Kinetic Basis of Carbohydrate-Mediated Inhibition of Human Glucokinase by the Glucokinase Regulatory Protein

Glucokinase (GCK) in diabetes: from molecular mechanisms to disease pathogenesis

Glucokinase (GCK), a key enzyme in glucose metabolism, plays a central role in glucose sensing and insulin secretion in pancreatic β-cells, as well as glycogen synthesis in the liver. Mutations in the GCK gene have been associated with various monogenic diabetes (MD) disorders, including permanent neonatal diabetes mellitus (PNDM) and maturity-onset diabetes of the young (MODY), highlighting its importance in maintaining glucose homeostasis. Additionally, GCK gain-of-function mutations lead to a rare congenital form of hyperinsulinism known as hyperinsulinemic hypoglycemia (HH), characterized by increased enzymatic activity and increased glucose sensitivity in pancreatic β-cells. This review offers a comprehensive exploration of the critical role played by the GCK gene in diabetes development, shedding light on its expression patterns, regulatory mechanisms, and diverse forms of associated monogenic disorders. Structural and mechanistic insights into GCK's involvement in glucose metabolism are discussed, emphasizing its significance in insulin secretion and glycogen synthesis. Animal models have provided valuable insights into the physiological consequences of GCK mutations, although challenges remain in accurately recapitulating human disease phenotypes. In addition, the potential of human pluripotent stem cell (hPSC) technology in overcoming current model limitations is discussed, offering a promising avenue for studying GCK-related diseases at the molecular level. Ultimately, a deeper understanding of GCK's multifaceted role in glucose metabolism and its dysregulation in disease states holds implications for developing targeted therapeutic interventions for diabetes and related disorders.

Research Progress on Saccharide Molecule Detection Based on Nanopores

Saccharides, being one of the fundamental molecules of life, play essential roles in the physiological and pathological functions of cells. However, their intricate structures pose challenges for detection. Nanopore technology, with its high sensitivity and capability for single-molecule-level analysis, has revolutionized the identification and structural analysis of saccharide molecules. This review focuses on recent advancements in nanopore technology for carbohydrate detection, presenting an array of methods that leverage the molecular complexity of saccharides. Biological nanopore techniques utilize specific protein binding or pore modifications to trigger typical resistive pulses, enabling the high-sensitivity detection of monosaccharides and oligosaccharides. In solid-state nanopore sensing, boronic acid modification and pH gating mechanisms are employed for the specific recognition and quantitative analysis of polysaccharides. The integration of artificial intelligence algorithms can further enhance the accuracy and reliability of analyses. Serving as a crucial tool in carbohydrate detection, we foresee significant potential in the application of nanopore technology for the detection of carbohydrate molecules in disease diagnosis, drug screening, and biosensing, fostering innovative progress in related research domains.

Characterization of Glucokinase Catalysis from a Pseudo-Dimeric View

Glucose phosphorylation by glucokinase exhibits a sigmoidal dependency on substrate concentration regardless of its simple structure. Dimorph mechanism suggested the existence of two enzymatic states with different catalytic properties, which has been shown to be plausible by structural analysis. However, the dimorph mechanism gives rise to a complicated or non-explicit non-closed mathematical form. It is neither feasible to apply the dimorph mechanism in effector characterizations. To improve the area of glucokinase study with stronger theoretical support and less complication in computation, we proposed the investigation of the enzyme from a pseudo-dimeric angle. The proposed mechanism started from the idealization of two monomeric glucokinase as a dimeric complex, which significantly simplified the glucose phosphorylation kinetics, while the differences in enzyme reconfiguration caused by variable substrates and effectors have been successfully characterized. The study presented a simpler and more reliable way in studying the properties of glucokinase and its effectors, providing guidelines of effector developments for hyperglycemia and hypoglycemia treatment.

Molecular and cellular regulation of human glucokinase

Glucose metabolism in humans is tightly controlled by the activity of glucokinase (GCK). GCK is predominantly produced in the pancreas, where it catalyzes the rate-limiting step of insulin secretion, and in the liver, where it participates in glycogen synthesis. A multitude of disease-causing mutations within the gck gene have been identified. Activating mutations manifest themselves in the clinic as congenital hyperinsulinism, while loss-of-function mutations produce several diabetic conditions. Indeed, pharmaceutical companies have shown great interest in developing GCK-associated treatments for diabetic patients. Due to its essential role in maintaining whole-body glucose homeostasis, GCK activity is extensively regulated at multiple levels. GCK possesses a unique ability to self-regulate its own activity via slow conformational dynamics, which allows for a cooperative response to glucose. GCK is also subject to a number of protein-protein interactions and post-translational modification events that produce a broad range of physiological consequences. While significant advances in our understanding of these individual regulatory mechanisms have been recently achieved, how these strategies are integrated and coordinated within the cell is less clear. This review serves to synthesize the relevant findings and offer insights into the connections between molecular and cellular control of GCK.

Functional characterization of MODY2 mutations in the nuclear export signal of glucokinase

Glucokinase (GCK) plays a key role in glucose homeostasis. Heterozygous inactivating mutations in the GCK gene cause the familial, mild fasting hyperglycaemia named MODY2. Besides its particular kinetic characteristics, glucokinase is regulated by subcellular compartmentation in hepatocytes. Glucokinase regulatory protein (GKRP) binds to GCK, leading to enzyme inhibition and import into the nucleus at fasting. When glucose concentration increases, GCK-GKRP dissociates and GCK is exported to the cytosol due to a nuclear export signal (NES). With the aim to characterize the GCK-NES, we have functionally analysed nine MODY2 mutations located within the NES sequence. Recombinant GCK mutants showed reduced catalytic activity and, in most cases, protein instability. Most of the mutants interact normally with GKRP, although mutations L306R and L309P impair GCK nuclear import in cotransfected cells. We demonstrated that GCK-NES function depends on exportin 1. We further showed that none of the mutations fully inactivate the NES, with the exception of mutation L304P, which likely destabilizes its α-helicoidal structure. Finally, we found that residue Glu300 negatively modulates the NES activity, whereas other residues have the opposite effect, thus suggesting that some of the NES spacer residues contribute to the low affinity of the NES for exportin 1, which is required for its proper functioning. In conclusion, our results have provided functional and structural insights regarding the GCK-NES and contributed to a better knowledge of the molecular mechanisms involved in the nucleo-cytoplasmic shuttling of glucokinase. Impairment of this regulatory mechanism by some MODY2 mutations might contribute to the hyperglycaemia in the patients.

Antidiabetic Disruptors of the Glucokinase-Glucokinase Regulatory Protein Complex Reorganize a Coulombic Interface

The glucokinase regulatory protein (GKRP) plays an essential role in glucose homeostasis by acting as a competitive inhibitor of glucokinase (GCK) and triggering its localization to the hepatocyte nucleus upon glucose deprivation. Metabolites such as fructose 6-phosphate and sorbitol 6-phosphate promote assembly of the GCK-GKRP complex, whereas fructose 1-phosphate and functionalized piperazines with potent in vivo antidiabetic activity disrupt the complex. Here, we establish the molecular basis by which these natural and synthetic ligands modulate the GCK-GKRP interaction. We demonstrate that a small-molecule disruptor of the protein-protein interaction utilizes a two-step conformational selection mechanism to associate with a rare GKRP conformation constituting 3% of the total population. Conformational heterogeneity of GKRP is localized to the N-terminus and deleting this region eliminates the ability of sorbitol 6-phosphate to promote the GCK-GKRP interaction. Stabilizing ligands favor an extended N-terminus, which sterically positions two arginine residues for optimal Coulombic interaction with a pair of carboxylate side chains from GCK. Conversely, disruptors promote a more compact N-terminus in which an interfacial arginine residue is stabilized in an unproductive orientation through a cation-π interaction with tyrosine 75. Eliminating the ability to sample this binding impaired conformation enhances the intrinsic inhibitory activity of GKRP. Elucidating the molecular basis of ligand-mediated control over the GCK-GKRP interaction is expected to impact the development and future refinement of therapeutic agents for diabetes and cardiovascular disease, which result from improper GKRP regulation of GCK.