Most proteins are enzymes that came into being rather early in evolution; this means that all cells, bacteria, animal and plant cells, have the same repertoire of enzymes. Everything summarized by the term primary metabolism, i.e. glycolysis, citric acid cycle, amino acid synthesis, carbohydrate synthesis, lipid and nucleotide synthesis is controlled by a set of enzymes that differ only slightly from one group of organism to another. If we want to talk about plant proteins here, then we do have to find out how plants differ from other organisms and whether the study of their proteins is helpful for the understanding of these special properties.
Most plants are multicellular organisms and special control mechanisms are necessary to co-ordinate the co-operation of the single cells. Cells have to be able to selectively take up and excrete certain molecules. This does often need energy. The performing enzymes are usually elements of the cellular membranes. They are transferases or pumps (like the sodium-potassium ion pump) that lead ions, metabolites and other substrates through the membrane. They have, just like enzymes, a substrate specificity, though they do not change it but release it at the other side of the membrane. Transport occurs often against a concentration gradient.
Other proteins again recognize special signals, physical factors like light of a certain wave length or chemical components like phytohormones or components of other cells' surfaces, for example. After signal recognition, they change their conformation and thus pass on the 'information'. Such proteins are called receptors and the signals effectors (or elicitors). Receptors may be located in the membrane, in the plasma or in the cell wall. Many of them, for example light receptors, are associated with co-factors (the classic example being phytochrome. Among their tasks is the transformation and the amplification of the signal. Animal cells could be shown to contain receptors connected in series. The result is a cascade-like amplification of the information elicited by the signal. An effector is usually not altered by binding to the receptor. It may therefore subsequently bind to several receptors, if no superordinate mechanisms for the elimination of surplus effectors exists like it does, for example, with acetylcholesterine esterase in the synaptic cleft between neurone and muscle. Among the best-known plant receptor molecules are the lectins, receptors that bind carbohydrates (more in the next section but one). Although lectins occur not exclusively in the plant kingdom, they are much more common here than in animals. They may therefore be regarded as the prototype of plant proteins.
Another group of typical plant proteins are storage proteins that can above all be found in seeds. Leguminoses are particularly rich in storage proteins. They became a focus of general interest because they are important for the human nutrition especially in third world countries. Plant proteins are usually poor in lysine and it was therefore searched for mutants (e.g. in maize) with a higher content of lysine. The second large problem is the overall content of proteins and the protein/carbohydrate ratio. Here, too, is it hoped (for example by genetic engineering) to obtain more valuable cultivated varieties in future.
Before advancing to lectins and storage proteins, we will embark on a short digression about the isolation and characterization of proteins , about alloenzymes, isoenzymes and finally about the use of proteins for the analysis of cellular structures and the localization of molecules within the cell.
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