Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Dr. Frédéric Marsolais

2nd Supervisor

Dr. Mark Bernards

Co-Supervisor

Abstract

With increasing food insecurity in the populated world, the number of people affected by chronic undernourishment is also increasing. Alone, protein energy malnutrition is linked to 6 million deaths annually. Despite being a good source of protein and dietary fibre, the quality of bean protein is limited because of 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-protein sulfur amino acid S-methyl-cysteine (S-methylCys) and dipeptide g-glutamyl-S-methyl-cysteine (g-Glu-S-methylCys).

One of the strategies to improve protein quality in bean is to redirect sulfur from S-methylCys and g-Glu-S-methylCys to the cysteine pool. In this thesis, elucidation of the unknown biochemical pathway of S-methylCys synthesis was accomplished using 13C and 15N labelled serine and cysteine, and revealed serine as the precursor of S-methylCys biosynthesis. Feeding developing seeds with 13C and 15N labelled methionine also suggested a role for methionine in S-methylCys biosynthesis. In the cytosol, methanethiol released during hydrolysis of methionine condensed with O-acetylserine to form S-methylCys. BSAS4;1, a b-substituted alanine synthase family member plays a key role in this reaction. BSAS4;1 is a cysteine synthase that utilizes O-acetylserine as a carbon backbone donor and methionine as a methyl donor to synthesize S-methylCys. In the case of g-Glu-S-methylCys, another pathway seems more active, whereby homoglutathione is transformed into S-methylhomoglutathione and g-Glu-S-methylCys is synthesized in the presence of a carboxypeptidase. This study identifies BSAS4;1 and methionine g-lyase (MGL) as candidate enzymes for S-methylCys biosynthesis in common bean.

According to the Canadian Food Inspection Agency, only the label “Good source of protein” can be used for beans and other legumes. In future, silencing of the S-methylCys pathway candidate 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”. Ultimately, the findings of this thesis could provide a helping hand to overcome the food insecurity in the growing world.

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