The pH dependence of the quaternary structure of pyruvate decarboxylase (EC 4.1.1.1) has recently been discovered [(1990) FEBS Lett. 266, 17–20; (1992) Biochemistry (in press)]. In the present study we have investigated the change in quaternary structure by observing the binding of the cofactor, thiamine pyrophosphate, using 31P NMR spectroscopy. The dissociation of the native tetramers into dimers when increasing the pH coincides with a weaker binding of the cofactor and loss of enzyme activity. The results provide further evidence that thiamine pyrophosphate is bound primarily via the β-phosphate moiety. In addition, a phosphoserine has been discovered in two of the four subunits.
