Botany online 1996-2004. No further update, only historical document of botanical science!
The short discussion of the amino acid syntheses aimed at demonstrating the complexity of the processes, their degrees of branching and the relations of the single components. It was shown in a few examples that different alternative pathways developed during evolution and that different groups of organisms chose different pathways.
It was an enormous work to elucidate the single steps experimentally and to proof the existence of all intermediates. Most of what is known today about amino acid biosynthesis is based on experiments performed with micro-organisms and animal cells. But it showed that the results were also largely true for plants although a number of details remain unclear.
In this context, plants should not only be regarded in contrast to micro-organisms and animals but it should also be kept in mind that the single plant groups may differ. Since the beginning of the 1940s when BEADLE AND TATUM initiated the genetic analysis of Neurospora crassa (a mould), it is known that a gene determines the generation of an enzyme and that this enzyme again catalyzes a reaction of the metabolism. The use of mutants with metabolic deficiencies proofed to be extremely helpful in the elucidation of single metabolic pathways and to provide safe evidence about the meaning of single intermediates.
After the metabolic pathways of micro-organisms were known and techniques were developed to cultivate haploid plant cells (protoplasts) according to microbiological methods, it was searched for mutants with metabolic deficiencies. Of special interest were those with defective amino acid syntheses (amino acid auxotrophic mutants).
The gain of results was disappointing. After treatment of the protoplasts with mutagenes, far less mutants were found than expected according to the experiences made in microbiology and with animal cells. The reason for this seems to be in the ability of plant cells to switch to an alternative biosynthetic pathway (polyploidy) in case of a genetic deficiency so that the defect shows no effect. It is very likely that such alternatives are not equal with respect to energetic costs so that the less favourable way is only used if the more efficient one is blocked. How costly amino acid syntheses actually are is understood best when remembering that many animals (including man) cannot synthesize a number of amino acids any more. The ability was lost in the course of evolution. Animals are heterotrophic and take up amino acids with their diet (as proteins). Normally, this is sufficient. Amino acids that cannot be synthesized by animals are called essential, the other, structurally more simple ones non-essential.
Contrary, plants are nearly completely autotrophic, i.e. they synthesize their organic matter from minerals, carbon dioxide and water. The biosynthetic pathways of the basic metabolism are consequently very important and the selection pressure to develop alternative paths seems to have been stronger than in organisms that could take up the end products with their diet. It seems as if the plant genome contains more information than expressed at a certain situation.
Control of amino acid syntheses. In the discussion of enzymes, the importance of allostery and end product inhibition was pointed out. Both mechanisms occur in numerous metabolic pathways of amino acids, i.e. single pathways can be switched off as soon as enough end product has been generated. (end product inhibition)