The purification of a detergent-soluble glucose-6-phosphatase from rat liver

Integrative analysis of pathogenic variants in glucose-6-phosphatase based on an AlphaFold2 model

Mediating the terminal reaction of gluconeogenesis and glycogenolysis, the integral membrane protein glucose-6-phosphate catalytic subunit 1 (G6PC1) regulates hepatic glucose production by catalyzing hydrolysis of glucose-6-phosphate (G6P) within the lumen of the endoplasmic reticulum. Consistent with its vital contribution to glucose homeostasis, inactivating mutations in G6PC1 causes glycogen storage disease (GSD) type 1a characterized by hepatomegaly and severe hypoglycemia. Despite its physiological importance, the structural basis of G6P binding to G6PC1 and the molecular disruptions induced by missense mutations within the active site that give rise to GSD type 1a are unknown. In this study, we determine the atomic interactions governing G6P binding as well as explore the perturbations imposed by disease-linked missense variants by subjecting an AlphaFold2 G6PC1 structural model to molecular dynamics simulations and in silico predictions of thermodynamic stability validated with robust in vitro and in situ biochemical assays. We identify a collection of side chains, including conserved residues from the signature phosphatidic acid phosphatase motif, that contribute to a hydrogen bonding and van der Waals network stabilizing G6P in the active site. The introduction of GSD type 1a mutations modified the thermodynamic landscape, altered side chain packing and substrate-binding interactions, and induced trapping of catalytic intermediates. Our results, which corroborate the high quality of the AF2 model as a guide for experimental design and to interpret outcomes, not only confirm the active-site structural organization but also identify previously unobserved mechanistic contributions of catalytic and noncatalytic side chains.

Molecular mechanisms of catalytic inhibition for active site mutations in glucose-6-phosphatase catalytic subunit 1 linked to glycogen storage disease

Mediating the terminal reaction of gluconeogenesis and glycogenolysis, the integral membrane protein G6PC1 regulates hepatic glucose production by catalyzing hydrolysis of glucose-6-phosphate (G6P) within the lumen of the endoplasmic reticulum. Consistent with its vital contribution to glucose homeostasis, inactivating mutations in G6PC1 cause glycogen storage disease (GSD) type 1a characterized by hepatomegaly and severe hypoglycemia. Despite its physiological importance, the structural basis of G6P binding to G6PC1 and the molecular disruptions induced by missense mutations within the active site that give rise to GSD type 1a are unknown. Exploiting a computational model of G6PC1 derived from the groundbreaking structure prediction algorithm AlphaFold2 (AF2), we combine molecular dynamics (MD) simulations and computational predictions of thermodynamic stability with a robust in vitro screening platform to define the atomic interactions governing G6P binding as well as explore the energetic perturbations imposed by disease-linked variants. We identify a collection of side chains, including conserved residues from the signature phosphatidic acid phosphatase motif, that contribute to a hydrogen bonding and van der Waals network stabilizing G6P in the active site. Introduction of GSD type 1a mutations into the G6PC1 sequence elicits changes in G6P binding energy, thermostability and structural properties, suggesting multiple pathways of catalytic impairment. Our results, which corroborate the high quality of the AF2 model as a guide for experimental design and to interpret outcomes, not only confirm active site structural organization but also suggest novel mechanistic contributions of catalytic and non-catalytic side chains.

Biophysical and functional properties of purified glucose-6-phosphatase catalytic subunit 1

Glucose-6-phosphatase catalytic subunit 1 (G6PC1) plays a critical role in hepatic glucose production during fasting by mediating the terminal step of the gluconeogenesis and glycogenolysis pathways. In concert with accessory transport proteins, this membrane-integrated enzyme catalyzes glucose production from glucose-6-phosphate (G6P) to support blood glucose homeostasis. Consistent with its metabolic function, dysregulation of G6PC1 gene expression contributes to diabetes, and mutations that impair phosphohydrolase activity form the clinical basis of glycogen storage disease type 1a. Despite its relevance to health and disease, a comprehensive view of G6PC1 structure and mechanism has been limited by the absence of expression and purification strategies that isolate the enzyme in a functional form. In this report, we apply a suite of biophysical and biochemical tools to fingerprint the in vitro attributes of catalytically active G6PC1 solubilized in lauryl maltose neopentyl glycol (LMNG) detergent micelles. When purified from Sf9 insect cell membranes, the glycosylated mouse ortholog (mG6PC1) recapitulated functional properties observed previously in intact hepatic microsomes and displayed the highest specific activity reported to date. Additionally, our results establish a direct correlation between the catalytic and structural stability of mG6PC1, which is underscored by the enhanced thermostability conferred by phosphatidylcholine and the cholesterol analog cholesteryl hemisuccinate. In contrast, the N96A variant, which blocks N-linked glycosylation, reduced thermostability. The methodologies described here overcome long-standing obstacles in the field and lay the necessary groundwork for a detailed analysis of the mechanistic structural biology of G6PC1 and its role in complex metabolic disorders.

Evidence for glucose-6-phosphate transport in rat liver microsomes

The existence of glucose-6-phosphate transport across the liver microsomal membrane is still controversial. In this paper, we show that S3483, a chlorogenic acid derivative known to inhibit glucose-6-phosphatase in intact microsomes, caused the intravesicular accumulation of glucose-6-phosphate when the latter was produced by glucose-6-phosphatase from glucose and carbamoyl-phosphate. S3483 also inhibited the conversion of glucose-6-phosphate to 6-phosphogluconate occurring inside microsomes in the presence of electron acceptors (NADP or metyrapone). These data indicate that liver microsomal membranes contain a reversible glucose-6-phosphate transporter, which furnishes substrate not only to glucose-6-phosphatase, but also to hexose-6-phosphate dehydrogenase.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.

New lessons in the regulation of glucose metabolism taught by the glucose 6-phosphatase system

The operation of glucose 6-phosphatase (EC 3.1.3.9) (Glc6Pase) stems from the interaction of at least two highly hydrophobic proteins embedded in the ER membrane, a heavily glycosylated catalytic subunit of m 36 kDa (P36) and a 46-kDa putative glucose 6-phosphate (Glc6P) translocase (P46). Topology studies of P36 and P46 predict, respectively, nine and ten transmembrane domains with the N-terminal end of P36 oriented towards the lumen of the ER and both termini of P46 oriented towards the cytoplasm. P36 gene expression is increased by glucose, fructose 2,6-bisphosphate (Fru-2,6-P2) and free fatty acids, as well as by glucocorticoids and cyclic AMP; the latter are counteracted by insulin. P46 gene expression is affected by glucose, insulin and cyclic AMP in a manner similar to P36. Accordingly, several response elements for glucocorticoids, cyclic AMP and insulin regulated by hepatocyte nuclear factors were found in the Glc6Pase promoter. Mutations in P36 and P46 lead to glycogen storage disease (GSD) type-1a and type-1 non a (formerly 1b and 1c), respectively. Adenovirus-mediated overexpression of P36 in hepatocytes and in vivo impairs glycogen metabolism and glycolysis and increases glucose production; P36 overexpression in INS-1 cells results in decreased glycolysis and glucose-induced insulin secretion. The nature of the interaction between P36 and P46 in controling Glc6Pase activity remains to be defined. The latter might also have functions other than Glc6P transport that are related to Glc6P metabolism.

Sepsis-induced depression of rat glucose-6-phosphatase gene expression and activity

Sepsis in rats decreases the hepatic expression of the gluconeogenic enzyme glucose-6-phosphatase (G6Pase). The aim of this study was to investigate the relationship among G6Pase transcription, mRNA, enzymatic activity, and serum glucose levels at different intervals during mild or fulminant sepsis. Both fulminant and mild sepsis immediately decreased hepatic G6Pase mRNA levels. In mild sepsis, levels began to recover late in the time course. Serum glucose levels were maintained in mild sepsis but decreased markedly in fulminant sepsis. G6Pase transcription after fulminant sepsis decreased and never recovered. A similar transcriptional decrease was noted in mild sepsis, but some recovery occurred in this state. Histochemistry after mild sepsis revealed a decrease in G6Pase protein and enzymatic activity that paralleled transcription. These studies suggest that changes in G6Pase transcription and activity are early markers for sepsis-induced alterations in hepatic function. Mechanisms other than gene expression and enzymatic activity serve to maintain glucose levels in mild sepsis, but in the fulminant disorder, compensatory mechanisms fail and hypoglycemia develops.

Evidence that the transit of glucose into liver microsomes is not required for functional glucose-6-phosphatase

We show that the production of glucose from glucose-6-phosphate hydrolysis outside microsomes is a function of glucose-6-phosphatase independent of its property to form glucose inside microsomes. Indeed, during development (before 1 day of age), mouse liver microsomes had glucose-6-phosphatase producing glucose solely outside microsomes. Furthermore, in vivo treatment of rats with the glucocorticoid analogue triamcinolone resulted in increased glucose-6-phosphatase activity outside but not inside microsomes and without change in the catalytic subunit 40 kDa glucose-6-phosphatase mRNA abundance or protein level, indicating that other factors induced by triamcinolone (e.g., altered membrane lipid environment and/or a regulatory protein) were responsible for the activity change. Triamcinolone treatment also lessened the inhibition of glucose-6-phosphatase by pyridoxal 5'-phosphate (PLP), but this effect was not due to an interaction of PLP with the active site. Accordingly, reversal of the inhibition was observed after permeabilization of the microsomes. The two distinct orientations of liver microsomal glucose-6-phosphate phosphohydrolase suggest different physiological roles played by this enzyme in the endoplasmic reticulum membrane.

Chlorogenic acid and synthetic chlorogenic acid derivatives: novel inhibitors of hepatic glucose-6-phosphate translocase

The enzyme system glucose-6-phosphatase (EC 3.1.3.9) plays a major role in the homeostatic regulation of blood glucose. It is responsible for the formation of endogenous glucose originating from gluconeogenesis and glycogenolysis. Recently, chlorogenic acid was identified as a specific inhibitor of the glucose-6-phosphate translocase component (Gl-6-P translocase) of this enzyme system in microsomes of rat liver. Glucose 6-phosphate hydrolysis was determined in the presence of chlorogenic acid or of new synthesized derivatives in intact rat liver microsomes in order to assess the inhibitory potency of the compounds on the translocase component. Variation in the 3-position of chlorogenic acid had only poor effects on inhibitory potency. Introduction of lipophilic side chain in the 1-position led to 100-fold more potent inhibitors. Functional assays on isolated perfused rat liver with compound 29i, a representative of the more potent derivatives, showed a dose-dependent inhibition of gluconeogenesis and glycogenolyosis, suggesting glucose-6-phosphatase as the locus of interference of the compound for inhibition of hepatic glucose production also in the isolated organ model. Gl-6-P translocase inhibitors may be useful for the reduction of inappropriately high rates of hepatic glucose output often found in non-insulin-dependent diabetes.

High levels of glucose-6-phosphatase gene and protein expression reflect an adaptive response in proliferating liver and diabetes

The regenerating liver after partial hepatectomy is one of the few physiologic models of cellular proliferation in the adult animal. During hepatic regeneration, the animal is able to maintain metabolic homeostasis despite the acute loss of two thirds of hepatic tissue. In examining the molecular mechanisms regulating hepatic regeneration, we isolated novel immediate-early genes that are rapidly induced as the remnant liver undergoes the transition from its normal quiescent state into the G1 phase of the cell cycle. One of the most rapidly and highly induced genes which we initially termed RL-1, encodes rat glucose-6-phosphatase (rG6Pase). G6Pase mRNA peaks at 30 min and 36-48 h after hepatectomy correlating with the first and second rounds of cell division. This finding is compatible with studies that showed that G6Pase enzyme activity increases during liver regeneration. However, the increase in G6Pase mRNA is much more dramatic, indicating that it is a more sensitive indicator of this regulation. G6Pase gene expression peaks in the perinatal time period in the liver and remains elevated during the first month of life. The expression of the G6Pase gene is also dramatically elevated in BB diabetic rats, again higher than the enzyme elevation, and its relative induction after partial hepatectomy is blunted in these animals. Insulin treatment of partially hepatectomized diabetic animals downregulates the expression of G6Pase mRNA. Using specific antibodies against G6Pase, we detect a 36-kD G6Pase protein, and its level is elevated in regenerating and diabetic livers. The pattern of G6Pase mRNA expression appears to reflect similar changes in insulin and glucagon levels which accompany diabetes and hepatic proliferation. The elevation of G6Pase expression in these conditions is indicative of its importance as a regulator of glucose homeostasis in normal and abnormal physiologic states.

Mutations in the glucose-6-phosphatase gene are associated with glycogen storage disease types 1a and 1aSP but not 1b and 1c

Glycogen storage disease (GSD) type 1, which is caused by the deficiency of glucose-6-phosphatase (G6Pase), is an autosomal recessive disease with heterogenous symptoms. Two models of G6Pase catalysis have been proposed to explain the observed heterogeneities. The translocase-catalytic unit model proposes that five GSD type 1 subgroups exist which correspond to defects in the G6Pase catalytic unit (1a), a stabilizing protein (1aSP), the glucose-6-P (1b), phosphate/pyrophosphate (1c), and glucose (1d) translocases. Conversely, the conformation-substrate-transport model suggests that G6Pase is a single multifunctional membrane channel protein possessing both catalytic and substrate (or product) transport activities. We have recently demonstrated that mutations in the G6Pase catalytic unit cause GSD type 1a. To elucidate whether mutations in the G6Pase gene are responsible for other GSD type 1 subgroups, we characterized the G6Pase gene of GSD type 1b, 1c, and 1aSP patients. Our results show that the G6Pase gene of GSD type 1b and 1c patients is normal, consistent with the translocase-catalytic unit model of G6Pase catalysis. However, a mutation in exon 2 that converts an Arg at codon 83 to a Cys (R83C) was identified in both G6Pase alleles of the type 1aSP patient. The R83C mutation was also demonstrated in one homozygous and five heterogenous GSD type 1a patients, indicating that type 1aSP is a misclassification of GSD type 1a. We have also analyzed the G6Pase gene of seven additional type 1a patients and uncovered two new mutations that cause GSD type 1a.

Glucose-6-phosphatase proteins of the endoplasmic reticulum

Hepatic glucose-6-phosphatase (G-6-Pase) catalyses the terminal step of hepatic glucose production and it plays a key role in the maintenance of blood glucose homeostasis. Hepatic G-6-Pase is an integral resident endoplasmic reticulum (ER) protein and it is part of a multicomponent system. Its active site is situated inside the lumen of the ER and transport proteins are needed to allow its substrates, glucose-6-phosphate (G-6-P) (and pyrophosphate), and its products, phosphate and glucose to cross the ER membrane. In addition, a calcium-binding protein is also associated with the G-6-Pase enzyme. Recent immunological studies have shown that G-6-Pase (which has conventionally been thought to be present only in the gluconeogenic organs) is present in minor cell types in a variety of human tissues and that its distribution changes dramatically during human development. In all the tissues, enzymatic analysis, direct transport assays and/or immunological detection of the ER glucose and phosphate transport proteins have been used to demonstrate the presence and activity of the whole G-6-Pase system. The G-6-Pase protein is very hydrophobic and has proved difficult to purify to homogeneity. Four proteins of the system have now been isolated and polyclonal antibodies have been raised against them; two have also been cloned. The available sequences, together with topological studies, have given some information about both the topology of the proteins in the ER and the probable mechanisms by which the proteins are retained in the ER.