Deciphering S-methylcysteine biosynthesis in common bean by isotopic tracking with mass spectrometry

Citation

Joshi, J., Renaud, J.B., Sumarah, M.W., Marsolais, F. (2019). Deciphering S-methylcysteine biosynthesis in common bean by isotopic tracking with mass spectrometry. Plant Journal, [online] 100(1), 176-186. http://dx.doi.org/10.1111/tpj.14438

Plain language summary

Common bean (dry bean) is an excellent dietary source of protein and fibre. However, its protein quality, defined as the balanced composition of nutritionally essential amino acids, is sub-optimal due to low levels of sulphur amino acids, methionine and cysteine. On the other hand, common bean accumulates high levels of a non-protein sulphur amino acid, S-methylcysteine. This study sought to identify the biochemical pathway leading to the accumulation of S-methylcysteine in seed. We used an approach which involved tracking the incorporation into sulphur metabolites of different amino acid precursors, labeled with stable isotopes of carbon and nitrogen, using high resolution mass spectrometry. Analysis of results revealed two distinct pathways. Free S-methylcysteine is formed early on during seed development, and a gene and enzyme responsible for this reaction was identified and characterized. The most prominent biosynthetic pathway involves the compound S-methylhomoglutathione as an intermediate. It takes place in plastids, which ensures a spatial separation from the formation of cysteine for protein synthesis which happens in the cytosol. This pathway shares similarities with the biosynthesis of flavor compounds in garlic and onion. This work identified key steps in the metabolism of sulphur amino acids in common bean and provides potential targets for nutritional improvement of this important crop.

Abstract

The suboptimal content of sulfur-containing amino acids methionine and cysteine prevents common bean (Phaseolus vulgaris) from being an excellent source of protein. Nutritional improvements to this significant crop require a better understanding of the biosynthesis of sulfur-containing compounds including the nonproteogenic amino acid S-methylcysteine and the dipeptide γ-glutamyl-S-methylcysteine, which accumulate in seed. In this study, seeds were incubated with isotopically labelled serine, cysteine or methionine and analyzed by reverse phase chromatography−high resolution mass spectrometry to track stable isotopes as they progressed through the sulfur metabolome. We determined that serine and methionine are the sole precursors of free S-methylcysteine in developing seeds, indicating that this compound is likely to be synthesized through the condensation of O-acetylserine and methanethiol. BSAS4;1, a cytosolic β-substituted alanine synthase preferentially expressed in developing seeds, catalyzed the formation of S-methylcysteine in vitro. A higher flux of labelled serine or cysteine was observed in a sequential pathway involving γ-glutamyl-cysteine, homoglutathione and S-methylhomoglutathione, a likely precursor to γ-glutamyl-S-methylcysteine. Preferential incorporation of serine over cysteine supports a subcellular compartmentation of this pathway, likely to be in the chloroplast. The origin of the methyl group in S-methylhomoglutathione was traced to methionine. There was substantial incorporation of carbons from methionine into the β-alanine portion of homoglutathione and S-methylhomoglutathione, suggesting the breakdown of methionine by methionine γ-lyase and conversion of α-ketobutyrate to β-alanine via propanoate metabolism. These findings delineate the biosynthetic pathways of the sulfur metabolome of common bean and provide an insight that will aid future efforts to improve nutritional quality.