and the consequent exit of malate from the vacuole, is more suppressive for the activity of the day-form, than it would have been for the unaltered night-form of the enzyme. An extension of our observations to other CAM species appears tO be necessary, however, before a generalization can be drawn, since the pH profiles of PEPC of CAM plants seem to be quite variable [5,8,11,13]. Our data, obtained with desalted extracts, are in agreement with earlier results [21,24] based on crude extracts; in these cases interference from the coextracted malate could not be avoided and so the validity of the results was rather questionable.…”

“…The data obtained for the less active day-form are quite interesting and novel, because the information available so far for PEPC from CAM plants [8,10,11,13,20] is appparently related to the stable activated state of the enzyme. The differences in kinetic behaviour, pH profiles and sensitivity to malate are not only pronounced but physiologically meaningful as well.…”

“…Kinetic and regulatory properties of PEPC in CAM plants have been studied mainly in partially purified preparations [t 1,13] ; in one case the enzyme was purified to homogeneity [8] and kinetic parameters for substrate and malate were studied [18]. Apparently, all the above studies refer to the active state of the enzyme, which might differ substantially from the alleged in vivo state in the light.…”

Comparative studies on immunological and molecular properties of phosphoenolpyruvate carboxylase in species of Sedum and Kalanchoë performing crassulacean acid metabolism (CAM)

“…A nutnber of different diurnal and seasonal CAM pattetns can be observed, A detailed atialysis of the tnechanistns involved in the control of CAM appears to be a pretequisite for explaining this CAM variability. The genus Sedutn offets a promising field for such apptoaches, since it includes both obligate and tacultative CAM plants (Kluge & Titig, 1978;Osmond, 1978), as well as species in which CAM is particularly susceptible to tnodulation by external factots (Kluge, 1977;Knopf & Kluge, 1979;Schuber & Kluge, unpublished),…”

“…Sitice the key enzytne of CAM, phosphoenolpyruvate earboxylase (PEP-c), is considered to be of particular itnportanee in the control of CAM (Ostnond, 1978;Dittrich, 1979), kinetic and tnolecular properties of this enzytne have been studied with respect to various CAM patterns (Knopf & Kluge, 1979). The results of these investigatiotis suggest that the PEP-c of those Sedum species perfortning obligate CAM is distitiguishable frotn others by its higher K,,,,,^, lower affinity for PEP, and lower sensitivity towards inhibition by malate.…”