The calcium binding sites of Bakers' Yeast Transketolase (TK) was elucidated
by estimating the pKa values of the functional groups that bind to calcium. These
pKa's were found to be 6.25 and 7.2 relating to the pKa's of the two immidazol
moieties of histidine residues on the enzyme. The rate of the binding of calcium to
the enzyme was obtained separately as a function of pH. Maximum values were
obtained at the pKa's of the immidazol rings of histidine correlating closely with the
values obtained by direct titration. The activity rate studies as a function of pH
showed that the enzyme, when bound to its cofactors, becomes further activated
over and above the regular increasing trend of the pH rate profile at similar pH's
which are optimal for calcium binding