A novel complex of a phenolic derivative with insulin: structural features related to the T-->R transition

Mutations at hypothetical binding site 2 in insulin and insulin-like growth factors 1 and 2

Elucidating how insulin and the related insulin-like growth factors 1 and 2 (IGF-1 and IGF-2) bind to their cellular receptors (IR and IGF-1R) and how the receptors are activated has been the holy grail for generations of scientists. However, deciphering the 3D structure of tyrosine kinase receptors and their hormone-bound complexes has been complicated by the flexible and dimeric nature of the receptors and the dynamic nature of their interaction with hormones. Therefore, mutagenesis of hormones and kinetic studies first became an important tool for studying receptor interactions. It was suggested that hormones could bind to receptors through two binding sites on the hormone surface called site 1 and site 2. A breakthrough in knowledge came with the solution of cryoelectron microscopy (cryoEM) structures of hormone-receptor complexes. In this chapter, we document in detail the mutagenesis of insulin, IGF-1, and IGF-2 with emphasis on modifications of the hypothetical binding site 2 in the hormones, and we discuss the results of structure-activity studies in light of recent cryoEM structures of hormone complexes with IR and IGF-1R.

Characterization of insulin crystalline form in isolated β-cell secretory granules

Insulin is stored in vivo inside the pancreatic β-cell insulin secretory granules. In vitro studies have led to an assumption that high insulin and Zn²⁺ concentrations inside the pancreatic β-cell insulin secretory granules should promote insulin crystalline state in the form of Zn²⁺-stabilized hexamers. Electron microscopic images of thin sections of the pancreatic β-cells often show a dense, regular pattern core, suggesting the presence of insulin crystals. However, the structural features of the storage forms of insulin in native preparations of secretory granules are unknown, because of their small size, fragile character and difficult handling. We isolated and investigated the secretory granules from MIN6 cells under near-native conditions, using cryo-electron microscopic (Cryo-EM) techniques. The analysis of these data from multiple intra-granular crystals revealed two different rhomboidal crystal lattices. The minor lattice has unit cell parameters (a ≃ b ≃ 84.0 Å, c ≃ 35.2 Å), similar to in vitro crystallized human 4Zn²⁺-insulin hexamer, whereas the largely prevalent unit cell has more than double c-axis (a ≃ b ≃ c ≃ 96.5 Å) that probably corresponds to two or three insulin hexamers in the asymmetric unit. Our experimental data show that insulin can be present in pancreatic MIN6 cell granules in a microcrystalline form, probably consisting of 4Zn²⁺-hexamers of this hormone.

Resveratrol as a nontoxic excipient stabilizes insulin in a bioactive hexameric form

Insulin aggregation is the leading cause of considerable reduction in the amount of active drug molecules in liquid formulations manufactured for diabetes management. Phenolic compounds, such as phenol and m-cresol, are routinely used to stabilize insulin in a hexameric form during its commercial preparation. However, long term usage of commercial insulin results in various adverse secondary responses, for which toxicity of the phenolic excipients is primarily responsible. In this study we aimed to find out a nontoxic insulin stabilizer. To that end, we have selected resveratrol, a natural polyphenol, as a prospective nontoxic insulin stabilizer because of its structural similarity with commercially used phenolic compounds. Atomic force microscopy visualization of resveratrol-treated human insulin revealed that resveratrol has a unique ability to arrest hINS in a soluble oligomeric form having discrete spherical morphology. Most importantly, resveratrol-treated insulin is nontoxic for HepG2 cells and it effectively maintains low blood glucose in a mouse model. Cryo-electron microscopy revealed 3D morphology of resveratrol-stabilized insulin that strikingly resembles crystal structures of insulin hexamer formulated with m-cresol. Significantly, we found that, in a condition inductive to amyloid fibrillation at physiological pH, resveratrol is capable of stabilizing insulin more efficiently than m-cresol. Thus, this study describes resveratrol as an effective nontoxic natural molecule that can be used for stabilizing insulin in a bioactive oligomeric form during its commercial formulation.

Stable Monomeric Insulin Formulations Enabled by Supramolecular PEGylation of Insulin Analogues

Current "fast-acting" insulin analogues contain amino acid modifications meant to inhibit dimer formation and shift the equilibrium of association states toward the monomeric state. However, the insulin monomer is highly unstable and current formulation techniques require insulin to primarily exist as hexamers to prevent aggregation into inactive and immunogenic amyloids. Insulin formulation excipients have thus been traditionally selected to promote insulin association into the hexameric form to enhance formulation stability. This study exploits a novel excipient for the supramolecular PEGylation of insulin analogues, including aspart and lispro, to enhance the stability and maximize the prevalence of insulin monomers in formulation. Using multiple techniques, it is demonstrated that judicious choice of formulation excipients (tonicity agents and parenteral preservatives) enables insulin analogue formulations with 70-80% monomer and supramolecular PEGylation imbued stability under stressed aging for over 100 h without altering the insulin association state. Comparatively, commercial "fast-acting" formulations contain less than 1% monomer and remain stable for only 10 h under the same stressed aging conditions. This simple and effective formulation approach shows promise for next-generation ultrafast insulin formulations with a short duration of action that can reduce the risk of post-prandial hypoglycemia in the treatment of diabetes.

In vitro assessment of the influence of intravenous extension set materials on insulin aspart drug delivery

Insulin is a frequently prescribed drug in hospitals and is usually administered by syringe pumps with an extension line which can be made of various materials. Two insulin solutions were studied: an insulin analogue, Novorapid^® which contains insulin aspart and two phenolic preservatives (e.g. phenol and metacresol) and Umuline rapide^® with human insulin and metacresol as preservative. Some studies have indicated interactions between insulin, polyvinyl chloride (PVC) and polyethylene (PE). The aim of this work was to study such interactions between Novorapid^® or Umuline rapide^® and infusion extension line materials (PVC, PE and coextruded (PE/PVC)). Insulin solution at 1 IU/mL was infused at 2 mL/h over 24 hours with 16 different extension lines (8 in PVC, 3 in PE and 5 in PE/PVC). Ultra-Fast Liquid Chromatography with diode array detection (UFLC-DAD) was performed to quantify insulin (human and aspart) and preservatives (metacresol and phenol). Limited human insulin sorption was observed thirty minutes after the onset of infusion: 24.3 ± 12.9%, 3.1 ± 1.6% and 18.6 ± 10.0% for PVC, PE and PE/PVC respectively. With insulin aspart, sorption of about 5% was observed at the onset of infusion for all materials. However, there were interactions between phenol and especially metacresol with PVC, but no interactions with PE and PE/PVC. This study shows that insulin interacts with PVC, PE and PE/PVC at the onset of infusion. It also demonstrates that insulin preservatives interact with PVC, which may result in problems of insulin conservation and conformation. Some more studies are required to understand the clinical impact of the latter during infusion.

In Quest for Improved Drugs against Diabetes: The Added Value of X-ray Powder Diffraction Methods

Human insulin (HI) is a well-characterized natural hormone which regulates glycose levels into the blood-stream and is widely used for diabetes treatment. Numerous studies have manifested that despite significant efforts devoted to structural characterization of this molecule and its complexes with organic compounds (ligands), there is still a rich diagram of phase transitions and novel crystalline forms to be discovered. Towards the improvement of drug delivery, identification of new insulin polymorphs from polycrystalline samples, simulating the commercially available drugs, is feasible today via macromolecular X-ray powder diffraction (XRPD). This approach has been developed, and is considered as a respectable method, which can be employed in biosciences for various purposes, such as observing phase transitions and characterizing bulk pharmaceuticals. An overview of the structural studies on human insulin complexes performed over the past decade employing both synchrotron and laboratory sources for XRPD measurements, is reported herein. This review aims to assemble all of the recent advances in the diabetes treatment field in terms of drug formulation, verifying in parallel the efficiency and applicability of protein XRPD for quick and accurate preliminary structural characterization in the large scale.

Computational and structural evidence for neurotransmitter-mediated modulation of the oligomeric states of human insulin in storage granules

Human insulin is a pivotal protein hormone controlling metabolism, growth, and aging and whose malfunctioning underlies diabetes, some cancers, and neurodegeneration. Despite its central position in human physiology, the in vivo oligomeric state and conformation of insulin in its storage granules in the pancreas are not known. In contrast, many in vitro structures of hexamers of this hormone are available and fall into three conformational states: T6, T3Rf3, and R6 As there is strong evidence for accumulation of neurotransmitters, such as serotonin and dopamine, in insulin storage granules in pancreatic β-cells, we probed by molecular dynamics (MD) and protein crystallography (PC) if these endogenous ligands affect and stabilize insulin oligomers. Parallel studies independently converged on the observation that serotonin binds well within the insulin hexamer (site I), stabilizing it in the T3R3 conformation. Both methods indicated serotonin binding on the hexamer surface (site III) as well. MD, but not PC, indicated that dopamine was also a good site III ligand. Some of the PC studies also included arginine, which may be abundant in insulin granules upon processing of pro-insulin, and stable T3R3 hexamers loaded with both serotonin and arginine were obtained. The MD and PC results were supported further by in solution spectroscopic studies with R-state-specific chromophore. Our results indicate that the T3R3 oligomer is a plausible insulin pancreatic storage form, resulting from its complex interplay with neurotransmitters, and pro-insulin processing products. These findings may have implications for clinical insulin formulations.

Contribution of TyrB26 to the Function and Stability of Insulin: STRUCTURE-ACTIVITY RELATIONSHIPS AT A CONSERVED HORMONE-RECEPTOR INTERFACE

Crystallographic studies of insulin bound to receptor domains have defined the primary hormone-receptor interface. We investigated the role of Tyr(B26), a conserved aromatic residue at this interface. To probe the evolutionary basis for such conservation, we constructed 18 variants at B26. Surprisingly, non-aromatic polar or charged side chains (such as Glu, Ser, or ornithine (Orn)) conferred high activity, whereas the weakest-binding analogs contained Val, Ile, and Leu substitutions. Modeling of variant complexes suggested that the B26 side chains pack within a shallow depression at the solvent-exposed periphery of the interface. This interface would disfavor large aliphatic side chains. The analogs with highest activity exhibited reduced thermodynamic stability and heightened susceptibility to fibrillation. Perturbed self-assembly was also demonstrated in studies of the charged variants (Orn and Glu); indeed, the Glu(B26) analog exhibited aberrant aggregation in either the presence or absence of zinc ions. Thus, although Tyr(B26) is part of insulin's receptor-binding surface, our results suggest that its conservation has been enjoined by the aromatic ring's contributions to native stability and self-assembly. We envisage that such classical structural relationships reflect the implicit threat of toxic misfolding (rather than hormonal function at the receptor level) as a general evolutionary determinant of extant protein sequences.

Human insulin polymorphism upon ligand binding and pH variation: the case of 4-ethylresorcinol

This study focuses on the effects of the organic ligand 4-ethylresorcinol on the crystal structure of human insulin using powder X-ray crystallography. For this purpose, systematic crystallization experiments have been conducted in the presence of the organic ligand and zinc ions within the pH range 4.50-8.20, while observing crystallization behaviour around the isoelectric point of insulin. High-throughput crystal screening was performed using a laboratory X-ray diffraction system. The most representative samples were selected for synchrotron X-ray diffraction measurements, which took place at the European Synchrotron Radiation Facility (ESRF) and the Swiss Light Source (SLS). Four different crystalline polymorphs have been identified. Among these, two new phases with monoclinic symmetry have been found, which are targets for the future development of microcrystalline insulin drugs.

Theoretical and computational studies of peptides and receptors of the insulin family

Synergistic interactions among peptides and receptors of the insulin family are required for glucose homeostasis, normal cellular growth and development, proliferation, differentiation and other metabolic processes. The peptides of the insulin family are disulfide-linked single or dual-chain proteins, while receptors are ligand-activated transmembrane glycoproteins of the receptor tyrosine kinase (RTK) superfamily. Binding of ligands to the extracellular domains of receptors is known to initiate signaling via activation of intracellular kinase domains. While the structure of insulin has been known since 1969, recent decades have seen remarkable progress on the structural biology of apo and liganded receptor fragments. Here, we review how this useful structural information (on ligands and receptors) has enabled large-scale atomically-resolved simulations to elucidate the conformational dynamics of these biomolecules. Particularly, applications of molecular dynamics (MD) and Monte Carlo (MC) simulation methods are discussed in various contexts, including studies of isolated ligands, apo-receptors, ligand/receptor complexes and intracellular kinase domains. The review concludes with a brief overview and future outlook for modeling and computational studies in this family of proteins.

X-ray investigation of gene-engineered human insulin crystallized from a solution containing polysialic acid

Attempts to crystallize the noncovalent complex of recombinant human insulin with polysialic acid were carried out under normal and microgravity conditions. Both crystal types belonged to the same space group, I2(1)3, with unit-cell parameters a = b = c = 77.365 A, alpha = beta = gamma = 90.00 degrees. The reported space group and unit-cell parameters are almost identical to those of cubic insulin reported in the PDB. The results of X-ray studies confirmed that the crystals obtained were cubic insulin crystals and that they contained no polysialic acid or its fragments. Electron-density maps were calculated using X-ray diffraction sets from earth-grown and microgravity-grown crystals and the three-dimensional structure of the insulin molecule was determined and refined. The conformation and secondary-structural elements of the insulin molecule in different crystal forms were compared.

Ligand escape pathways and (un)binding free energy calculations for the hexameric insulin-phenol complex

Cooperative binding of phenolic species to insulin hexamers is known to stabilize pharmaceutical preparations of the hormone. Phenol dissociation is rapid on hexamer dissolution timescales, and phenol unbinding upon dilution is likely the first step in the conversion of (pharmaceutical) hexameric insulin to the active monomeric form upon injection. However, a clear understanding of the determinants of the rates of phenol unbinding remains obscure, chiefly because residues implicated in phenol exchange as determined by NMR are not all associated with likely unbinding routes suggested by the best-resolved hexamer structures. We apply random acceleration molecular dynamics simulation to identify potential escape routes of phenol from hydrophobic cavities in the hexameric insulin-phenol complex. We find three major pathways, which provide new insights into (un)binding mechanisms for phenol. We identify several residues directly participating in escape events that serve to resolve ambiguities from recent NMR experiments. Reaction coordinates for dissociation of phenol are developed based on these exit pathways. Potentials of mean force along the reaction coordinate for each pathway are resolved using multiple independent steered molecular dynamics simulations with second-order cumulant expansion of Jarzynski's equality. Our results for DeltaF agree reasonably well within the range of known experimental and previous simulation magnitudes of this quantity. Based on structural analysis and energetic barriers for each pathway, we suggest a plausible preferred mechanism of phenolic exchange that differs from previous mechanisms. Several weakly-bound metastable states are also observed for the first time in the phenol dissociation reaction.

The crystallization of biological macromolecules under microgravity: a way to more accurate three-dimensional structures?

The crystallization of proteins and other biological particles (including nucleic acids, nucleo-protein complexes and large assemblies such as nucleosomes, ribosomal subunits or viruses) in a microgravity environment can produce crystals having lesser defects than crystals prepared under normal gravity on earth. Such microgravity-grown crystals can diffract X-rays to a higher resolution and have a lower mosaic spread. The inferred electron density maps can be richer in details owing to which more accurate three-dimensional structure models can be built. Major results reported in this field of research are reviewed. Novel ones obtained with the Advanced Protein Crystallization Facility are presented. For structural biology, practical applications and implications associated with crystallization and crystallography onboard the International Space Station are discussed.

Preparation of a microcrystalline suspension formulation of Lys(B28)Pro(B29)-human insulin with ultralente properties

The monomeric analogue, Lys(B28)Pro(B29)-human insulin (LysPro), has been crystallized using similar conditions employed to prepare extended-acting insulin ultralente formulations. In the presence of zinc ions, sodium acetate and sodium chloride, but without phenolic preservative, LysPro surprisingly forms small rhombohedral crystals with similar morphology to human insulin ultralente crystals with a mean particle size of 20 +/- 1 microm. X-ray powder diffraction studies on the LysPro crystals prior to dilution in ultralente vehicle ([NaCl] = 1.2 M) revealed the presence of T(3)R(3)(f) hexamers. Consistent with human insulin ultralente preparations, LysPro crystals formulated as an ultralente suspension ([NaCl] = 0. 12 M) contain T(6) hexamers indicating that a conformational change occurs in the hexamer units of the crystals upon dilution of the salt concentration. The pharmacological properties of subcutaneously administered ultralente LysPro (ULP) were compared to ultralente human insulin (UHI) using a conscious dog model (n = 5) with glucose levels clamped at basal. There were no statistically significant differences between the kinetic and dynamic responses of ULP compared to UHI [C(max) (ng/mL): 3.58 +/- 0.76, ULP and 3.61 +/- 0. 66, UHI; T(max) (min): 226 +/- 30, ULP and 185 +/- 42, UHI; R(max) (mg/kg min): 11.2 +/- 1.9, ULP and 13.3 +/- 2.0, UHI; and T(Rmax) (min): 336 +/- 11, ULP and 285 +/- 57, UHI]. Although the Pro to Lys sequence inversion destabilizes insulin self-assembly and greatly alters the time action of soluble LysPro preparations, this modification has now been found neither to prevent the formation of ultralente crystals in the absence of phenolics nor to compromise the protracted activity of the insulin analogue suspension.

Quaternary structure of the insulin-insulin receptor complex

The three-dimensional (3D) structure of the intrinsically dimeric insulin receptor bound to its ligand, insulin, was determined by electron cryomicroscopy. Gold-labeled insulin served to locate the insulin-binding domain. The 3D structure was then fitted with available known high-resolution domain substructures to obtain a detailed contiguous model for this heterotetrameric transmembrane receptor. The 3D reconstruction indicates that the two alpha subunits jointly participate in insulin binding and that the kinase domains in the two beta subunits are in a juxtaposition that permits autophosphorylation of tyrosine residues in the first step of insulin receptor activation.

Binding of phenol to R6 insulin hexamers

Small amounts of phenolic compounds are being used as preservatives in pharmaceutical insulin preparations. It has been shown previously that these compounds bind to specific sites on the insulin hexamer and act as allosteric effectors, inducing a transformation of the T6 hexamer to the R6 hexamer, via a T3R3 intermediate. In this article, the crystal structures of eight different insulin derivatives, all in the phenol-containing R6 form, are analyzed with respect to their phenol-binding sites. While six phenol molecules are normally bound per insulin hexamer, one of the engineered insulins appears to contain only three phenols but yet exists in an R6 conformation. This observation provides additional evidence for an inherent nonequivalence of the two trimers in the insulin hexamer. The unusual observation of a seventh phenol molecule bound to the hexamer of crystalline A21Gly-B31,B32Arg2 insulin (HOE 901), a long-acting derivative currently undergoing phase III clinical trials, provides a partial explanation for its protracted activity.

Three-dimensional structure analysis of PROSITE patterns

Pattern matches for each of the sequence patterns in PROSITE, a database of sequence patterns, were searched in all protein sequences in the Brookhaven Protein Data Bank (PDB). The three-dimensional structures of the pattern matches for the 20 patterns with the largest numbers of hits were analysed. We found that the true positives have a common three-dimensional structure for each pattern; the structures of false positives, found for six of the 20 patterns, were clearly different from those of the true positives. The results suggest that the true pattern matches each have a characteristic common three-dimensional structure, which could be used to create a template to define a three-dimensional functional pattern.

Insulin and insulin mutants stimulate glucose uptake in rat adipocytes

A simple method to determine the in vitro biological activity of insulin by measuring glucose uptake in the rat adipocytes is presented here. In the presence of insulin, the glucose uptake is 5-6 times more than the basal control. And the uptake of D-[3-(3)H]-glucose is linear as the logarithm of insulin concentration from 0.2 microg/L to 1.0 microg/L. Glucose and 3-O-methyl-glucose inhibit D-[3-(3)H]-glucose uptake into adipocytes. By this method, the in vitro biological activity of [B(2)-Lys]-insulin and [B(3)-Lys]-insulin was measured to be 61.6% and 154% respectively, relative to that of insulin.