Determination Of Essentiality

Numerous solution culture and pot experiments with K+-free substrates have shown that plants do not grow without K+. As soon as the potassium reserves of the seed are exhausted, plants die. This condition may also occur on strongly K+-fixing soils. In contrast to other plant nutrients such as N, S, and P, there are hardly any organic constituents known with K+ as a building element. Potassium ions activate various enzymes, which may also be activated by other univalent cationic species with a similar size and water mantle such as NH4+, Rb+, and Cs+ (7). These other species, however, play no major role under natural conditions as the concentrations of Cs+, Rb+, and also NH4+ in the tissues are low and will not reach the activation concentration required. In vitro experiments have shown that maximum activation is obtained within a concentration range of 0.050 to 0.080 M K+. Ammonium may attain high concentrations in the soil solution of flooded soils, and ammonium uptake rates of plant species such as rice (Oryza sativa L.) are very high. In the cytosol, however, no high NH4+ concentrations build up because NH4+ is assimilated rapidly, as was shown for rice (8). Activation of enzymes in vivo may occur at the same high K+ concentration as seen in in vitro experiments, as was shown for ribulose bisphosphate carboxylase (9).

It is assumed that K+ binds to the enzyme surface, changing the enzymic conformation and thus leading to enzyme activation. Recent research has shown that in the enzyme dialkyl-glycine car-boxylase, K+ is centered in an octahedron with O atoms at the six corners. As shown in Figure 4.1, these O atoms are provided by three amino acyls, one water molecule, and the O of hydroxyl groups of each of serine and aspartate (10). As compared with Na+, the K+ binding is very selective because the dehydration energy required for K+ is much lower than for Na+. If the latter binds to the enzyme, the natural conformation of the enzyme is distorted, and the access of the substrate to the binding site is blocked. Lithium ions (Li+) inactivate the enzyme in an analogous way. It is supposed that in most K+-activated enzymes, the required conformation change is brought about by the central position of K+ in the octahedron, where its positive charge attracts the negative site of the O atom located at each corner of the octahedron. This conformation is a unique structure that gives evidence of the unique function of K+. In this context, it is of interest that the difference between K+ and Na+ binding to the enzyme is analogous to the adsorption of the cationic species to the

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