2Martin-Luther-University Halle-Wittenberg, Institute of Biochemistry/Biotechnology, Kurt-Mothes-Strasse 3, Halle/Saale 06120, Germany
* This paper is devoted to the 80-th anniversary of the birth of Prof. A. Schellenberger (Martin-Luther-University Halle-Wittenberg, Institute of Biochemistry/Biotechnology, Halle, Germany).
** To whom correspondence should be addressed.
Received July 22, 2008; Revision received August 29, 2008
In this work, we investigated the rate of formation of the central intermediate of the transketolase reaction with thiamine diphosphate (ThDP) or 4′-methylamino-ThDP as cofactors and its stability using stopped-flow spectroscopy and circular dichroism (CD) spectroscopy. The intermediates of the transketolase reaction were analyzed by NMR spectroscopy. The kinetic stability of the intermediate was shown to be dependent on the state of the amino group of the coenzyme. The rates of the intermediate formation were the same in the case of the native and methylated ThDP, but the rates of the protonation or oxidation of the complex in the ferricyanide reaction were significantly higher in the complex with methylated ThDP. A new negative band was detected in the CD spectrum of the complex transketolase–4′-methylamino-ThDP corresponding to the protonated dihydroxyethyl-4′-methylamino-ThDP released from the active sites of the enzyme. These data suggest that transketolase in the complex with the NH2-methylated ThDP exhibits dihydroxyethyl-4′-methylamino-ThDP-synthase activity. Thus, the 4′-amino group of the coenzyme provides kinetic stability of the central intermediate of the transketolase reaction, dihydroxyethyl-ThDP.
KEY WORDS: transketolase, thiamine diphosphate, dihydroxyethyl-thiamine diphosphate, 4′-methylamino-thiamine diphosphate, stopped-flow spectroscopy, CD, NMR