CAM Causes Problems in Biochemical Regulation
Various instructive reaction schemes were developed to show the biochemistry of CAM (see Liittge et al. 1994; Heldt 1997) and therefore need not be presented here.
The understanding of CAM as PEPC-mediated dark fixation of carbon dioxide, more exactly of HCO3, the transient storage in the form of malic acid in the vacuole and the release by the malic enzyme for the final assimilation by Rubisco, assumes a complicated network of regulatory characteristics and processes.
Two of these metabolic processes will be discussed in more detail, namely the storage of malic acid in the vacuole and the competition for C02 between PEPC and Rubisco.
Storage of Malic Acid in Vacuoles The tonoplast is the membrane that solves the problem of regulating transitory malic acid storage in the vacuole: During dark fixation of carbon dioxide, malic acid must be imported against the concentration gradient into the vacuole (and be retained there), but, during daytime, flow (probably even controlled) in the opposite direction must be enabled. The import of malic acid is well understood, but export still poses certain unsolved questions. A pH value of 7 (to perhaps 8) is assumed in the cytosol; at those values malic acid is dissociated as the pH value of both carboxyl groups is in the acidic range (CI: at pH 3.1; C4: at pH 5.1). Import of divalent malate ions into the vacuole would soon come to an end and can only be maintained if the negative charges are balanced by positive charges. This is achieved through the proton-ATPase of the tonoplast (V-ATPase), which is supplied with energy from glycolysis together with a pyrophosphate-dependent proton pump. Uptake of the divalent malate ion into the vacuole takes place via dicarboxylate channels which transport malate as well as fumarate. Because of the acidification of the vacuole (to pH 3.5-3.0) malate becomes increasingly protonated and finally only malic acid is present. This is also important for osmotic reasons, because three osmotically active solutes (2xH+ and malate) unify. Little is known about the efflux of malic acid from the vacuole. The undissociated acid could permeate the tonoplast via the "lipid path" (see Sect. 1.5.1). Some results also suggest a carrier in the tonoplast which catalyses a ma-late/2H+ symport. However, why this carrier is not active during dark fixation has not yet been elucidated. It is interesting that a decrease in temperature during the night is an absolute requirement for acidification: During warm nights, there is no accumulation of malic acid in vacuoles (Liittge et al. 1996).
The CAM-PEP Carboxylase as Pacemaker Enzyme
PEPC is a cytosolic enzyme which binds bicarbonate to PEP in an irreversible reaction:
Diverse metabolic effectors
(CAM: oscillation) (C4) Night
(CAM: oscillation) (C4) Night
Sensitive towards malate Less sensitive towards glucose-6-P Weakly active (CAM: during the day; C4: nocturnal)
Fig. 1.5.16. Molecular model of the light/ dark (C4) or day/night (CAM) regulation of the sensitivity of the photosynthetic PEP-carboxy-lase to effectors. Sensitivity changes with the reversible phosphorylation of a serine residue close to the N-terminus of the 110-kDa subunit. (After Chollet et al. 1996)
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