Bogumil Zelent

Glucokinase activators for diabetes therapy: May 2010 status report.

Type 2 diabetes is characterized by elevated blood glucose levels resulting from a pancreatic β-cell secretory insufficiency combined with insulin resistance, most significantly manifested in skeletal muscle and liver (1). If untreated, diabetic complications develop that cause loss of vision, peripheral neuropathy, impaired kidney function, heart disease, and stroke. The disease has a polygenic basis because numerous genes (the latest count exceeding 20) participate in its pathogenesis, but modern lifestyle characterized by limited physical activity and excessive caloric intake are critical precipitating factors for the current epidemic of type 2 diabetes worldwide (2). It appears that available treatments, including attempts at lifestyle alterations and drug therapies including insulin, are insufficient to stem the tide. Therefore, new approaches, including the development of therapeutic agents with novel mechanisms of action, are needed. Selection of new drug targets to treat type 2 diabetes has to be guided primarily by consideration of established physiological chemistry of glucose homeostasis and of prevailing views about the pathophysiology of type 2 diabetes because the genetics of the disease that could serve as another guiding principle remain prohibitively perplexing. The glucose-phosphorylating enzyme glucokinase (GK) was identified as an outstanding drug target for developing antidiabetic medicines because it has an exceptionally high impact on glucose homeostasis because of its glucose sensor role in pancreatic β-cells and as a rate-controlling enzyme for hepatic glucose clearance and glycogen synthesis, both processes that are impaired in type 2 diabetes (3). Milestones in the 45-year history of GK research are listed in Supplementary Table 1 (Supplementary References S1–S27). In the mid-1990s, Hoffmann La-Roche scientists conducted a high-throughput screen in search of small molecules that could reverse the inhibition of GK by its regulatory protein (GKRP, see further discussion below) and identified a hit molecule that reversed GKRP inhibition by directly stimulating GK (4 …

Research and Development of Glucokinase Activators for Diabetes Therapy: Theoretical and Practical Aspects

Glucokinase Glucokinase (GK GK ; EC 2.7.1.1.) phosphorylates and regulates glucose metabolism in insulin-producing pancreatic beta-cells, hepatocytes, and certain cells of the endocrine and nervous systems allowing it to play a central role in glucose homeostasis glucose homeostasis . Most importantly, it serves as glucose sensor glucose sensor in pancreatic beta-cells mediating glucose-stimulated insulin biosynthesis and release and it governs the capacity of the liver to convert glucose to glycogen. Activating and inactivating mutations of the glucokinase gene cause autosomal dominant hyperinsulinemic hypoglycemia and hypoinsulinemic hyperglycemia in humans, respectively, illustrating the preeminent role of glucokinase in the regulation of blood glucose and also identifying the enzyme as a potential target for developing antidiabetic drugs antidiabetic drugs . Small molecules called glucokinase activators (GKAs) glucokinase activators (GKAs) which bind to an allosteric activator allosteric activator site of the enzyme have indeed been discovered and hold great promise as new antidiabetic agents. GKAs increase the enzymes affinity for glucose and also its maximal catalytic rate. Consequently, they stimulate insulin biosynthesis and secretion, enhance hepatic glucose uptake, and augment glucose metabolism and related processes in other glucokinase-expressing cells. Manifestations of these effects, most prominently a lowering of blood glucose, are observed in normal laboratory animals and man but also in animal models of diabetes and patients with type 2 diabetes mellitus (T2DM T2DM ) type 2 diabetes mellitus (T2DM) . These compelling concepts and results sustain a strong R&D effort by many pharmaceutical companies to generate GKAs with characteristics allowing for a novel drug treatment of T2DM.

Time-resolved and steady-state fluorescence quenching of N-acetyl-L-tryptophanamide by acrylamide and iodide.

We examined the time-resolved and steady-state fluorescence quenching of N-acetyl-L-tryptophanamide (NATA) by acrylamide and iodide, over a range of viscosities in propylene glycol. The quenching of NATA by acrylamide and iodide results in heterogeneity of the intensity decay which increases with the quencher concentration. We attribute the complex decays of NATA to transient effects in diffusion and the nature of the fluorophore-quencher interaction. These data were compared using the phenomenological radiation boundary condition (RBC) and distance-dependent quenching (DDQ) models for collisional quenching. We used global analysis of the time-resolved frequency-domain and steady-state data to select between the models. Consideration of both the frequency-domain and steady state data demonstrate that the quenching rate depends exponentially on the fluorophore-quencher distance, indicating the validity of the DDQ model. The rate constants for acrylamide and iodide quenching, at the constant distance of 5 A, were found to be near 10(13) s-1 and 10(9) s-1, respectively. These rates reflect electron transfer and exchange interactions as the probable quenching mechanisms, respectively.

DISTANCE-DEPENDENT FLUORESCENCE QUENCHING OF TRYPTOPHAN BY ACRYLAMIDE

Abstract We used GHz frequency‐domain fluorometry to investigate the time‐dependent intensity decays of N‐acetyl ‐L‐trytophanamide (NATA) when collisionally quenched by acrylamide in propylene glycol at 20°C. The intensity decays of NATA became increasingly heterogeneous in the presence of acrylamide. The NATA intensity decays were not consistent with the Collins‐Kimball radiation boundary condition (RBC) model for quenching. The steady‐state Stern‐Volmer plots show significant upward curvature. At low temperature in vitrified propylene glycol (‐60%), where translational diffusion cannot occur during the lifetime of the excited state, quenching of NATA by acrylamide was observed. The Smoluchowski and RBC quenching models do not predict any quenching in the absence of translational diffusion. Hence, these frequency‐domain and steady‐state data indicate a through‐space quenching interaction between NATA and acrylamide. The rate for quenching of NAT A by acrylamide appears to depend exponentially on the fluorophore‐quencher separation distance. Comparison of the time‐resolved and steady‐state data provides a sensitive method to determine the distance dependence of the fluorophore‐quencher interaction. The distance‐dependent rate of quenching also explains the upward curvature of the Stern‐Volmer plot, which is often observed for quenching by acrylamide. These results suggest that the distance‐dependent quenching rates need to be considered in the interpretation of quenching data of proteins by acrylamide.

Sugar binding to recombinant wild-type and mutant glucokinase monitored by kinetic measurement and tryptophan fluorescence

Tryptophan fluorescence was used to study GK (glucokinase), an enzyme that plays a prominent role in glucose homoeostasis which, when inactivated or activated by mutations, causes diabetes mellitus or hypoglycaemia in humans. GK has three tryptophan residues, and binding of D-glucose increases their fluorescence. To assess the contribution of individual tryptophan residues to this effect, we generated GST-GK [GK conjugated to GST (glutathione transferase)] and also pure GK with one, two or three of the tryptophan residues of GK replaced with other amino acids (i.e. W99C, W99R, W167A, W167F, W257F, W99R/W167F, W99R/W257F, W167F/W257F and W99R/W167F/W257F). Enzyme kinetics, binding constants for glucose and several other sugars and fluorescence quantum yields (varphi) were determined and compared with those of wild-type GK retaining its three tryptophan residues. Replacement of all three tryptophan residues resulted in an enzyme that retained all characteristic features of GK, thereby demonstrating the unique usefulness of tryptophan fluorescence as an indicator of GK conformation. Curves of glucose binding to wild-type and mutant GK or GST-GK were hyperbolic, whereas catalysis of wild-type and most mutants exhibited co-operativity with D-glucose. Binding studies showed the following order of affinities for the enzyme variants: N-acetyl-D-glucosamine>D-glucose>D-mannose>D-mannoheptulose>2-deoxy-D-glucose>>L-glucose. GK activators increased sugar binding of most enzymes, but not of the mutants Y214A/V452A and C252Y. Contributions to the fluorescence increase from Trp(99) and Trp(167) were large compared with that from Trp(257) and are probably based on distinct mechanisms. The average quantum efficiency of tryptophan fluorescence in the basal and glucose-bound state was modified by activating (Y214A/V452A) or inactivating (C213R and C252Y) mutations and was interpreted as a manifestation of distinct conformational states.

Mutational analysis of allosteric activation and inhibition of glucokinase.

GK (glucokinase) is activated by glucose binding to its substrate site, is inhibited by GKRP (GK regulatory protein) and stimulated by GKAs (GK activator drugs). To explore further the mechanisms of these processes we studied pure recombinant human GK (normal enzyme and a selection of 31 mutants) using steady-state kinetics of the enzyme and TF (tryptophan fluorescence). TF studies of the normal binary GK-glucose complex corroborate recent crystallography studies showing that it exists in a closed conformation greatly different from the open conformation of the ligand-free structure, but indistinguishable from the ternary GK-glucose-GKA complex. GKAs did activate and GKRP did inhibit normal GK, whereas its TF was doubled by glucose saturation. However, the enzyme kinetics, GKRP inhibition, TF enhancement by glucose and responsiveness to GKA of the selected mutants varied greatly. Two predominant response patterns were identified accounting for nearly all mutants: (i) GK mutants with a normal or close to normal response to GKA, normally low basal TF (indicating an open conformation), some variability of kinetic parameters (k(cat), glucose S(0.5), h and ATP K(m)), but usually strong GKRP inhibition (13/31); and (ii) GK mutants that are refractory to GKAs, exhibit relatively high basal TF (indicating structural compaction and partial closure), usually show strongly enhanced catalytic activity primarily due to lowering of the glucose S(0.5), but with reduced or no GKRP inhibition in most cases (14/31). These results and those of previous studies are best explained by envisioning a common allosteric regulator region with spatially non-overlapping GKRP- and GKA-binding sites.

[Aladan3]TIPP: A fluorescent δ-opioid antagonist with high δ-receptor binding affinity and δ selectivity†

Fluorescent analogues of the potent and highly selective δ‐opioid antagonist TIPP (HTyrTicPhePheOH) and TIP (HTyrTicPheOH) containing the exceptionally environmentally sensitive fluorescent amino acid β‐(6′‐dimethylamino‐2′‐naphthoyl)alanine (Aladan [Ald]) in place of Phe3 were synthesized. The Ald3‐ and D‐Ald3 analogues of TIPP and TIP all retained δ‐opioid antagonist properties. The most potent analogue, [Ald3]TIPP, showed a Ke value of 2.03 nM in the mouse vas deferens assay and five times higher δ vs. μ selectivity (K  μi /K  δi = 7930) than the TIPP parent peptide in the opioid receptor binding assays. Theoretical conformational analyses of [Ald3]TIPP and [Ald3]TIP using molecular mechanics calculations resulted in a number of low‐energy conformers, including some showing various patterns of aromatic ring stacking and others with the Ald side chain and a carbonyl group (fluorescence quencher) in close proximity. These ensembles of low‐energy conformers are in agreement with the results of steady‐state fluorescence experiments (fluorescence emission maxima and quantum yields) and fluorescence decay measurements (fluorescence lifetime components), which indicated that the fluorophore was either engaged in intramolecular hydrophobic interactions or in proximity of a fluorescence quencher (e.g., a carbonyl group). These fluorescent TIP(P) δ‐opioid antagonists represent valuable pharmacological tools for various applications, including studies on membrane interactions, binding to receptors, cellular uptake and intracellular distribution, and tissue distribution.

DISTANCE-DEPENDENT QUENCHING OF ANTHRACENE FLUORESCENCE BY N,N-DIETHYLANILINE OBSERVED BY FREQUENCY-DOMAIN FLUOROMETRY

Fluorescence quenching of Nile Blue by amines is thought to be due to electron transfer to the excited dye molecule from the amine electron donor. We used electron transfer quenching of Nile blue byN,N-diethylaniline in propylene glycol as a model system for an interaction which depends exponentially on distance. We investigated the time dependence of the presumed distance-dependent process using gigahertz harmonic-content frequency-domain fluorometry. The frequency-domain data and the steady-state quantum yield were analyzed globally based on either the Smoluchowski-Collins-Kimball radiation boundary condition (RBC) model or the distancedependent quenching (DDQ) model, in which the rate of quenching depends exponentially on the flourophore-quencher distance. We performed a global analysis which included both the frequencydomain time-resolved decays and the steady-state intensities. The latter were found to be particularly sensitive to the model and parameter values. The data cannot be satisfactorily analyzed using the RBC model for quenching. The analysis shows the excellent agreement of the DDQ model with the experimental data, supporting the applicability of the DDQ model to describe the quenching by the electron transfer process, which depends exponentially on the donor-acceptor distance.

Thermal stabilty of glucokinase (GK) as influenced by the substrate glucose, an allosteric glucokinase activator drug (GKA) and the osmolytes glycerol and urea

We investigated how glycerol, urea, glucose and a GKA influence kinetics and stability of wild-type and mutant GK. Glycerol and glucose stabilized GK additively. Glycerol barely affected the TF spectra of all GKs but decreased k(cat), glucose S(0.5) and K(D) values and ATP K(M) while leaving cooperativity unchanged. Glycerol sensitized all GKs to GKA as shown by TF. Glucose increased TF of GKs without influence of glycerol on the effect. Glycerol and GKA affected kinetics and binding additively. The activation energies for thermal denaturation of GK were a function of glucose with K(D)s of 3 and 1mM without and with glycerol, respectively. High urea denatured wild type GK reversibly at 20 and 60°C and urea treatment of irreversibly heat denatured GK allowed refolding as demonstrated by TF including glucose response. We concluded: Glycerol stabilizes GK indirectly without changing the folding structure of the apoenzyme, by restructuring the surface water of the protein, whereas glucose stabilizes GK directly by binding to its substrate site and inducing a compact conformation. Glucose or glycerol (alone or combined) is unable to prevent irreversible heat denaturation above 40°C. However, urea denatures GK reversibly even at 60°C by binding to the protein backbone and directly interacting with hydrophobic side chains. It prevents irreversible aggregation allowing complete refolding when urea is removed. This study establishes the foundation for exploring numerous instability mutants among the more than 600 variant GKs causing diabetes in animals and humans.

Pyrene-1-Carboxylate in Water and Glycerol Solutions: Origin of the Change of pK Upon Excitation

Pyrene-1-carboxy acid has a pK of 4 in the ground state, and a pK of 8 in the excited state. Fluorescence spectra of the acid and base forms are presented as a function of solvent and temperature. Ab initio quantum calculations indicate that the bond between the ring system and the carboxyl group has aromatic character that becomes stronger upon excitation. This stabilization helps to account for the increase in pK upon excitation.