Abstract
BackgroundThiamine diphosphate (ThDP), an indispensable cofactor for oxidative energy metabolism, is synthesized through the reaction thiamine + ATP ⇆ ThDP + AMP, catalyzed by thiamine pyrophosphokinase 1 (TPK1), a cytosolic dimeric enzyme. It was claimed that the equilibrium of the reaction is in favor of the formation of thiamine and ATP, at odds with thermodynamic calculations. Here we show that this discrepancy is due to feedback inhibition by the product ThDP. MethodsWe used a purified recombinant mouse TPK1 to study reaction kinetics in the forward (physiological) and for the first time also in the reverse direction. ResultsKeq values reported previously are strongly underestimated, due to the fact the reaction in the forward direction rapidly slows down and reaches a pseudo-equilibrium as ThDP accumulates. We found that ThDP is a potent non-competitive inhibitor (Ki ≈ 0.4 μM) of the forward reaction. In the reverse direction, a true equilibrium is reached with a Keq of about 2 × 10−5, strongly in favor of ThDP formation. In the reverse direction, we found a very low Km for ThDP (0.05 μM), in agreement with a tight binding of ThDP to the enzyme. General significanceInhibition of TPK1 by ThDP explains why intracellular ThDP levels remain low after administration of even very high doses of thiamine. Understanding the consequences of this feedback inhibition is essential for developing reliable methods for measuring TPK activity in tissue extracts and for optimizing the therapeutic use of thiamine and its prodrugs with higher bioavailability under pathological conditions.
Highlights
Thiamine diphosphate (ThDP), the diphosphorylated derivative of thiamine, is a coenzyme for several essential mammalian enzymes and enzyme com plexes that play a key role in energy metabolism [1].In animals, which are unable to synthesize thiamine, the vitamin is taken up by specific transporters [1]
Thiamine is pyrophos phorylated to the coenzyme ThDP by cytosolic thiamine diphosphoki nase according to the reaction: thiamine + ATP ⇆ ThDP + AMP
Using for the first time the reverse reaction, we were able to estimate a Keq for the thiamine pyrophosphokinase 1 (TPK1) reaction and we show that this equilibrium strongly favors the synthesis of the coenzyme ThDP
Summary
Thiamine diphosphate (ThDP), the diphosphorylated derivative of thiamine (vitamin B1), is a coenzyme for several essential mammalian enzymes (transketolase, 2-hydroxyacyl-CoA lyase 1) and enzyme com plexes (pyruvate, 2-oxoglutarate and 2-oxoacid dehydrogenase com plexes) that play a key role in energy metabolism [1].In animals, which are unable to synthesize thiamine, the vitamin is taken up by specific transporters [1]. Thiamine is pyrophos phorylated to the coenzyme ThDP by cytosolic thiamine diphosphoki nase (or pyrophosphokinase, TPK, EC 2.7.6.2) according to the reaction: thiamine + ATP ⇆ ThDP + AMP. Thiamine diphosphate (ThDP), an indispensable cofactor for oxidative energy metabolism, is syn thesized through the reaction thiamine + ATP ⇆ ThDP + AMP, catalyzed by thiamine pyrophosphokinase 1 (TPK1), a cytosolic dimeric enzyme. Methods: We used a purified recombinant mouse TPK1 to study reaction kinetics in the forward (physiological) and for the first time in the reverse direction. Results: Keq values reported previously are strongly underestimated, due to the fact the reaction in the forward direction rapidly slows down and reaches a pseudo-equilibrium as ThDP accumulates. Understanding the consequences of this feedback inhibition is essential for developing reliable methods for measuring TPK activity in tissue extracts and for optimizing the therapeutic use of thiamine and its prodrugs with higher bioavailability under pathological conditions
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