of Tk-BpsA in the presence or absence of the substrates spermidine and dcSAM revealed that a large, negatively charged cavity is responsible for the binding of branched-chain substrates. The binding is different from that in the activesite of linear polyamine spermidine/spermine synthases, and loop-closures occur upon the binding of spermidine. Based on structural analyses, further kinetic studies
to two of five aspartate/glutamate residues (Glu339 , Asp342 , Asp343 , Glu344 , and Glu345 ) in the C-terminal flexible region. Mutating each of these five amino acid residues to asparagine/glutamine resulted in a slight decrease in activity. The quadruple mutant D342N/D343N/E344Q/E345Q exhibited a severe reduction in catalytic efficiency, suggesting that these aspartate/glutamate residues function