The Biosynthesis of Non-protein Sulfur Amino Acid in Developing Seeds of Common Bean

Zixuan Lu, The University of Western Ontario

Abstract

The protein quality of common bean (Phaseolus vulgaris) is associated with the level of dietary essential sulphur amino acids – methionine and cysteine. Extra sulphur that cannot be stored in the protein pool accumulates as the non-protein amino acid S-methylcysteine (S-methylCys) and its dipeptide γ-glutamyl-S-methylcysteine (γ-Glu-S-methylCys). Previous studies have indicated that S-methylhomoglutathione (S-methylhGSH) is present in the developing seed of common bean. It is hypothesized that S-methylhGSH is the key intermediate in the biosynthetic pathway of the γ-Glu-S-methylCys that leads to the accumulation of this dipeptide. This project elucidated the unknown biochemical pathway of S-methylhGSH synthesis using ³⁴S labeled methionine and ¹³C labeled sodium thiomethoxide in feeding experiments with developing seeds. The results suggest S-methylhGSH is synthesized by methylation of homoglutathione (hGSH). Biochemical assay with seed extract suggested γ-glutamyl transferase (GGT) is likely to catalyzing the reaction between hGSH and S-methylCys to produce γ-Glu-S-methylCys. Furthermore, benzoic acid was identified as the inhibitor for the enzyme catalyzed the synthesis of S-methylCys, BSAS4;1. The findings delineate the biosynthetic pathways of the sulphur metabolome and provide potential approach to improve nutritional quality of common bean.