Enzyme Kinetics and Metabolic Regulation in a CAM Plant Under Salinity
You are studying a facultative CAM plant, Mesembryanthemum crystallinum, under salinity. Control plants (0 mM NaCl) show C3 photosynthesis with daytime stomatal opening. Salt-treated plants (300 mM NaCl) shift to CAM, with nighttime CO2 uptake and daytime stomatal closure. You measure the activity of phosphoenolpyruvate carboxylase (PEPC) in leaf extracts at midnight and noon. In control, PEPC activity is 2 µmol min-1 mg-1 protein at noon and 0.5 at midnight. In salt-treated, it is 1.5 at noon and 4.0 at midnight. You also measure malate concentration: control 5 mM at noon, 2 mM at midnight; salt-treated 15 mM at noon, 8 mM at midnight. The enzyme PEPC is known to be regulated by phosphorylation and by malate inhibition. You add 10 mM malate to the assay and find that midnight PEPC from salt-treated plants is inhibited by only 20%, whereas noon PEPC from control is inhibited by 70%.
1. Explain the biochemical basis for the shift in PEPC activity and malate sensitivity. How does this support CAM?
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In salt-treated plants, PEPC activity at midnight is high (4.0) and low at noon (1.5), opposite to the control. This indicates a shift to nighttime CO2 fixation, characteristic of CAM. The midnight PEPC is less sensitive to malate inhibition (20% vs 70%), which is crucial because malate accumulates during the night and would otherwise feedback-inhibit PEPC. The reduced sensitivity is likely due to phosphorylation of PEPC by PEPC kinase, which also reduces malate inhibition. In control plants, daytime PEPC is active and sensitive to malate, consistent with C3. The higher malate concentration in salt-treated plants (15 mM at noon) reflects storage in the vacuole, which is typical of CAM. The biochemical regulation allows PEPC to function at night despite malate accumulation, and the daytime decarboxylation of malate provides CO2 for photosynthesis behind closed stomata, conserving water under salinity.
2. Design an experiment to test whether the change in malate sensitivity is due to phosphorylation. Include controls and expected results.
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Treat leaf discs from salt-treated plants with a protein phosphatase inhibitor (e.g., okadaic acid) or a kinase inhibitor (e.g., K-252a) and measure PEPC activity and malate inhibition at midnight. Control: untreated discs. Also, perform in vitro dephosphorylation with alkaline phosphatase and test malate sensitivity. Expected: if phosphorylation causes the reduced malate sensitivity, then dephosphorylation should increase malate inhibition to ~70%, and kinase inhibition should reduce midnight PEPC activity and increase malate sensitivity. Include a western blot with anti-phosphoPEPC antibody to confirm phosphorylation status. Use three biological replicates and assay at saturating PEP and bicarbonate. The results would link the kinetic change to covalent modification. Additionally, measure PEPC kinase activity in vitro. This experiment would confirm that the shift is post-translational, not due to new enzyme synthesis.