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Thesis Format

Monograph

Degree

Master of Science

Program

Biology

Supervisor

Marsolais, Frédéric

2nd Supervisor

Bernards, Mark

Co-Supervisor

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.

Summary for Lay Audience

Common bean (Phaseolus vulgaris) is one of the most consumed legume crops in the world. This is especially in developing countries. However, the protein quality in common bean is poor. This is due to the low level of essential sulphur amino acids, methionine and cysteine. Previous studies showed that instead of making methionine and cysteine, common beans make other sulphur containing compounds called non-protein sulphur amino acids. These non-protein sulphur amino acids are stored in common bean seeds. One way to improve protein quality in common bean seed would be to increase the amount of methionine and cysteine. This could be done by shifting sulphur away from non-protein sulphur amino acids and into methionine and cysteine. To be able to do this, we need to better understand the way non-protein sulphur amino acids are formed. Only then can we prevent them from being formed.

In my study, the biosynthesis of an important component in the formation of one non-protein sulphur amino acid was revealed. This new information provides clues to the identity of the enzyme involved in this pathway. Another enzyme, γ-glutamyl transferase (GGT), was shown to be involved in the biosynthesis a different non-protein sulphur amino acid. I also showed that GGT present in the seed of common bean accumulates in cell cytoplasm. This suggests a possible site of action for GGT. A third enzyme involved in non-protein sulphur amino acid, called BSAS4;1, was shown to be inhibited by the compound benzoic acid. This was true both in test tube reactions and in live bean seed cells. Altogether, my research findings could aid future efforts to improve the nutritional quality of common bean seeds, and contribute to the relief of malnutrition in the growing world.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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