Biosynthesis of essential amino acids
Mammals are unable to synthesize metabolically various amino acids necessary for protein synthesis and should incorporate foods from the diet. These amino acids are called essential. Photosynthetic organisms can synthesize it. Has long been accepted that the metabolic pathways involved in the biosynthesis of amino acids in plants are identical to those in microorganisms. However, recent results indicate significant differences and a higher level of complexity due to the existence of multiple enzymes that are distributed in different subcellular compartments.
Lysine, threonine, methionine and isoleucine are essential amino acids that share the common precursor aspartate and which are synthesized in the chloroplasts of plant leaves or in the plastids of non-photosynthetic organs, such as seeds and roots. Until now it was believed that the aspartate was synthesized by the action of aspartate aminotransferase similar to that in mammals, although gene knockout mutants did not show any phenotype.
A study carried out in the Department of Biochemistry and Molecular Biology, University of Malaga has demonstrated the existence in plastids of plants of another form of aspartate aminotransferase enzyme very similar to cyanobacteria and has greater affinity for the enzyme glutamate previously characterized. In an article published in Plant Journal, the authors propose that the physiological role of this new enzyme is the conversion of glutamate to aspartate for the biosynthesis of lysine, threonine, methionine and isoleucine. It is estimated that about 18% of plant genes derived from cyanobacteria. This gene transfer to plants by endosymbiotic event would explain the absence of this enzyme in animals and the essential nature of these amino acids in the diet.