Chiral mutagenesis of insulin. Contribution of the B20-B23 beta-turn to activity and stability

Insulin contains a beta-turn (residues B20-B23) interposed between two receptor-binding elements, the central alpha-helix of the B chain (B9-B19) and its C-terminal beta-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 phi dihedral angles within a classic type-I beta-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) phi 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 (delta deltaG(u) 2.0 kcal/mole) than at B23 (delta deltaG(u) 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).     

Shortened insulin analogues: marked changes in biological activity resulting from replacement of TyrB26 and N-methylation of peptide bonds in the C-terminus of the B-chain

The role of three highly conserved insulin residues PheB24, PheB25, and TyrB26 was studied to better understand the subtleties of the structure-function relationship between insulin and its receptor. Ten shortened insulin analogues with modifications in the beta-strand of the B-chain were synthesized by trypsin-catalyzed coupling of des-octapeptide (B23-B30)-insulin with synthetic peptides. Insulin analogues with a single amino acid substitution in the position B26 and/or single N-methylation of the peptide bond at various positions were all shortened in the C-terminus of the B-chain by four amino acids. The effect of modifications was followed by two types of in vitro assays, i.e., by the binding to the receptor of rat adipose plasma membranes and by the stimulation of the glucose transport into the isolated rat adipocytes. From our results, we can deduce several conclusions: (i) the replacement of tyrosine in the position B26 by phenylalanine has no significant effect on the binding affinity and the stimulation of the glucose transport of shortened analogues, whereas the replacement of TyrB26 by histidine affects the potency highly positively; [HisB26]-des-tetrapeptide (B27-B30)-insulin-B26-amide and [NMeHisB26]-des-tetrapeptide (B27-B30)-insulin-B26-amide show binding affinity 529 and 5250%, respectively, of that of human insulin; (ii) N-methylation of the B24-B25 peptide bond exhibits a disruptive effect on the potency of analogues in both in vitro studies regardless the presence of amino acid in the position B26; (iii) N-methylation of the B23-B24 peptide bond markedly reduces the binding affinity and the glucose transport of respective analogue [NMePheB24]-des-tetrapeptide (B27-B30)-insulin-B26-amide.     

Evolution of the insulin molecule: insights into structure-activity and phylogenetic relationships.

The conformation of insulin in the crystalline state has been known for more than 30 years but there remains uncertainty regarding the biologically active conformation and the structural features that constitute the receptor-binding domain. The primary structure of insulin has been determined for at least 100 vertebrate species. In addition to the invariant cysteines, only ten amino acids (GlyA1, IleA2, ValA3, TyrA19, LeuB6, GlyB8, LeuB11, ValB12, GlyB23 and PheB24) have been fully conserved during vertebrate evolution. This observation supports the hypothesis derived from alanine-scanning mutagenesis studies that five of these invariant residues (IleA2, ValA3, TyrA19, GlyB23, and Phe24) interact directly with the receptor and five additional conserved residues (LeuB6, GlyB8, LeuB11, GluB13 and PheB25) are important in maintaining the receptor-binding conformation. With the exception of the hagfish, only conservative substitutions are found at B13 (Glu --> Asp) and B25(Phe --> Tyr). In contrast, amino acid residues that were also considered to be important in receptor binding based upon the crystal structure of insulin (GluA4, GlnA5, AsnA21, TyrB16, TyrB26) have been much less well conserved and are probably not components of the receptor-binding domain. The hypothesis that LeuA13 and LeuB17 form part of a second receptor-binding site in the insulin molecule finds some support in terms of their conservation during vertebrate evolution, although the site is probably absent in some hystricomorph insulins. In general, the amino acid sequences of insulins are not useful in cladistic analyses especially when evolutionary distant taxa are compared but, among related species in a particular order or family, the presence of unusual structural features in the insulin molecule may permit a meaningful phylogenetic inference. For example, analysis of insulin sequences supports monophyletic status for Dipnoi, Elasmobranchii, Holocephali and Petromyzontiformes.     

A new B-chain mutant of insulin: comparison with the insulin crystal structure and role of sulfonate groups in the B-chain structure.

The solution structure of a new B-chain mutant of bovine insulin, in which the cysteines B7 and B19 are replaced by two serines, has been determined by circular dichroism, 2D-NMR and molecular modeling. This structure is compared with that of the oxidized B-chain of bovine insulin [Hawkins et al. (1995) Int. J. Peptide Protein Res.46, 424-433]. Circular dichroism spectroscopy showed in particular that a higher percentage of helical secondary structure for the B-chain mutant is estimated in trifluoroethanol solution in comparison with the oxidized B-chain. 2D-NMR experiments confirmed, among multiple conformations, that the B-chain mutant presents defined secondary structures such as a alpha-helix between residues B9 and B19, and a beta-turn between amino acids B20 and B23 in aqueous trifluoroethanol. The 3D structures, which are consistent with NMR data and were obtained using a simulated annealing protocol, showed that the tertiary structure of the B-chain mutant is better resolved and is more in agreement with the insulin crystal structure than the oxidized B-chain structure described by Hawkins et al. An explanation could be the presence of two sulfonate groups in the oxidized insulin B-chain. Either by their charges and/or their size, such chemical groups could play a destructuring effect and thus could favor peptide flexibility and conformational averaging. Thus, this study provides new insights on the folding of isolated B-chains.     

Crystal structure of destripeptide (B28-B30) insulin: implications for insulin dissociation

Destripeptide (B28-B30) insulin (DTRI) is an insulin analogue that has much weaker association ability than native insulin but keeps most of its biological activity. It can be crystallized from a solution containing zinc ions at near-neutral pH. Its crystal structure has been determined by molecular replacement and refined at 1.9 A resolution. DTRI in the crystal exists as a loose hexamer compared with 2Zn insulin. The hexamer only contains one zinc ion that coordinates to the B10 His residues of three monomers. Although residues B28-B30 are located in the monomer-monomer interface within a dimer, the removal of them can simultaneously weaken both the interactions between monomers within the dimer and the interactions between dimers. Because the B-chain C-terminus of insulin is very flexible, we take the DTRI hexamer as a transition state in the native insulin dissociation process and suggest a possible dissociation process of the insulin hexamer based on the DTRI structure.     

Semisynthetic des-(B27-B30)-insulins with modified B26-tyrosine

Semisynthetic des-(B27-B30)-insulins containing modified B26-tyrosine residues were prepared to refine the understanding of the importance of position B26 with regard to biological and structural properties of the hormone. The following shortened insulin analogues were synthesized by trypsin-catalysed peptide-bond formation between the C-terminal amino acid ArgB22 of des-(B23-B30)-insulin and synthetic tetrapeptides as amino components: des-(B27-B30)-insulin, des-(B27-B30)-insulin-B26-methyl ester, -B26-carboxamide with varying C-terminal hydrophobicity of the B-chain, and [Tyr(NH2)B26]-, [Tyr(NO2)B26]-, [Tyr(I2)B26]-, [D-TyrB26]des-(B27-B30)-insulin-B26-carboxamide containing non-proteinogenic amino acids in position B26. Starting from insulin and an excess of synthetic Gly-Phe-Phe-Tyr-OMe as nucleophile, des-(B27-B30)-insulin-B26-methyl ester--the formal transpeptidation product at ArgB22--was formed in one step. Biological in vitro properties (binding to cultured human IM-9 lymphocytes, relative lipogenic potency in isolated rat adipocytes) of all semisynthetic analogues are reported, ranging from slightly decreased to two-fold receptor affinity and nearly three-fold biopotency relative to insulin. If the C-terminal tetrapeptide B27-B30 is removed, full relative insulin activity is still preserved, while the shortening results in the loss of ability to associate in solution. Only after carboxamidation or methyl esterification of TyrB26 the self-association typical of native insulin can be observed, and the CD-spectral effects in the near UV spectrum related to association and hexamerization of the native hormone are qualitatively reestablished. The results of this investigation underline the importance of position B26 to the modulation of hormonal properties and solution structure of the shortened insulins.     

Role of the phenylalanine B24 side chain in directing insulin interaction with its receptor. Importance of main chain conformation

We have investigated (by use of semisynthetic insulin analogs and isolated canine hepatocytes) the role of invariant residue PheB24 in determining the affinity of insulin-receptor interactions. Our results confirm that replacement of PheB24 by D-Phe is not detrimental to ligand binding to receptor, show that D-Ala is well tolerated at position B24 (whereas Ala is not), and demonstrate that [GlyB24]insulin retains as much as 78% of the receptor binding potency of native insulin. Additional findings show that replacement of PheB24 by D-Pro or by alpha-aminoisobutyric acid results in analogs with severely decreased binding potency, and that the COOH-terminal domain containing residues B26-B30 plays a positive role in determining receptor binding potency in GlyB24-substituted insulin (whereas it plays a negative role in determining the receptor binding potency of its GlyB25-substituted counterpart). We interpret our results as identifying (a) a critical role for the insulin main chain near residue B24 in determining the affinity of receptor for ligand, (b) the importance of main chain flexibility in achieving a high affinity state of receptor-bound hormone, and (c) a potential interaction of the PheB24 side chain with receptor which initiates main chain structural changes in the natural hormone, but which does not itself confer affinity to ligand-receptor interactions.     

Two-dimensional NMR and photo-CIDNP studies of the insulin monomer: assignment of aromatic resonances with application to protein folding, structure, and dynamics

The aromatic 1H NMR resonances of the insulin monomer are assigned at 500 MHz by comparative studies of chemically modified and genetically altered variants, including a mutant insulin (PheB25----Leu) associated with diabetes mellitus. The two histidines, three phenylalanines, and four tyrosines are observed to be in distinct local environments; their assignment provides sensitive markers for studies of tertiary structure, protein dynamics, and protein folding. The environments of the tyrosine residues have also been investigated by photochemically induced dynamic nuclear polarization (photo-CIDNP) and analyzed in relation to packing constraints in the crystal structures of insulin. Dimerization involving specific B-chain interactions is observed with increasing protein concentration and is shown to depend on temperature, pH, and solvent composition. In the monomer large variations are observed in the line widths of amide resonances, suggesting intermediate exchange among conformational substates; such substates may relate to conformational changes observed in different crystal states and proposed to occur in the hormone-receptor complex. Additional evidence for multiple conformations in solution is provided by comparative studies of an insulin analogue containing a peptide bond between residues B29 and A1 (mini-proinsulin). This analogue forms dimers and higher-order oligomers under conditions in which native insulin is monomeric, suggesting that the B29-A1 peptide bond stabilizes a conformational substate favorable for dimerization. Such stabilization is not observed in corresponding studies of native proinsulin, in which a 35-residue connecting peptide joins residues B30 and A1; this extended tether is presumably too flexible to constrain the conformation of the B-chain. The differences between proinsulin and mini-proinsulin suggest a structural mechanism for the observation that the fully reduced B29-A1 analogue folds more efficiently than proinsulin to form the correct pattern of disulfide bonds. These results are discussed in relation to molecular mechanics calculations of insulin based on the available crystal structures.     

Flexibility and bioactivity of insulin: an NMR investigation of the solution structure and folding of an unusually flexible human insulin mutant with increased biological activity

The structure and folding of a novel human insulin mutant, [Thr(B27) --> Pro, Pro(B28) --> Thr]insulin (PT insulin), in aqueous solution and in mixtures of water and 2,2,2-trifluoroethanol (TFE) have been studied by NMR spectroscopy. It was found that PT insulin has a highly flexible structure in pure water and is present in at least two different conformations, although with an overall tertiary structure similar to that of native insulin. Furthermore, the native helical structures are poorly defined. Surprisingly, the mutant has a biological activity about 50% higher than native insulin. In contrast, in TFE/water solution the mutant reveals a propensity of forming a well-defined structure at the secondary structure level, similar to monomeric native insulin. Thus, as shown by a detailed determination of the structure from 208 distance restraints and 52 torsion angle restraints by distance geometry, simulated annealing, and restrained energy minimization, the native insulin helices (A2-A7, A13-A19, and B10-B19) as well as the beta-turn (B20-B23) are formed in 35% TFE. However, the amount of tertiary structure is decreased significantly in TFE/water solution. The obtained results suggest that only an overall tertiary fold, as observed for PT insulin in pure water, is necessary for expressing the biological activity of insulin, as long as the molecule is flexible and retains the propensity to form the secondary structure required for its receptor binding. In contrast, a compact secondary structure, as found for native insulin in solution, is unnecessary for the biological activity. A model for the receptor binding of insulin is suggested that relates the increased bioactivity to the enhanced flexibility of the mutant.     

A protein caught in a kinetic trap: structures and stabilities of insulin disulfide isomers

Proinsulin contains six cysteines whose specific pairing (A6-A11, A7-B7, and A20-B19) is a defining feature of the insulin fold. Pairing information is contained within A and B domains as demonstrated by studies of insulin chain recombination. Two insulin isomers containing non-native disulfide bridges ([A7-A11,A6-B7,A20-B19] and [A6-A7,A11-B7,A20-B19]), previously prepared by directed chemical synthesis, are metastable and biologically active. Remarkably, the same two isomers are preferentially formed from native insulin or proinsulin following disulfide reassortment in guanidine hydrochloride. The absence of other disulfide isomers suggests that the observed species exhibit greater relative stability and/or kinetic accessibility. The structure of the first isomer ([A7-A11,A6-B7,A20-B19], insulin-swap) has been described [Hua, Q. X., Gozani, S. N., Chance, R. E., Hoffmann, J. A., Frank, B. H., and Weiss, M. A. (1995) Nat. Struct. Biol. 2, 129-138]. Here, we demonstrate that the second isomer (insulin-swap2) is less ordered than the first. Nativelike elements of structure are retained in the B chain, whereas the A chain is largely disordered. Thermodynamic studies of guanidine denaturation demonstrate the instability of the isomers relative to native insulin (DeltaDeltaG(u) > 3 kcal/mol). In contrast, insulin-like growth factor I (IGF-I) and the corresponding isomer IGF-swap, formed as alternative products of a bifurcating folding pathway, exhibit similar cooperative unfolding transitions. The insulin isomers are similar in structure and stability to two-disulfide analogues whose partial folds provide models of oxidative folding intermediates. Each exhibits a nativelike B chain and less-ordered A chain. This general asymmetry is consistent with a hierarchical disulfide pathway in which nascent structure in the B chain provides a template for folding of the A chain. Structures of metastable disulfide isomers provide probes of the topography of an energy landscape.     

Inactive conformation of an insulin despite its wild-type sequence.

The peptide group between residues B24 and B25 of insulin was replaced by an ester bond. This modification only in the backbone was meant to eliminate a structurally important H-bond between the amide proton of B25 and the carbonyl oxygen of A19, and consequently to enhance detachment of the C-terminal B-chain from the body of the molecule, exposing the underlying A-chain. According to a model derived from the effects of side-chain substitutions, main-chain shortening, and crosslinking, this conformational change is prerequisite for receptor binding. Contrary to the expectation that increased flexibility would increase receptor binding and activity, depsi-insulin ([B24-B25 CO-O]insulin) has turned out be only 3-4% potent. In search of an explanation for this observation, the solution structure of depsi-insulin was determined by two-dimensional 1H-NMR spectroscopy. It was found that the loss of the B25-A19 H-bond does not entail detachment of the C-terminal B-chain. On the contrary, it is overcompensated by a gain in hydrophobic interaction achieved by insertion of the Phe B25 side chain into the molecule's core. This is possible because of increased rotational freedom in the backbone owing to the ester bond. Distortion of the B20-B23 turn and an altered direction of the distal B-chain are consequences that also affect self-association. The exceptional position of the B25 side chain is thus the key feature of the depsi-insulin structure. Being buried in the interior, it is not available for guiding the interaction with the receptor, a crucial role attributed to it by the model. This seems to be the main reason why the structure of depsi-insulin represents an inactive conformation.     

去七肽胰岛素的分子动力学研究

本文用分子动力学的方法对去七肽胰岛素(DHPI)分子的构象进行了研究,首先用分子动力学方法对晶体胰岛素分子的构象能进行了优化,然后除去B链C端的最后七个残基(B24—B30),做分子动力学模拟,得到了DHPI的平衡构象和均方差波动。胰岛素分子的X射线晶体衍射结构和能量优化构象之间的均方根偏差为0.1;所得DHPI构象和胰岛素能量优化构象间C原子间的均方根偏差为1.8。变化最大的区域是A8—A10,A18—A21,B1—B41和B18—B23。     

Non-equivalent role of inter- and intramolecular hydrogen bonds in the insulin dimer interface

Apart from its role in insulin receptor (IR) activation, the C terminus of the B-chain of insulin is also responsible for the formation of insulin dimers. The dimerization of insulin plays an important role in the endogenous delivery of the hormone and in the administration of insulin to patients. Here, we investigated insulin analogues with selective N-methylations of peptide bond amides at positions B24, B25, or B26 to delineate their structural and functional contribution to the dimer interface. All N-methylated analogues showed impaired binding affinities to IR, which suggests a direct IR-interacting role for the respective amide hydrogens. The dimerization capabilities of analogues were investigated by isothermal microcalorimetry. Selective N-methylations of B24, B25, or B26 amides resulted in reduced dimerization abilities compared with native insulin (K(d) = 8.8 μM). Interestingly, although the N-methylation in [NMeTyrB26]-insulin or [NMePheB24]-insulin resulted in K(d) values of 142 and 587 μM, respectively, the [NMePheB25]-insulin did not form dimers even at high concentrations. This effect may be attributed to the loss of intramolecular hydrogen bonding between NHB25 and COA19, which connects the B-chain β-strand to the core of the molecule. The release of the B-chain β-strand from this hydrogen bond lock may result in its higher mobility, thereby shifting solution equilibrium toward the monomeric state of the hormone. The study was complemented by analyses of two novel analogue crystal structures. All examined analogues crystallized only in the most stable R(6) form of insulin oligomers (even if the dimer interface was totally disrupted), confirming the role of R(6)-specific intra/intermolecular interactions for hexamer stability.     

Replacement of the interchain disulfide bridge-forming amino acids A7 and B7 by glutamate impairs the structure and activity of insulin

Insulin contains three disulfide bonds, one intrachain bond, A6-A11, and two interchain bonds, A7-B7 and A20-B19. Site-directed mutagenesis results (the two cysteine residues of disulfide A7-B7 were replaced by serine) showed that disulfide A7-B7 is crucial to both the structure and activity of insulin. However, chemical modification results showed that the insulin analogs still retained relatively high biological activity when A7Cys and B7Cys were modified by chemical groups with a negative charge. Did the negative charge of the modification groups restore the loss of activity and/or the disturbance of structure of these insulin analogs caused by deletion of disulfide A7-B7? To answer this question, an insulin analog with both A7Cys and B7Cys replaced by Glu, which has a long side-chain and a negative charge, was prepared by protein engineering, and its structure and activity were analyzed. Both the structure and activity of the present analog are very similar to that of the mutant with disulfide A7-B7 replaced by Ser, but significantly different from that of wild-type insulin. The present results suggest that removal of disulfide A7-B7 will result in serious loss of biological activity and the native conformation of insulin, even if the disulfide is replaced by residues with a negative charge.     

Molecular Evolution of Insulin in Non-Mammalian Vertebrates

The traditional view, based primarily on X-ray crystallographicdata, is that the amino acid residues at positions B12, B16,B23-B26, A1-A5, A19 and A21 in the insulin molecule comprisethe receptor-binding domain. More recently, however, it hasbeen proposed that the conformation adopted by insulin in thecrystal structure is an inactive one. The results of alanine-scanningmutagenesis studies suggest that GlyB23, PheB24, IleA2, ValA3,and TyrA19 interact directly with the receptor with LeuB6, GlyB8,LeuB11, GluB13 and PheB25, although not part of the bindingepitope, being important in maintaining the receptor-bindingconformation. A comparison of the primary structures of insulinsfrom a wide range of non-mammalian vertebrates, from hagfishto birds, provides support for this revised view by demonstratingthat strong evolutionary pressure has acted to conserve thoseamino acids postulated to be important in the biologically activeconformation. In addition to the cysteine residues, the aminoacids at B6, B8, B11, B23, B24, A2, A3, and A19 are invariantin all species yet studied with only conservative substitutions(Glu Asp) at B13 and (Phe Tyr) at B25. In contrast, severalinsulins containing substitutions at positions B16, A5 and A21,sites of importance in maintaining the crystal structure conformation,have been identified. Although the amino acid sequences of insulinare not generally useful as molecular markers for inferringphylogenetic relationships between species, the presence ofcommon structural features in insulins from closely relatedspecies may permit a valid inference. For example, the presenceof an N-terminal pentapeptide extension to the B-chains of insulinsisolated from both holarctic and southern hemisphere lampreyssupports the monophyletic status of the Petromyzontiformes.     

Effects of cysteine to serine substitutions in the two inter-chain disulfide bonds of insulin

Using site-directed mutagenesis we deleted the two inter-chain disulfide bonds of insulin, separately or both, by substitution of the cysteine residues with serine. Deletion of A20-B19 or both of the two inter-chain disulfide bonds resulted in the complete loss of secretion of the mutant single-chain porcine insulin precursor (PIP) from Saccharomyces cerevisiae cells. Removal of the A7-B7 disulfide bond resulted in a large reduction of secretion, but we could obtain the mutant for analysis of its biological and some physico-chemical properties. The A7-B7 disulfide bond deleted insulin mutant retained only 0.1% receptor-binding activity compared with porcine insulin, and its in vivo biological potency measured by mouse convulsion assay was also very low. We also studied some physico-chemical properties of the mutant using circular dichroism, native polyacrylamide gel electrophoresis and reversed-phase HPLC, which revealed some structural changes of the mutant peptides compared to native insulin. The present study shows that the two inter-chain disulfide bonds are important for efficient in vivo folding/secretion of PIP from yeast, especially the A20-B19 disulfide bond, and that the A7-B7 disulfide bond is crucial for maintaining the native conformation and biological activity of insulin.     

TyrB26位点改变的人类胰岛素类似物的化学合成与体外活性

摘要：为了研究人类胰岛素B链第26位的酪氨酸对胰岛素和受体之间的结合的影响,包括单独的氨基酸替换或化合物替换的不同的胰岛素类似物被合成,其中化合物替代的类似物的B链C末端都减少了4个氨基酸。在对它们与胰岛素受体的亲和力进行研究中,结果发现它们与胰岛素受体的亲和力没有丢失, HisB26类似物和N-MeHisB26类似物的结合能力与胰岛素相比改变不大,分别是胰岛素的72 %和107 %。N-MeGluB26类似物,AadB26类似物和Phe (4-carboxy) B26类似物的结合能力有很大的提高,分别是130 %, 234 %和160 %。     

Mutations at the dimer, hexamer, and receptor-binding surfaces of insulin independently affect insulin-insulin and insulin-receptor interactions.

Mutagenesis of the dimer- and hexamer-forming surfaces of insulin yields analogues with reduced tendencies to aggregate and dramatically altered pharmacokinetic properties. We recently showed that one such analogue, HisB10----Asp, ProB28----Lys, LysB29----Pro human insulin (DKP-insulin), has enhanced affinity for the insulin receptor and is useful for studying the structure of the insulin monomer under physiologic solvent conditions [Weiss, M. A., Hua, Q. X., Lynch, C. S., Frank, B. H., & Shoelson, S. E. (1991) Biochemistry 30, 7373-7389]. DKP-insulin retains native secondary and tertiary structure in solution and may therefore provide an appropriate baseline for further studies of related analogues containing additional substitutions within the receptor-binding surface of insulin. To test this, we prepared a family of DKP analogues having potency-altering substitutions at the B24 and B25 positions using a streamlined approach to enzymatic semisynthesis which negates the need for amino-group protection. For comparison, similar analogues of native human insulin were prepared by standard semisynthetic methods. The DKP analogues show a reduced tendency to self-associate, as indicated by 1H-NMR resonance line widths. In addition, CD spectra indicate that (with one exception) the native insulin fold is retained in each analogue; the exception, PheB24----Gly, induces similar perturbations in both native insulin and DKP-insulin backgrounds. Notably, analogous substitutions exhibit parallel trends in receptor-binding potency over a wide range of affinities: D-PheB24 greater than unsubstituted greater than GlyB24 greater than SerB24 greater than AlaB25 greater than LeuB25 greater than SerB25, whether the substitution was in a native human or DKP-insulin background. Such "template independence" reflects an absence of functional interactions between the B24 and B25 sites and additional substitutions in DKP-insulin and demonstrates that mutations in discrete surfaces of insulin have independent effects on protein structure and function. In particular, the respective receptor-recognition (PheB24, PheB25), hexamer-forming (HisB10), and dimer-forming (ProB28, LysB29) surfaces of insulin may be regarded as independent targets for protein design. DKP-insulin provides an appropriate biophysical model for defining structure-function relationships in a monomeric template.     

Processing and presentation of insulin. I. Analysis of immunogenic peptides and processing requirements for insulin A loop-specific T cells

The processing and presentation of insulin by B hybridoma cells to insulin A loop-specific T cell hybridomas was investigated. We found that the activation of these T cells requires insulin to be processed in a manner that permits unfolding of the molecule and prevents extensive proteolysis. An analysis of insulin peptides formed by either enzymatic digestion in vitro or solid phase synthesis revealed that a conformational determinant comprised of residues A1-A14 disulfide-linked to B7-B15 is most immunogenic to these T cells. Reduction and/or proteolysis of this peptide markedly decreases its immunogenicity. The pork insulin A1-A14/B7-B15 peptide differs only at residue A4 from its mouse insulin homolog. Thus, Glu A4 forms part of the antigenic site recognized by a pork insulin/I-Ad-specific mouse T cell. This insulin peptide can be induced to assume an alpha-helical configuration in a hydrophobic environment. In addition, virtually all of the residues of this peptide are identical with those predicted to be situated in amphipathic regions of the native insulin molecule. N-Ethylmaleimide and bacitracin, which inhibit the activity of two cytosolic enzymes that cleave insulin, enhance the antigen presentation of insulin. This suggests that these enzymes may participate in the nonlysosomal antigen processing of insulin by a B lymphocyte. A comparison of the relative avidity of several T cell hybridomas, which have the same apparent specificity for this insulin peptide, showed that an increase in their avidity was associated with a degeneracy in their fine specificity. Our data demonstrate that the efficiency of processing and presentation of a given antigenic determinant is related to the conformation of the determinant and the specificity and avidity of the T cell.     

Toward the insulin-IGF-I intermediate structures: functional and structural properties of the [TyrB25NMePheB26] insulin mutant

The origins of differentiation of insulin from insulin-like growth factor I (IGF-I) are still unknown. To address the problem of a structural and biological switch from the mostly metabolic hormonal activity of insulin to the predominant growth factor activities of IGF-I, an insulin analogue with IGF-I-like structural features has been synthesized. Insulin residues Phe(B25) and Tyr(B26) have been swapped with the IGF-I-like Tyr(24) and Phe(25) sequence with a simultaneous methylation of the peptide nitrogen of residue Phe(B26). These modifications were expected to introduce a substantial kink in the main chain, as observed at residue Phe(25) in the IGF-I crystal structure. These alterations should provide insight into the structural origins of insulin-IGF-I structural and functional divergence. The [Tyr(B25)NMePhe(B26)] mutant has been characterized, and its crystal structure has been determined. Surprisingly, all of these changes are well accommodated within an insulin R6 hexamer. Only one molecule of each dimer in the hexamer responds to the structural alterations, the other remaining very similar to wild-type insulin. All alterations, modest in their scale, cumulate in the C-terminal part of the B-chain (residues B23-B30), which moves toward the core of the insulin molecule and is associated with a significant shift of the A1 helix toward the C-terminus of the B-chain. These changes do not produce the expected bend of the main chain, but the fold of the mutant does reflect some structural characteristics of IGF-1, and in addition establishes the CO(A19)-NH(B25) hydrogen bond, which is normally characteristic of T-state insulin.