The preceding has reviewed and updated the role of the bifunctional enzyme and F-2,6-P2 in the acute response of the cardiac and hepatic metabolic fluxes to blood-borne signals. These were shown to be primarily determined by regulatory serine phosphorylation, albeit in response to different signals, which serves to modulate the K/B of the bifunctional enzyme and thus acutely change cellular F-2,6-P2. Despite the emerging consensus that F-2,6-P2 does not inhibit FDPase-1 in vivo, the primary target of the PFK-2/FDPase-2/F-2,6-P2 system is PFK-1 and the net flux through the F-6-P/F-1,6-P2 cycle. This remains consistent with Fig. 1, at least in spirit. It should be noted that the report of Jin et al. showed no change in gluconeogenic flux during the acute phase (Jin et al., 2003). Even so, it is well known that the liver is a net consumer of glucose in response to re-feeding. Although Jin et al. did not determine glycolytic rates, we can infer an up-regulation from the reported 10-fold increase in hepatic F-2,6-P 2. This implies that net hepatic output was reduced by coordinate up-regulation of GK and PFK-1 by the bifunctional enzyme and the biofactor, respectively, according to the models we propose here. This description signals a significant shift in the paradigm of metabolic regulation by PFK-2/FDPase-2 and F-2,6-P2 in the liver, as opposed to the classical view, where activation of PFK-1 complimented the inhibition of FDPase-1 to drive glycolysis in response to increasing F-2,6-P2. The new paradigm suggests that the F-2,6-P2 activation PFK-1 is coordinated with the PFK-2/FDPase-2 activation of GK to overcome the persistent gluconeogenic flux and acutely reduce net hepatic glucose output. Thus, the newly described activation of GK by PFK-2/FDPase-2 helps to fulfill its moniker, the bifunctional enzyme, at the cellular level because this regulatory system is now seen to function in two ways to up-regulate glycolysis, activation of both GK and PFK-1. At the same time, we have adopted a new perspective of the bifunctional enzyme by making the case that the separation of the active sites, especially the extreme stability of the His-P intermediate at the FDPase-2 site, constitutes a regulatory strategy that imparts switch-like behavior to PFK-2/FDPase-2. This switch-like action is directly related to the linkage of the phosphorylation state of Ser-32 to that of His-258, which provides for acute modulation of cellular F-2,6-P2. Moreover, the same regulatory Ser-P site also impacts the activation of GK by PFK-2/FDPase-2. In this light, the detailed biophysical studies of the molecular mechanisms that underlie the change in the enzyme's K/B in response to de/phosphorylation of Ser-32 gain significance, especially with regard to therapeutic modulation of cellular F-2,6-P2. These are most clearly evident with regard to diabetes (Okar et al., 1998); however, recent observations that reducing F-2,6-P2 in cancer cells can weaken them towards chemotherapeutics (Hirata et al., 2000) suggests that the PFK-2/FDPase-2/F-2,6-P2 system may provide pharmaceutical targets beyond the liver and diabetes. The observation that the hepatic PFK-2/FDPase-2/F-2,6-P2 system works with GK to coordinate the acute up-regulation of glucose phosphorylation and glycolysis implies that the net effect is to commit the incoming sugar to glycolysis and the preferential oxidation of serum glucose by the liver in the fasted-to-fed transition. This is intriguingly consistent with observations by others that liver glycogen is repleted from 3 carbon precursors after re-feeding (McGarry et al., 1987). When considered in the context of the report from Cascante et al. that F-2,6-P 2 can influence the substrate channeling from glucose to trioses (Cascante et al., 2000) and the observations of the existence of macromolecular complexes within the cytosol that bring together the enzymes of glycogen synthesis (Newgard et al., 2000), we are encouraged to suggest that hepatic glycolysis and gluconeogenesis may take place within physically distinct complexes of metabolic enzymes and regulators. The existence of distinct metabolic fates of the G-6-P produced by hexokinase 1 and GK (hexokinase IV) has suggested that the pathways of carbohydrate metabolism may be spatially distinct (Seoane et al., 1996). Although the interpretation of Seoane et al. is based on substrate "pools", we consider these equivalent to our hypothetical macromolecular complexes. In this light, their observations are consistent with distinct complexes for glycolysis and glycogenesis with which HK1 and HK4 (GK) differentially interact. The same is true of the binding to PFK-2/FDPase-2; HK1 apparently, does not bind to the bifunctional enzyme (Baltrusch et al., 2000). We are not the first to ponder such possibilities (Srere et al., 1987; Ureta, 1978), yet we contend that the strength of the recent data and the functional context provided by the in vivo studies of F-2,6-P2 and GK binding, brings new impetus to such considerations (Baltrusch et al., 2001; Wu et al., 2001b). Regardless of whether such subcellular complexes actually exist in the cytosol, the coordinating role of the bifunctional enzyme with regard to activation of PFK-1 (via F-2,6-P 2) and of GK (via protein:protein interaction) described herein is a novel mechanism for the acute response of the liver to re-feeding. From the perspective of molecular switches, the interaction between GK and the bifunctional enzyme brings together two powerful regulators of liver fuel metabolism (Aiston et al., 2001; Matschinsky, 1990; Van Schaftingen et al., 1997), as GK too has substantial switch-like properties, but that discussion is beyond the scope of the current article. The modulation of global lipid metabolism promoted by increased hepatic F-2,6-P2 (Wu et al., 2001b, 2002) may reflect a similar coordinating role for F-2,6-P2 and the bifunctional enzyme isozymes in systemic fuel metabolism. Those discussions, no doubt, shall be the topic of future studies of this enigmatic and powerful regulator of fuel metabolism.
Bibliographical noteFunding Information:
The authors wish to thank the National Institutes of Health (DK38354-16 to AJL) and the Department of Veterans Affairs (Merit Award to DAO) for funding this work.