**9.1 Toxicity due to malformed selenoproteins**

Malformed selenoproteins result from the substitution of seCys/seMet into the protein chain in place of Cys/Met, these se amino acids are unstably unfavorable to protein functioning. Cysteine plays a primary role in the structure and function of a protein chain, disulfide bond formation, chemical catalysis and also functions a metal-binding site. Substitution of Cys with seCys produces result in alteration to the protein structure and capacity due to the seCys being bigger, responsive and more effectively deprotonated than cysteine [79], as in the case of methionine sulfoxide reductase enzyme which lost its function as a result of the substitution of SeCys [19]. SeCys substitution mutilates the tertiary structure of protein because of its large diselenide bridge formation and modified redox potential affect enzyme kinetics [79]. Fe-S group proteins of chloroplast and mitochondrial electron transport chain [80] are inclined to SeCys substitution for instance as in the event of chloroplast NifS-like protein [81]. Fe-Se bunch are bigger in size and do not fit appropriately in apoproteins.

### **9.2 Selenium toxicity due to oxidative stress**

A high dosage of selenium acts as a pro-oxidant and creates receptive oxygen species which cause oxidative stress in plants. Under selenium-induced stress, glutathione is diminished [82], except for Se-tolerant plants where raised level of glutathione is increased [83]. Previously studied plants such as *Arabidopsis* and *Viciafaba* have shown that reactive oxygen species accumulation under Se stress increased lipid peroxidation, cell mortality [20].
