Unveiling sulfur amino acid metabolism in common bean seeds (Phaseolus vulgaris).

Approximately 870 million of the world’s population suffer from chronic undernourishment. Protein energy malnutrition is the most prevalent form of undernourishment. Despite being a good source of protein and dietary fibre, common bean has sub optimal levels of essential sulfur amino acids: methionine and cysteine. Levels of cysteine and methionine in developing seeds have an inverse relationship with the non-proteinogenic sulphur amino acid S-methyl-cysteine (SMC). One of the strategies to improve protein quality can be to engineer a system to block or reduce the biosynthesis of SMC, which will redirect the sulfur from the SMC to the cysteine pool. To elucidate the unknown biochemical pathway of SMC synthesis, an isotope-labeling experiment was performed to track the precursor of SMC by generating high resolution mass spectra using an Orbitrap mass spectrometer. In this study, incubating developing seeds with either 13C ,15N labeled serine or cysteine suggested that serine is the precursor of SMC in common bean. In cytoplasm, methanethiol released the hydrolysis of methionine might be condensed with O-acetylserine to form SMC. Another pathway seems more active, whereby homoglutathione is transformed into S-methylhomoglutathione. According to the Canadian Food Inspection Agency, the label “Good source of protein” can be used for beans and other legumes. In future silencing of SMC pathway genes or development of TILLING lines with high cysteine and methionine levels may change bean from a good source to an “excellent source of protein”. Hence our findings would provide a helping hand to overcome the food insecurity in the growing world.