Human insulin analogues modified at the B26 site reveal a hormone conformation that is undetected in the receptor complex

State of the Science on Brain Insulin Resistance and Cognitive Decline Due to Alzheimer's Disease

Type 2 diabetes mellitus (T2DM) is common and increasing in prevalence worldwide, with devastating public health consequences. While peripheral insulin resistance is a key feature of most forms of T2DM and has been investigated for over a century, research on brain insulin resistance (BIR) has more recently been developed, including in the context of T2DM and non-diabetes states. Recent data support the presence of BIR in the aging brain, even in non-diabetes states, and found that BIR may be a feature in Alzheimer's disease (AD) and contributes to cognitive impairment. Further, therapies used to treat T2DM are now being investigated in the context of AD treatment and prevention, including insulin. In this review, we offer a definition of BIR, and present evidence for BIR in AD; we discuss the expression, function, and activation of the insulin receptor (INSR) in the brain; how BIR could develop; tools to study BIR; how BIR correlates with current AD hallmarks; and regional/cellular involvement of BIR. We close with a discussion on resilience to both BIR and AD, how current tools can be improved to better understand BIR, and future avenues for research. Overall, this review and position paper highlights BIR as a plausible therapeutic target for the prevention of cognitive decline and dementia due to AD.

Combinatorial Libraries of Bipodal Binders of the Insulin Receptor

The binding process of insulin to its transmembrane receptor entails a sophisticated interplay between two proteins, each possessing two binding sites. Given the difficulties associated with the use of insulin in the treatment of diabetes, despite its remarkable efficacy, there is interest in smaller and more stable compounds than the native hormone that would effectively activate the receptor. Our study adopts a strategy focused on synthesizing extensive combinatorial libraries of bipodal compounds consisting of two distinct peptides linked to a molecular scaffold. These constructs, evaluated in a resin bead-bound format, were designed to assess their binding to the insulin receptor. Despite notable nonspecific binding, our approach successfully generated and tested millions of compounds. Rigorous evaluations via flow cytometry and specific antibodies revealed peptide sequences with specific interactions at either receptor binding Site 1 or 2. Notably, these sequences bear similarity to peptides discovered through phage display by other researchers. This convergence of chemical and biological methods underscores nature's beauty, revealing general principles in peptide binding to the insulin receptor. Overall, our study deepens the understanding of molecular interactions in ligand binding to the insulin receptor, highlighting the challenges of targeting large proteins with small synthetic peptides.

Omniligase-1-Mediated Phage-Peptide Library Modification and Insulin Engineering

Chemical and enzymatic modifications of peptide-displayed libraries have been successfully employed to expand the phage display library. However, the requirement of specific epitopes and scaffolds has limited the scope of protein engineering using phage display. In this study, we present a novel approach utilizing omniligase-1-mediated selective and specific ligation on the phage pIII protein, offering a high conversion rate and compatibility with commercially available phage libraries. We applied this method to perform high-throughput engineering of insulin analogues with randomized B chain C-terminal regions. Insulin analogues with different B chain C-terminal segments were selected and exhibited biological activity equivalent to that of human insulin. Molecular dynamics studies of insulin analogues revealed a novel interaction between the insulin B27 residue and insulin receptor L1 domain. In summary, our findings highlight the potential of omniligase-1-mediated phage display in the development and screening of disulfide-rich peptides and proteins. This approach holds promise for the creation of novel insulin analogues with enhanced therapeutic properties and exhibits potential for the development of other therapeutic compounds.

Non-glycosylated IGF2 prohormones are more mitogenic than native IGF2

Insulin-like Growth Factor-2 (IGF2) is important for the regulation of human embryonic growth and development, and for adults' physiology. Incorrect processing of the IGF2 precursor, pro-IGF2(156), leads to the formation of two IGF2 proforms, big-IGF2(87) and big-IGF2(104). Unprocessed and mainly non-glycosylated IGF2 proforms are found at abnormally high levels in certain diseases, but their mode of action is still unclear. Here, we found that pro-IGF2(156) has the lowest ability to form its inactivating complexes with IGF-Binding Proteins and has higher proliferative properties in cells than IGF2 and other IGF prohormones. We also showed that big-IGF2(104) has a seven-fold higher binding affinity for the IGF2 receptor than IGF2, and that pro-IGF2(87) binds and activates specific receptors and stimulates cell growth similarly to the mature IGF2. The properties of these pro-IGF2 forms, especially of pro-IGF2(156) and big-IGF2(104), indicate them as hormones that may be associated with human diseases related to the accumulation of IGF-2 proforms in the circulation.

Progress in Simulation Studies of Insulin Structure and Function

Insulin is a peptide hormone known for chiefly regulating glucose level in blood among several other metabolic processes. Insulin remains the most effective drug for treating diabetes mellitus. Insulin is synthesized in the pancreatic β-cells where it exists in a compact hexameric architecture although its biologically active form is monomeric. Insulin exhibits a sequence of conformational variations during the transition from the hexamer state to its biologically-active monomer state. The structural transitions and the mechanism of action of insulin have been investigated using several experimental and computational methods. This review primarily highlights the contributions of molecular dynamics (MD) simulations in elucidating the atomic-level details of conformational dynamics in insulin, where the structure of the hormone has been probed as a monomer, dimer, and hexamer. The effect of solvent, pH, temperature, and pressure have been probed at the microscopic scale. Given the focus of this review on the structure of the hormone, simulation studies involving interactions between the hormone and its receptor are only briefly highlighted, and studies on other related peptides (e.g., insulin-like growth factors) are not discussed. However, the review highlights conformational dynamics underlying the activities of reported insulin analogs and mimetics. The future prospects for computational methods in developing promising synthetic insulin analogs are also briefly highlighted.

Molecular Aspects of Insulin Aggregation and Various Therapeutic Interventions

Protein aggregation leading to the formation of amyloid fibrils has various adverse effects on human health ranging from fatigue and numbness to organ failure and death in extreme cases. Insulin, a peptide hormone commonly used to treat diabetes, undergoes aggregation at the site of repeated injections in diabetic patients as well as during its industrial production and transport. The reduced bioavailability of insulin due to aggregation hinders the proper control of glucose levels in diabetic patients. Thus, it is necessary to develop rational approaches for inhibiting insulin aggregation, which in turn requires a detailed understanding of the mechanism of fibrillation. Given the relative simplicity of insulin and ease of access, insulin has also served as a model system for studying amyloids. Approaches to inhibit insulin aggregation have included the use of natural molecules, synthetic peptides or small molecules, and bacterial chaperone machinery. This review focuses on insulin aggregation with an emphasis on its mechanism, the structural features of insulin fibrils, and the reported inhibitors that act at different stages in the aggregation pathway. We discuss molecules that can serve as leads for improved inhibitors for use in commercial insulin formulations. We also discuss the aggregation propensity of fast- and slow-acting insulin biosimilars, commonly administered to diabetic patients. The development of better insulin aggregation inhibitors and insights into their mechanism of action will not only aid diabetic therapies, but also enhance our knowledge of protein amyloidosis.

Insulin Analogues with Altered Insulin Receptor Isoform Binding Specificities and Enhanced Aggregation Stabilities

Insulin is a lifesaver for millions of diabetic patients. There is a need for new insulin analogues with more physiological profiles and analogues that will be thermally more stable than human insulin. Here, we describe the chemical engineering of 48 insulin analogues that were designed to have changed binding specificities toward isoforms A and B of the insulin receptor (IR-A and IR-B). We systematically modified insulin at the C-terminus of the B-chain, at the N-terminus of the A-chain, and at A14 and A18 positions. We discovered an insulin analogue that has Cα-carboxyamidated Glu at B31 and Ala at B29 and that has a more than 3-fold-enhanced binding specificity in favor of the "metabolic" IR-B isoform. The analogue is more resistant to the formation of insulin fibrils at 37 °C and is also more efficient in mice than human insulin. Therefore, [AlaB29,GluB31,amideB31]-insulin may be interesting for further clinical evaluation.

Analysis of insulin glulisine at the molecular level by X-ray crystallography and biophysical techniques

This study concerns glulisine, a rapid-acting insulin analogue that plays a fundamental role in diabetes management. We have applied a combination of methods namely X-ray crystallography, and biophysical characterisation to provide a detailed insight into the structure and function of glulisine. X-ray data provided structural information to a resolution of 1.26 Å. Crystals belonged to the H3 space group with hexagonal (centred trigonal) cell dimensions a = b = 82.44 and c = 33.65 Å with two molecules in the asymmetric unit. A unique position of D21Glu, not present in other fast-acting analogues, pointing inwards rather than to the outside surface was observed. This reduces interactions with neighbouring molecules thereby increasing preference of the dimer form. Sedimentation velocity/equilibrium studies revealed a trinary system of dimers and hexamers/dihexamers in dynamic equilibrium. This new information may lead to better understanding of the pharmacokinetic and pharmacodynamic behaviour of glulisine which might aid in improving formulation regarding its fast-acting role and reducing side effects of this drug.

Radiolabeled hormones in insulin research, a minireview

Preparation of both 125 I-labeled insulin and insulin-like growth factor 1 (IGF-1) was critical because it enabled a detailed characterization of binding properties of these important hormones towards their cognate transmembrane receptors. Binding modes of hundreds of hormone derivatives were analyzed using competition radioligand binding assays. This effort has resulted in development of six insulin analogs that are today clinically used for the treatment of diabetes. Here, we will briefly summarize a history of insulin research employing iodinated hormones.

A radioligand binding assay for the insulin-like growth factor 2 receptor

Insulin-like growth factors 2 and 1 (IGF2 and IGF1) and insulin are closely related hormones that are responsible for the regulation of metabolic homeostasis, development and growth of the organism. Physiological functions of insulin and IGF1 are relatively well-studied, but information about the role of IGF2 in the body is still sparse. Recent discoveries called attention to emerging functions of IGF2 in the brain, where it could be involved in processes of learning and memory consolidation. It was also proposed that these functions could be mediated by the receptor for IGF2 (IGF2R). Nevertheless, little is known about the mechanism of signal transduction through this receptor. Here we produced His-tagged domain 11 (D11), an IGF2-binding element of IGF2R; we immobilized it on the solid support through a well-defined sandwich, consisting of neutravidin, biotin and synthetic anti-His-tag antibodies. Next, we prepared specifically radiolabeled [125I]-monoiodotyrosyl-Tyr2-IGF2 and optimized a sensitive and robust competitive radioligand binding assay for determination of the nanomolar binding affinities of hormones for D11 of IGF2. The assay will be helpful for the characterization of new IGF2 mutants to study the functions of IGF2R and the development of new compounds for the treatment of neurological disorders.

SQM/COSMO Scoring Function: Reliable Quantum-Mechanical Tool for Sampling and Ranking in Structure-Based Drug Design

Quantum mechanical (QM) methods have been gaining importance in structure-based drug design where a reliable description of protein-ligand interactions is of utmost significance. However, strategies i. e. QM/MM, fragmentation or semiempirical (SQM) methods had to be pursued to overcome the unfavorable scaling of QM methods. Various SQM-based approaches have significantly contributed to the accuracy of docking and improvement of lead compounds. Parametrizations of SQM and implicit solvent methods in our laboratory have been instrumental to obtain a reliable SQM-based scoring function. The experience gained in its application for activity ranking of ligands binding to tens of protein targets resulted in setting up a faster SQM/COSMO scoring approach, which outperforms standard scoring methods in native pose identification for two dozen protein targets with ten thousand poses. Recently, SQM/COSMO was effectively applied in a proof-of-concept study of enrichment in virtual screening. Due to its superior performance, feasibility and chemical generality, we propose the SQM/COSMO approach as an efficient tool in structure-based drug design.

Mutations at hypothetical binding site 2 in insulin and insulin-like growth factors 1 and 2 result in receptor- and hormone-specific responses

Information on how insulin and insulin-like growth factors 1 and 2 (IGF-1 and -2) activate insulin receptors (IR-A and -B) and the IGF-1 receptor (IGF-1R) is crucial for understanding the difference in the biological activities of these peptide hormones. Cryo-EM studies have revealed that insulin uses its binding sites 1 and 2 to interact with IR-A and have identified several critical residues in binding site 2. However, mutagenesis studies suggest that Ile-A10, Ser-A12, Leu-A13, and Glu-A17 also belong to insulin's site 2. Here, to resolve this discrepancy, we mutated these insulin residues and the equivalent residues in IGFs. Our findings revealed that equivalent mutations in the hormones can result in differential biological effects and that these effects can be receptor-specific. We noted that the insulin positions A10 and A17 are important for its binding to IR-A and IR-B and IGF-1R and that A13 is important only for IR-A and IR-B binding. The IGF-1/IGF-2 positions 51/50 and 54/53 did not appear to play critical roles in receptor binding, but mutations at IGF-1 position 58 and IGF-2 position 57 affected the binding. We propose that IGF-1 Glu-58 interacts with IGF-1R Arg-704 and belongs to IGF-1 site 1, a finding supported by the NMR structure of the less active Asp-58–IGF-1 variant. Computational analyses indicated that the aforementioned mutations can affect internal insulin dynamics and inhibit adoption of a receptor-bound conformation, important for binding to receptor site 1. We provide a molecular model and alternative hypotheses for how the mutated insulin residues affect activity.

Insulin and Insulin Receptors in Adipose Tissue Development

Insulin is a major endocrine hormone also involved in the regulation of energy and lipid metabolism via the activation of an intracellular signaling cascade involving the insulin receptor (INSR), insulin receptor substrate (IRS) proteins, phosphoinositol 3-kinase (PI3K) and protein kinase B (AKT). Specifically, insulin regulates several aspects of the development and function of adipose tissue and stimulates the differentiation program of adipose cells. Insulin can activate its responses in adipose tissue through two INSR splicing variants: INSR-A, which is predominantly expressed in mesenchymal and less-differentiated cells and mainly linked to cell proliferation, and INSR-B, which is more expressed in terminally differentiated cells and coupled to metabolic effects. Recent findings have revealed that different distributions of INSR and an altered INSR-A:INSR-B ratio may contribute to metabolic abnormalities during the onset of insulin resistance and the progression to type 2 diabetes. In this review, we discuss the role of insulin and the INSR in the development and endocrine activity of adipose tissue and the pharmacological implications for the management of obesity and type 2 diabetes.

A versatile insulin analog with high potency for both insulin and insulin-like growth factor 1 receptors: Structural implications for receptor binding

Insulin and insulin-like growth factor 1 (IGF-1) are closely related hormones involved in the regulation of metabolism and growth. They elicit their functions through activation of tyrosine kinase–type receptors: insulin receptors (IR-A and IR-B) and IGF-1 receptor (IGF-1R). Despite similarity in primary and three-dimensional structures, insulin and IGF-1 bind the noncognate receptor with substantially reduced affinity. We prepared [d-His^B24, Gly^B31, Tyr^B32]-insulin, which binds all three receptors with high affinity (251 or 338% binding affinity to IR-A respectively to IR-B relative to insulin and 12.4% binding affinity to IGF-1R relative to IGF-1). We prepared other modified insulins with the aim of explaining the versatility of [d-His^B24, Gly^B31, Tyr^B32]-insulin. Through structural, activity, and kinetic studies of these insulin analogs, we concluded that the ability of [d-His^B24, Gly^B31, Tyr^B32]-insulin to stimulate all three receptors is provided by structural changes caused by a reversed chirality at the B24 combined with the extension of the C terminus of the B chain by two extra residues. We assume that the structural changes allow the directing of the B chain C terminus to some extra interactions with the receptors. These unusual interactions lead to a decrease of dissociation rate from the IR and conversely enable easier association with IGF-1R. All of the structural changes were made at the hormones' Site 1, which is thought to interact with the Site 1 of the receptors. The results of the study suggest that merely modifications of Site 1 of the hormone are sufficient to change the receptor specificity of insulin.

Insulin Receptor Isoforms in Physiology and Disease: An Updated View

The insulin receptor (IR) gene undergoes differential splicing that generates two IR isoforms, IR-A and IR-B. The physiological roles of IR isoforms are incompletely understood and appear to be determined by their different binding affinities for insulin-like growth factors (IGFs), particularly for IGF-2. Predominant roles of IR-A in prenatal growth and development and of IR-B in metabolic regulation are well established. However, emerging evidence indicates that the differential expression of IR isoforms may also help explain the diversification of insulin and IGF signaling and actions in various organs and tissues by involving not only different ligand-binding affinities but also different membrane partitioning and trafficking and possibly different abilities to interact with a variety of molecular partners. Of note, dysregulation of the IR-A/IR-B ratio is associated with insulin resistance, aging, and increased proliferative activity of normal and neoplastic tissues and appears to sustain detrimental effects. This review discusses novel information that has generated remarkable progress in our understanding of the physiology of IR isoforms and their role in disease. We also focus on novel IR ligands and modulators that should now be considered as an important strategy for better and safer treatment of diabetes and cancer and possibly other IR-related diseases.

Computational study of the activity, dynamics, energetics and conformations of insulin analogues using molecular dynamics simulations: Application to hyperinsulinemia and the critical residue B26

Due to the increasing prevalence of diabetes, finding therapeutic analogues for insulin has become an urgent issue. While many experimental studies have been performed towards this end, they have limited scope to examine all aspects of the effect of a mutation. Computational studies can help to overcome these limitations, however, relatively few studies that focus on insulin analogues have been performed to date. Here, we present a comprehensive computational study of insulin analogues-three mutant insulins that have been identified with hyperinsulinemia and three mutations on the critical B26 residue that exhibit similar binding affinity to the insulin receptor-using molecular dynamics simulations with the aim of predicting how mutations of insulin affect its activity, dynamics, energetics and conformations. The time evolution of the conformers is studied in long simulations. The probability density function and potential of mean force calculations are performed on each insulin analogue to unravel the effect of mutations on the dynamics and energetics of insulin activation. Our conformational study can decrypt the key features and molecular mechanisms that are responsible for an enhanced or reduced activity of an insulin analogue. We find two key results: 1) hyperinsulinemia may be due to the drastically reduced activity (and binding affinity) of the mutant insulins. 2) Y26_BS and Y26_BE are promising therapeutic candidates for insulin as they are more active than WT-insulin. The analysis in this work can be readily applied to any set of mutations on insulin to guide development of more effective therapeutic analogues.

Structural Perspectives of Insulin Receptor Isoform-Selective Insulin Analogs

A significant drawback of the exogenous administration of insulin to diabetics is the non-physiological profile of insulin action resulting in the insufficient suppression of hepatic glucose production, which is the main contributing factor to diabetic hyperglycemia under fasting conditions and the basis of the challenge to restore a more physiological glucose profile in diabetes. The insulin receptor (IR) exists in two alternatively spliced variants, IR-A and IR-B, with different tissue distribution. While peripheral tissues contain different proportions of both isoforms, hepatic cells almost exclusively contain IR-B. In this respect, IR-B-selective insulin analogs would be of great interest for their potential to restore more natural metabolic homeostasis in diabetes. Recent advances in the structural biology of insulin and IR have provided new clues for understanding the interaction of both proteins. This article discusses and offers some structural perspectives for the design of specific insulin analogs with a preferential binding to IR-B.

Synthesis and Evaluation of a Library of Trifunctional Scaffold-Derived Compounds as Modulators of the Insulin Receptor

We designed a combinatorial library of trifunctional scaffold-derived compounds, which were derivatized with 30 different in-house-made azides. The compounds were proposed to mimic insulin receptor (IR)-binding epitopes in the insulin molecule and bind to and activate this receptor. This work has enabled us to test our synthetic and biological methodology and to prove its robustness and reliability for the solid-phase synthesis and testing of combinatorial libraries of the trifunctional scaffold-derived compounds. Our effort resulted in the discovery of two compounds, which were able to weakly induce the autophosphorylation of IR and weakly bind to this receptor at a 0.1 mM concentration. Despite these modest biological results, which well document the well-known difficulty in modulating protein-protein interactions, this study represents a unique example of targeting the IR with a set of nonpeptide compounds that were specifically designed and synthesized for this purpose. We believe that this work can open new perspectives for the development of next-generation insulin mimetics based on the scaffold structure.

Elucidating the Activation Mechanism of the Insulin-Family Proteins with Molecular Dynamics Simulations

The insulin-family proteins bind to their own receptors, but insulin-like growth factor II (IGF-II) can also bind to the A isoform of the insulin receptor (IR-A), activating unique and alternative signaling pathways from those of insulin. Although extensive studies of insulin have revealed that its activation is associated with the opening of the B chain-C terminal (BC-CT), the activation mechanism of the insulin-like growth factors (IGFs) still remains unknown. Here, we present the first comprehensive study of the insulin-family proteins comparing their activation process and mechanism using molecular dynamics simulations to reveal new insights into their specificity to the insulin receptor. We have found that all the proteins appear to exhibit similar stochastic dynamics in their conformational change to an active state. For the IGFs, our simulations show that activation involves two opening locations: the opening of the BC-CT section away from the core, similar to insulin; and the additional opening of the BC-CT section away from the C domain. Furthermore, we have found that these two openings occur simultaneously in IGF-I, but not in IGF-II, where they can occur independently. This suggests that the BC-CT section and the C domain behave as a unified domain in IGF-I, but as two independent domains in IGF-II during the activation process, implying that the IGFs undergo different activation mechanisms for receptor binding. The probabilities of the active and inactive states of the proteins suggest that IGF-II is hyperactive compared to IGF-I. The hinge residue and the hydrophobic interactions in the core are found to play a critical role in the stability and activity of IGFs. Overall, our simulations have elucidated the crucial differences and similarities in the activation mechanisms of the insulin-family proteins, providing new insights into the molecular mechanisms responsible for the observed differences between IGF-I and IGF-II in receptor binding.

Probing Receptor Specificity by Sampling the Conformational Space of the Insulin-like Growth Factor II C-domain

Insulin and insulin-like growth factors I and II are closely related protein hormones. Their distinct evolution has resulted in different yet overlapping biological functions with insulin becoming a key regulator of metabolism, whereas insulin-like growth factors (IGF)-I/II are major growth factors. Insulin and IGFs cross-bind with different affinities to closely related insulin receptor isoforms A and B (IR-A and IR-B) and insulin-like growth factor type I receptor (IGF-1R). Identification of structural determinants in IGFs and insulin that trigger their specific signaling pathways is of increasing importance in designing receptor-specific analogs with potential therapeutic applications. Here, we developed a straightforward protocol for production of recombinant IGF-II and prepared six IGF-II analogs with IGF-I-like mutations. All modified molecules exhibit significantly reduced affinity toward IR-A, particularly the analogs with a Pro-Gln insertion in the C-domain. Moreover, one of the analogs has enhanced binding affinity for IGF-1R due to a synergistic effect of the Pro-Gln insertion and S29N point mutation. Consequently, this analog has almost a 10-fold higher IGF-1R/IR-A binding specificity in comparison with native IGF-II. The established IGF-II purification protocol allowed for cost-effective isotope labeling required for a detailed NMR structural characterization of IGF-II analogs that revealed a link between the altered binding behavior of selected analogs and conformational rearrangement of their C-domains.

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.

Rational steering of insulin binding specificity by intra-chain chemical crosslinking

Insulin is a key hormone of human metabolism with major therapeutic importance for both types of diabetes. New insulin analogues with more physiological profiles and better glycemic control are needed, especially analogues that preferentially bind to the metabolic B-isoform of insulin receptor (IR-B). Here, we aimed to stabilize and modulate the receptor-compatible conformation of insulin by covalent intra-chain crosslinking within its B22-B30 segment, using the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction of azides and alkynes. This approach resulted in 14 new, systematically crosslinked insulin analogues whose structures and functions were extensively characterized and correlated. One of the analogues, containing a B26-B29 triazole bridge, was highly active in binding to both IR isoforms, with a significant preference for IR-B. Our results demonstrate the potential of chemistry-driven modulation of insulin function, also shedding new light on the functional importance of hormone's B-chain C-terminus for its IR-B specificity.

Molecular Dynamics Simulations of Insulin: Elucidating the Conformational Changes that Enable Its Binding

A sequence of complex conformational changes is required for insulin to bind to the insulin receptor. Recent experimental evidence points to the B chain C-terminal (BC-CT) as the location of these changes in insulin. Here, we present molecular dynamics simulations of insulin that reveal new insights into the structural changes occurring in the BC-CT. We find three key results: 1) The opening of the BC-CT is inherently stochastic and progresses through an open and then a “wide-open” conformation—the wide-open conformation is essential for receptor binding, but occurs only rarely. 2) The BC-CT opens with a zipper-like mechanism, with a hinge at the Phe24 residue, and is maintained in the dominant closed/inactive state by hydrophobic interactions of the neighboring Tyr26, the critical residue where opening of the BC-CT (activation of insulin) is initiated. 3) The mutation Y26N is a potential candidate as a therapeutic insulin analogue. Overall, our results suggest that the binding of insulin to its receptor is a highly dynamic and stochastic process, where initial docking occurs in an open conformation and full binding is facilitated through interactions of insulin receptor residues with insulin in its wide-open conformation.

Structural and functional study of the GlnB22-insulin mutant responsible for maturity-onset diabetes of the young

The insulin gene mutation c.137G>A (R46Q), which changes an arginine at the B22 position of the mature hormone to glutamine, causes the monogenic diabetes variant maturity-onset diabetes of the young (MODY). In MODY patients, this mutation is heterozygous, and both mutant and wild-type (WT) human insulin are produced simultaneously. However, the patients often depend on administration of exogenous insulin. In this study, we chemically synthesized the MODY mutant [GlnB22]-insulin and characterized its biological and structural properties. The chemical synthesis of this insulin analogue revealed that its folding ability is severely impaired. In vitro and in vivo tests showed that its binding affinity and biological activity are reduced (both approximately 20% that of human insulin). Comparison of the solution structure of [GlnB22]-insulin with the solution structure of native human insulin revealed that the most significant structural effect of the mutation is distortion of the B20-B23 β-turn, leading to liberation of the B chain C-terminus from the protein core. The distortion of the B20-B23 β-turn is caused by the extended conformational freedom of the GlnB22 side chain, which is no longer anchored in a hydrogen bonding network like the native ArgB22. The partially disordered [GlnB22]-insulin structure appears to be one reason for the reduced binding potency of this mutant and may also be responsible for its low folding efficiency in vivo. The altered orientation and flexibility of the B20-B23 β-turn may interfere with the formation of disulfide bonds in proinsulin bearing the R46Q (GlnB22) mutation. This may also have a negative effect on the WT proinsulin simultaneously biosynthesized in β-cells and therefore play a major role in the development of MODY in patients producing [GlnB22]-insulin.