Shi Quan Hu

Design of an Active Ultrastable Single-chain Insulin Analog SYNTHESIS, STRUCTURE, AND THERAPEUTIC IMPLICATIONS

Single-chain insulin (SCI) analogs provide insight into the inter-relation of hormone structure, function, and dynamics. Although compatible with wild-type structure, short connecting segments (<3 residues) prevent induced fit upon receptor binding and so are essentially without biological activity. Substantial but incomplete activity can be regained with increasing linker length. Here, we describe the design, structure, and function of a single-chain insulin analog (SCI-57) containing a 6-residue linker (GGGPRR). Native receptor-binding affinity (130 ± 8% relative to the wild type) is achieved as hindrance by the linker is offset by favorable substitutions in the insulin moiety. The thermodynamic stability of SCI-57 is markedly increased (ΔΔGu = 0.7 ± 0.1 kcal/mol relative to the corresponding two-chain analog and 1.9 ± 0.1 kcal/mol relative to wild-type insulin). Analysis of inter-residue nuclear Overhauser effects demonstrates that a native-like fold is maintained in solution. Surprisingly, the glycine-rich connecting segment folds against the insulin moiety: its central Pro contacts ValA3 at the edge of the hydrophobic core, whereas the final Arg extends the A1-A8 α-helix. Comparison between SCI-57 and its parent two-chain analog reveals striking enhancement of multiple native-like nuclear Overhauser effects within the tethered protein. These contacts are consistent with wild-type crystal structures but are ordinarily attenuated in NMR spectra of two-chain analogs, presumably due to conformational fluctuations. Linker-specific damping of fluctuations provides evidence for the intrinsic flexibility of an insulin monomer. In addition to their biophysical interest, ultrastable SCIs may enhance the safety and efficacy of insulin replacement therapy in the developing world.

Supramolecular Protein Engineering DESIGN OF ZINC-STAPLED INSULIN HEXAMERS AS A LONG ACTING DEPOT

Bottom-up control of supramolecular protein assembly can provide a therapeutic nanobiotechnology. We demonstrate that the pharmacological properties of insulin can be enhanced by design of “zinc staples” between hexamers. Paired (i, i+4) His substitutions were introduced at an α-helical surface. The crystal structure contains both classical axial zinc ions and novel zinc ions at hexamer-hexamer interfaces. Although soluble at pH 4, the combined electrostatic effects of the substitutions and bridging zinc ions cause isoelectric precipitation at neutral pH. Following subcutaneous injection in a diabetic rat, the analog effected glycemic control with a time course similar to that of long acting formulation Lantus®. Relative to Lantus, however, the analog discriminates at least 30-fold more stringently between the insulin receptor and mitogenic insulin-like growth factor receptor. Because aberrant mitogenic signaling may be associated with elevated cancer risk, such enhanced specificity may improve safety. Zinc stapling provides a general strategy to modify the pharmacokinetic and biological properties of a subcutaneous protein depot.

Decoding the Cryptic Active Conformation of a Protein by Synthetic Photoscanning INSULIN INSERTS A DETACHABLE ARM BETWEEN RECEPTOR DOMAINS

Proteins evolve in a fitness landscape encompassing a complex network of biological constraints. Because of the interrelation of folding, function, and regulation, the ground-state structure of a protein may be inactive. A model is provided by insulin, a vertebrate hormone central to the control of metabolism. Whereas native assembly mediates storage within pancreatic β-cells, the active conformation of insulin and its mode of receptor binding remain elusive. Here, functional surfaces of insulin were probed by photocross-linking of an extensive set of azido derivatives constructed by chemical synthesis. Contacts are circumferential, suggesting that insulin is encaged within its receptor. Mapping of photoproducts to the hormone-binding domains of the insulin receptor demonstrated alternating contacts by the B-chain β-strand (residues B24-B28). Whereas even-numbered probes (at positions B24 and B26) contact the N-terminal L1 domain of the α-subunit, odd-numbered probes (at positions B25 and B27) contact its C-terminal insert domain. This alternation corresponds to the canonical structure of aβ-strand (wherein successive residues project in opposite directions) and so suggests that the B-chain inserts between receptor domains. Detachment of a receptor-binding arm enables photo engagement of surfaces otherwise hidden in the free hormone. The arm and associated surfaces contain sites also required for nascent folding and self-assembly of storage hexamers. The marked compression of structural information within a short polypeptide sequence rationalizes the diversity of diabetes-associated mutations in the insulin gene. Our studies demonstrate that photoscanning mutagenesis can decode the active conformation of a protein and so illuminate cryptic constraints underlying its evolution.

Toward the Active Conformation of Insulin: Stereospecific modulation of a structural switch in the B chain.

How insulin binds to the insulin receptor has long been a subject of speculation. Although the structure of the free hormone has been extensively characterized, a variety of evidence suggests that a conformational change occurs upon receptor binding. Here, we employ chiral mutagenesis, comparison of corresponding d and l amino acid substitutions, to investigate a possible switch in the B-chain. To investigate the interrelation of structure, function, and stability, isomeric analogs have been synthesized in which an invariant glycine in a β-turn (GlyB8) is replaced by d- or l-Ser. The d substitution enhances stability (ΔΔGu 0.9 kcal/mol) but impairs receptor binding by 100-fold; by contrast, the l substitution markedly impairs stability (ΔΔGu -3.0 kcal/mol) with only 2-fold reduction in receptor binding. Although the isomeric structures each retain a native-like overall fold, the l-SerB8 analog exhibits fewer helix-related and long range nuclear Overhauser effects than does the d-SerB8 analog or native monomer. Evidence for enhanced conformational fluctuations in the unstable analog is provided by its attenuated CD spectrum. The inverse relationship between stereospecific stabilization and receptor binding strongly suggests that the B7-B10 β-turn changes conformation on receptor binding.

Enhancing the activity of a protein by stereospecific unfolding. Conformational life cycle of insulin and its evolutionary origins

A central tenet of molecular biology holds that the function of a protein is mediated by its structure. An inactive ground-state conformation may nonetheless be enjoined by the interplay of competing biological constraints. A model is provided by insulin, well characterized at atomic resolution by x-ray crystallography. Here, we demonstrate that the activity of the hormone is enhanced by stereospecific unfolding of a conserved structural element. A bifunctional β-strand mediates both self-assembly (within β-cell storage vesicles) and receptor binding (in the bloodstream). This strand is anchored by an invariant side chain (PheB24); its substitution by Ala leads to an unstable but native-like analog of low activity. Substitution by d-Ala is equally destabilizing, and yet the protein diastereomer exhibits enhanced activity with segmental unfolding of the β-strand. Corresponding photoactivable derivatives (containing l- or d-para-azido-Phe) cross-link to the insulin receptor with higher d-specific efficiency. Aberrant exposure of hydrophobic surfaces in the analogs is associated with accelerated fibrillation, a form of aggregation-coupled misfolding associated with cellular toxicity. Conservation of PheB24, enforced by its dual role in native self-assembly and induced fit, thus highlights the implicit role of misfolding as an evolutionary constraint. Whereas classical crystal structures of insulin depict its storage form, signaling requires engagement of a detachable arm at an extended receptor interface. Because this active conformation resembles an amyloidogenic intermediate, we envisage that induced fit and self-assembly represent complementary molecular adaptations to potential proteotoxicity. The cryptic threat of misfolding poses a universal constraint in the evolution of polypeptide sequences.

Chiral mutagenesis of insulin.contribution of the B20-B23 β -turn to activity and stability

Insulin contains a β-turn (residues B20-B23) interposed between two receptor-binding elements, the central α-helix of the B chain (B9-B19) and its C-terminal β-strand (B24-B28). The turn contains conserved glycines at B20 and B23. Although insulin exhibits marked conformational variability among crystal forms, these glycines consistently maintain positive φ dihedral angles within a classic type-I β-turn. Because the Ramachandran conformations of GlyB20 and GlyB23 are ordinarily forbidden to l-amino acids, turn architecture may contribute to structure or function. Here, we employ “chiral mutagenesis,” comparison of corresponding d- and l-Ala substitutions, to investigate this turn. Control substitutions are introduced at GluB21, a neighboring residue exhibiting a conventional (negative) φ angle. The d- and l-Ala substitutions at B23 are associated with a marked stereospecific difference in activity. Whereas the d-AlaB23 analog retains native activity, the l analog exhibits a 20-fold decrease in receptor binding. By contrast, d- and l-AlaB20 analogs each exhibit high activity. Stereospecific differences between the thermodynamic stabilities of the analogs are nonetheless more pronounced at B20 (ΔΔGu 2.0 kcal/mole) than at B23 (ΔΔGu 0.7 kcal/mole). Control substitutions at B21 are well tolerated without significant stereospecificity. Chiral mutagenesis thus defines the complementary contributions of these conserved glycines to protein stability (GlyB20) or receptor recognition (GlyB23).

The structure of a mutant insulin uncouples receptor binding from protein allostery. An electrostatic block to the TR transition.

The zinc insulin hexamer undergoes allosteric reorganization among three conformational states, designated T6, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{T}_{3}\mathrm{R}_{3}^{\mathrm{f}}\) \end{document}, and R6. Although the free monomer in solution (the active species) resembles the classical T-state, an R-like conformational change is proposed to occur upon receptor binding. Here, we distinguish between the conformational requirements of receptor binding and the crystallographic TR transition by design of an active variant refractory to such reorganization. Our strategy exploits the contrasting environments of HisB5 in wild-type structures: on the T6 surface but within an intersubunit crevice in R-containing hexamers. The TR transition is associated with a marked reduction in HisB5 pKa, in turn predicting that a positive charge at this site would destabilize the R-specific crevice. Remarkably, substitution of HisB5 (conserved among eutherian mammals) by Arg (occasionally observed among other vertebrates) blocks the TR transition, as probed in solution by optical spectroscopy. Similarly, crystallization of ArgB5-insulin in the presence of phenol (ordinarily a potent inducer of the TR transition) yields T6 hexamers rather than R6 as obtained in control studies of wild-type insulin. The variant structure, determined at a resolution of 1.3Å, closely resembles the wild-type T6 hexamer. Whereas ArgB5 is exposed on the protein surface, its side chain participates in a solvent-stabilized network of contacts similar to those involving HisB5 in wild-type T-states. The substantial receptor-binding activity of ArgB5-insulin (40% relative to wild type) demonstrates that the function of an insulin monomer can be uncoupled from its allosteric reorganization within zinc-stabilized hexamers.

Contribution of residue B5 to the folding and function of insulin and IGF I. Constraints and fine tuning in the evolution of a protein family

Proinsulin exhibits a single structure, whereas insulin-like growth factors refold as two disulfide isomers in equilibrium. Native insulin-related growth factor (IGF)-I has canonical cystines (A6—A11, A7–B7, and A20—B19) maintained by IGF-binding proteins; IGF-swap has alternative pairing (A7–A11, A6—B7, and A20—B19) and impaired activity. Studies of mini-domain models suggest that residue B5 (His in insulin and Thr in IGFs) governs the ambiguity or uniqueness of disulfide pairing. Residue B5, a site of mutation in proinsulin causing neonatal diabetes, is thus of broad biophysical interest. Here, we characterize reciprocal B5 substitutions in the two proteins. In insulin, HisB5 → Thr markedly destabilizes the hormone (ΔΔGu 2.0 ± 0.2 kcal/mol), impairs chain combination, and blocks cellular secretion of proinsulin. The reciprocal IGF-I substitution ThrB5 → His (residue 4) specifies a unique structure with native 1H NMR signature. Chemical shifts and nuclear Overhauser effects are similar to those of native IGF-I. Whereas wild-type IGF-I undergoes thiol-catalyzed disulfide exchange to yield IGF-swap, HisB5-IGF-I retains canonical pairing. Chemical denaturation studies indicate that HisB5 does not significantly enhance thermodynamic stability (ΔΔGu 0.2 ± 0.2 kcal/mol), implying that the substitution favors canonical pairing by destabilizing competing folds. Whereas the activity of ThrB5-insulin is decreased 5-fold, HisB5-IGF-I exhibits 2-fold increased affinity for the IGF receptor and augmented post-receptor signaling. We propose that conservation of ThrB5 in IGF-I, rescued from structural ambiguity by IGF-binding proteins, reflects fine-tuning of signal transduction. In contrast, the conservation of HisB5 in insulin highlights its critical role in insulin biosynthesis.

Deciphering the Hidden Informational Content of Protein Sequences FOLDABILITY OF PROINSULIN HINGES ON A FLEXIBLE ARM THAT IS DISPENSABLE IN THE MATURE HORMONE

Protein sequences encode both structure and foldability. Whereas the interrelationship of sequence and structure has been extensively investigated, the origins of folding efficiency are enigmatic. We demonstrate that the folding of proinsulin requires a flexible N-terminal hydrophobic residue that is dispensable for the structure, activity, and stability of the mature hormone. This residue (PheB1 in placental mammals) is variably positioned within crystal structures and exhibits 1H NMR motional narrowing in solution. Despite such flexibility, its deletion impaired insulin chain combination and led in cell culture to formation of non-native disulfide isomers with impaired secretion of the variant proinsulin. Cellular folding and secretion were maintained by hydrophobic substitutions at B1 but markedly perturbed by polar or charged side chains. We propose that, during folding, a hydrophobic side chain at B1 anchors transient long-range interactions by a flexible N-terminal arm (residues B1–B8) to mediate kinetic or thermodynamic partitioning among disulfide intermediates. Evidence for the overall contribution of the arm to folding was obtained by alanine scanning mutagenesis. Together, our findings demonstrate that efficient folding of proinsulin requires N-terminal sequences that are dispensable in the native state. Such arm-dependent folding can be abrogated by mutations associated with β-cell dysfunction and neonatal diabetes mellitus.