Oscillation in fructose 2,6-bisphosphate levels and in the phosphorylation states of fructose 6-phosphate,2-kinase:fructose-2,6-bisphosphatase in ischemic rat liver

Short- and Long-Term Adaptation to Altered Levels of Glucose: Fifty Years of Scientific Adventure

My graduate and postdoctoral training in metabolism and enzymology eventually led me to study the short- and long-term regulation of glucose and lipid metabolism. In the early phase of my career, my trainees and I identified, purified, and characterized a variety of phosphofructokinase enzymes from mammalian tissues. These studies led us to discover fructose 2,6-P₂, the most potent activator of phosphofructokinase and glycolysis. The discovery of fructose 2,6-P₂ led to the identification and characterization of the tissue-specific bifunctional enzyme 6-phosphofructo-2-kinase:fructose 2,6-bisphosphatase. We discovered a glucose signaling mechanism by which the liver maintains glucose homeostasis by regulating the activities of this bifunctional enzyme. With a rise in glucose, a signaling metabolite, xylulose 5-phosphate, triggers rapid activation of a specific protein phosphatase (PP2ABδC), which dephosphorylates the bifunctional enzyme, thereby increasing fructose 2,6-P₂ levels and upregulating glycolysis. These endeavors paved the way for us to initiate the later phase of my career in which we discovered a new transcription factor termed the carbohydrate response element binding protein (ChREBP). Now ChREBP is recognized as the masterregulator controlling conversion of excess carbohydrates to storage of fat in the liver. ChREBP functions as a central metabolic coordinator that responds to nutrients independently of insulin. The ChREBP transcription factor facilitates metabolic adaptation to excess glucose, leading to obesity and its associated diseases.

Tissue-specific structure/function differentiation of the liver isoform of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

The crystal structures of the human liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in three different liganding states were determined and compared with those of the rat testis isozyme. A set of amino acid sequence heterogeneity from the two distinct genes encoding the two different tissue isozymes leads to both global and local conformational differences that may cause the differences in catalytic properties of the two isozymes. The sequence differences in a beta-hairpin loop in the kinase domain causes a translational shift of several hydrophobic interactions in the dimeric contact region, and its propagation to the domains interface results in a 5 degrees twist of the entire bisphosphatase domain relative to the kinase domain. The bisphosphatase domain twist allows the dimeric interactions between the bisphosphatase domains, which are negligible in the testis enzyme, and as a result, the conformational stability of the domain is increased. Sequence polymorphisms also confer small but significant structural dissimilarities in the substrate-binding loops, allowing the differentiated catalytic properties between the two different tissue-type isozymes. Whereas the polymorphic sequence at the bisphosphatase-active pocket suggests a more suitable substrate binding, a similar extent of sequence differences at the kinase-active pocket confers a different mechanism of substrates bindings to the kinase-active pocket. It includes the ATP-sensitive unwinding of the switch helix alpha5, which is a characteristic ATP-dependent conformational change in the testis form. The sequence-dependent structural difference disallows the liver kinase to follow the ATP-switch mechanism. Altogether these suggest that the liver isoform has structural features more appropriate for an elevated bisphosphatase activity, compared with that of the testis form. The structural predisposition for bisphosphatase activity in the liver isozyme is consistent with the liver-unique glucose metabolic pathway, gluconeogenesis.

Critical switch of the metabolic fluxes by phosphofructo-2-kinase:fructose-2,6-bisphosphatase. A kinetic model

A kinetic model for the bifunctional enzyme, phosphofructo-2-kinase:fructose-2,6-bisphosphatase, is analysed by application of the graph-theoretical method, considering comparable levels for all participants. Certain elementary reactions, distributed on the enzyme surface, are considered to be co-ordinated in a single conformational transition (a model of parallel molecular operations). The method allows us to identify in the kinetic scheme its destabilising sub-scheme as a branched cycle of elementary reactions. Under certain conditions this sub-scheme induces critical phenomena (bistability or oscillations). The computer calculations for the estimated parameter values fit well the experimental observations for this system. The model explains the periodic or bistable counterphase changes of the two opposing activities of this enzyme, observed after glucose perfusion of rat hepatic enzyme samples, and predicts drastic critical changes in kinetic behaviour induced by small external signals. The model also shows the necessity of the phosphoryl intermediate in the mechanism of the bisphosphatase for the critical kind of kinetic behaviour.

Regulation of energy metabolism in macrophages during hypoxia. Roles of fructose 2,6-bisphosphate and ribose 1,5-bisphosphate

Macrophages can adapt to the absence of oxygen by switching to anaerobic glycolysis. In this study, we investigated (a) the roles of fructose 2,6-bisphosphate (Fru-2,6-P2) and ribose 1,5-bisphosphate (Rib-1,5-P2), potent activators of phosphofructokinase, (b) the enzymes responsible for the synthesis of Rib-1,5-P2, and (c) the mechanisms of regulation of these enzymes in H36.12j macrophages during the initial phase of hypoxia. Within 1 min after initiating hypoxia, glycolysis was activated through activation of phosphofructokinase. Over the same period, Fru-2,6-P2 decreased 50% and recovered completely upon reoxygenation. Similar changes in cAMP levels were observed. In contrast, the Rib-1,5-P2 concentration rapidly increased to a maximum level of 8.0 +/- 0.9 nmol/g cell 30 s after hypoxia. Thus, Rib-1,5-P2 was the major factor increasing the rate of glycolysis during the initial phase of hypoxia. Moreover, we found that Rib-1,5-P2 was synthesized by two steps: the ribose-phosphate pyrophosphokinase (5-phosphoribosyl-1-pyrophosphate synthetase; PRPP synthetase) reaction (EC ) catalyzing the reaction, Rib-5-P + ATP --> PRPP + AMP and a new enzyme, "PRPP pyrophosphatase" catalyzing the reaction, PRPP --> Rib-1,5-P2 + P(i). Both PRPP synthetase and PRPP pyrophosphatase were significantly activated 30 s after hypoxia. Pretreatment with 1-octadecyl-2-methyl-rac-glycero-3-phosphocholine and calphostin C prevented the activation of ribose PRPP synthetase and PRPP pyrophosphatase as well as increase in Rib-1,5-P2 and activation of phosphofructokinase 30 s after hypoxia. These data suggest that the activation of the above enzymes was mediated by protein kinase C acting via activation of phosphatidylinositol specific phospholipase C in the macrophages during hypoxia.

Hepatic preconditioning preserves energy metabolism during sustained ischemia

We evaluated the possibility that ischemic preconditioning could modify hepatic energy metabolism during ischemia. Accordingly, high-energy nucleotides and their degradation products, glycogen and glycolytic intermediates and regulatory metabolites, were compared between preconditioned and nonpreconditioned livers. Preconditioning preserved to a greater extent ATP, adenine nucleotide pool, and adenylate energy charge; the accumulation of adenine nucleosides and bases was much lower in preconditioned livers, thus reflecting slower adenine nucleotide degradation. These effects were associated with a decrease in glycogen depletion and reduced accumulation of hexose 6-phosphates and lactate. 6-Phosphofructo-2-kinase decreased in both groups, reducing the availability of fructose-2, 6-bisphosphate. Preconditioning sustained metabolite concentration at higher levels although this was not correlated with an increased glycolytic rate, suggesting that adenine nucleotides and cAMP may play the main role in the modulation of glycolytic pathway. Preconditioning attenuated the rise in cAMP and limited the accumulation of hexose 6-phosphates and lactate, probably by reducing glycogen depletion. Our results suggest the induction of metabolic arrest and/or associated metabolic downregulation as energetic cost-saving mechanisms that could be induced by preconditioning.

Irreversible metabolic transitions: the glucose 6-phosphate metabolism in yeast cell-free extracts

The steady-state and dynamic behavior of a partial glycolytic reaction sequence are investigated in cell-free extracts of yeast. Pyruvate kinase, adenylate kinase and glucose 6-phosphate isomerase cooperate to a multienzyme system centered around the 6-phosphofructokinase (6-PFK) and fructose 1,6-bisphosphatase (FBPase) cycle. The reaction system operates under thermodynamically open conditions maintained by a continuous supply of substrates, i.e., glucose 6-phosphate (Glc6P), ATP and phosphoenolpyruvate (PPrv) in a flow-through reaction chamber. Appropriate conditions lead to the occurrence of (two) coexisting and markedly different time-independent states in the metabolite concentrations and fluxes. For particular experimental conditions, changes in the influx adenylic energy charge, [AEC]IN, may cause transitions between these alternative steady states which are either reversible as it occurs in classical hysteresis phenomena, or, more importantly, irreversible (irreversible transitions, IT) where the system is not able to switch back to its previous state even when the perturbation is reverted. The emergence of these irreversible transitions do not result from artificial or non-realistic experimental constraints, but are a potential intrinsic property of any non-linear dynamic system exhibiting bi- or multistability. These one-way transitions may well have important biological implications with respect to switching, adaptation and memory phenomena.

Fructose 2,6-bisphosphate induces irreversible transitions in cell-free extracts of rat liver

The effect of fructose 2,6-bisphosphate on the dynamics of the 6-phosphofructo-1-kinase/fructose-1,6-bisphosphatase cycle is investigated in a cell-free extract of rat liver under steady-state conditions. Bistability emerges on the basis of the reciprocal allosteric modulation of 6-phosphofructo-1-kinase and fructose-1,6-bisphosphatase. Under conditions of bistability fructose 2,6-bisphosphate may cause transitions between alternative steady states. However, in contrast to what is frequently observed in bistable systems, within a broad range of experimental conditions these transitions proceed irreversibly from states with high ATP to states characterized by low ATP.

Changes in glucose transporter 2 and carbohydrate-metabolizing enzymes in the liver during cold preservation and warm ischemia

In order to examine glucose metabolism in liver grafts during cold preservation (24 and 48 hr), warm ischemia (60 and 120 min), a combination of the two and reperfusion, the amount of protein and mRNA of glucose transporter 2 and the activities of enzymes in glycolysis (glucokinase, phosphofructokinase, pyruvatekinase), gluconeogenesis (glucose 6-phosphatase, fructose 1,6-bisphosphatase), and the pentose phosphate pathway (glucose 6-phosphate dehydrogenase) were measured. It appeared that glucose transport, the pentose phosphate pathway, and gluconeogenesis were maintained during cold preservation and warm ischemia. The activity of glucokinase significantly decreased from the control value of 1.33 +/- 0.23 IU/g protein to 0.70 +/- 0.17 (24 hr, P<0.05) and 0.57 +/- 0.12 (48 hr, P<0.01) only during cold preservation. However, the activity of phosphofructokinase significantly decreased from the control value of 4.37 +/- 0.06 IU/g protein to 2.67 +/- 0.15 (60 min, P<0.0001) and 1.53 +/- 0.06 (120 min, P<0.0001) only during warm ischemia. This indicates that glycolysis deteriorates during both cold preservation and warm ischemia and demonstrates further that the balance between glycolysis and gluconeogenesis shifts to gluconeogenesis. Even when cold preservation was combined with warm ischemia, the activity of glucokinase decreased only during cold preservation and the activity of phosphofructokinase decreased only during warm ischemia. Furthermore, these changes were time-dependent. It is suggested that they can be used as a clock to measure the durations of cold preservation and warm ischemia separately and that the magnitude of an ischemic injury to a liver and a liver graft's viability can be indirectly estimated before transplantation.

Bistability and damped oscillations in the fructose 6-phosphate/fructose 1,6-bisphosphate cycle in cell-free extracts from rat liver

The dynamics of the fructose 6-phosphate/fructose 1,6-bisphosphate substrate cycle was investigated in cell-free extracts from rat liver. Under conditions of continuous substrate supply different types of stationary states and damped oscillations were observed experimentally and found to be in qualitative agreement with theoretical predictions. Changing the adenylate energy charge of the substrate supply, bistability was shown to be related to irreversible transitions between functionally different branches of stable stationary states.