A structure-activity study of thaumatin using pyridoxal 5′-phosphate (PLP) as a probe

Abstract

The structural features responsible for the sensory properties of the sweet protein, thaumatin, have been investigated by side chain modification of amino acid residues using pyridoxal 5′-phosphate (PLP). PLP molecules bind covalently to proteins by reacting with the α-amino group and the ε-amino group of lysine residues. Spectral and sensory studies have been performed on thaumatin-PLP derivatives prepared at various molar ratios. The incorporation of one mole of PLP into thaumatin causes substantial modification of the sensory properties which include generation of astringency, an unpleasant taste and the loss of sweetness intensity. The introduction of more than one mole of PLP has no further effect on the gustatory properties of thaumatin. Removal by alkaline phosphatase of the phosphate group of PLP bound to thaumatin has no influence on the ability of PLP to modify the sensory characteristics of thaumatin. This suggests that the sensory alteration caused by PLP cannot be ascribed to the changes in the net charge of the protein, but is likely to be due to the modification of specific lysine residue(s) which are thus implicated in the sweet site.