Substrate inhibition of transketolase

New Role of Water in Transketolase Catalysis

Transketolase catalyzes the interconversion of keto and aldo sugars. Its coenzyme is thiamine diphosphate. The binding of keto sugar with thiamine diphosphate is possible only after C2 deprotonation of its thiazole ring. It is believed that deprotonation occurs due to the direct transfer of a proton to the amino group of its aminopyrimidine ring. Using mass spectrometry, it is shown that a water molecule is directly involved in the deprotonation process. After the binding of thiamine diphosphate with transketolase and its subsequent cleavage, a thiamine diphosphate molecule is formed with a mass increased by one oxygen molecule. After fragmentation, a thiamine diphosphate molecule is formed with a mass reduced by one and two hydrogen atoms, that is, HO and H₂O are split off. Based on these data, it is assumed that after the formation of holotransketolase, water is covalently bound to thiamine diphosphate, and carbanion is formed as a result of its elimination. This may be a common mechanism for other thiamine enzymes. The participation of a water molecule in the catalysis of the one-substrate transketolase reaction and a possible reason for the effect of the acceptor substrate on the affinity of the donor substrate for active sites are also shown.

Dynamic Acetylation of Phosphoenolpyruvate Carboxykinase Toggles Enzyme Activity between Gluconeogenic and Anaplerotic Reactions

Cytosolic phosphoenolpyruvate carboxykinase (PCK1) is considered a gluconeogenic enzyme; however, its metabolic functions and regulatory mechanisms beyond gluconeogenesis are poorly understood. Here, we describe that dynamic acetylation of PCK1 interconverts the enzyme between gluconeogenic and anaplerotic activities. Under high glucose, p300-dependent hyperacetylation of PCK1 did not lead to protein degradation but instead increased the ability of PCK1 to perform the anaplerotic reaction, converting phosphoenolpyruvate to oxaloacetate. Lys91 acetylation destabilizes the active site of PCK1 and favors the reverse reaction. At low energy input, we demonstrate that SIRT1 deacetylates PCK1 and fully restores the gluconeogenic ability of PCK1. Additionally, we found that GSK3β-mediated phosphorylation of PCK1 decreases acetylation and increases ubiquitination. Biochemical evidence suggests that serine phosphorylation adjacent to Lys91 stimulates SIRT1-dependent deacetylation of PCK1. This work reveals an unexpected capacity of hyperacetylated PCK1 to promote anaplerotic activity, and the intersection of post-translational control of PCK1 involving acetylation, phosphorylation, and ubiquitination.