Sequence-specific 1H-NMR assignments for the aromatic region of several biologically active, monomeric insulins including native human insulin

The aromatic region of the 1H-FT-NMR spectrum of the biologically fully-potent, monomeric human insulin mutant, B9 Ser----Asp, B27 Thr----Glu has been investigated in D2O. At 1 to 5 mM concentrations, this mutant insulin is monomeric above pH 7.5. Coupling and amino acid classification of all aromatic signals is established via a combination of homonuclear one- and two-dimensional methods, including COSY, multiple quantum filters, selective spin decoupling and pH titrations. By comparisons with other insulin mutants and with chemically modified native insulins, all resonances in the aromatic region are given sequence-specific assignments without any reliance on the various crystal structures reported for insulin. These comparisons also give the sequence-specific assignments of most of the aromatic resonances of the mutant insulins B16 Tyr----Glu, B27 Thr----Glu and B25 Phe----Asp and the chemically modified species des-(B23-B30) insulin and monoiodo-Tyr A14 insulin. Chemical dispersion of the assigned resonances, ring current perturbations and comparisons at high pH have made possible the assignment of the aromatic resonances of human insulin, and these studies indicate that the major structural features of the human insulin monomer (including those critical to biological function) are also present in the monomeric mutant.     

Demonstration of three protonated forms of poly(A): potentiometric and conductometric study of isoionic solutions

It is shown that there are three parts on the potentiometric titration curves of isoionic solutions of poly(A) ascribed to the three protonated structures. Double-helical protonated structures are especially stable in isoionic solution. These parts on potentiometric curves are attributed to the single-stranded poly(A), to the completely protonated double-stranded poly(A+).poly(A+), and to the semiprotonated poly(A+).poly(A) structures: D, A, B forms of poly(A), respectively. pK0 values of these forms are calculated. The D form portion is found to be about 18% in isoionic solution, 40% in KCl solution (from 0.01 to 1.0 M), 40% in solution, containing 1.2 X 10(-3) M MgCl2 and 70% in 8 X 10(-4) M MgCl2 solution. The increase of MgCl2 concentration up to 8 X 10(-4) M leads to complete degradation of the double-helical structure. Only single-stranded D form exists in 5 X 10(-3) M MgCl2 solution. About 5-7% of all protons become inaccessible for titration in all solutions containing KCl and in the presence of small amounts of MgCl2. This phenomenon can not be explained by aggregation of poly(A), because all protons become accessible for titration in more concentrated MgCl2 solution when aggregation of poly(A) is significant and accompanied by the precipitation of sediment insoluble in NaOH. The supposition is made, that unprotonated double-stranded poly(A) can exist in salt-free solution at neutral pH. It is this form that is protonated with decrease of pH.     

Quaternary conformational stability: The effect of reversible self‐association on the fibrillation of two insulin analogs

Under conditions relevant to the manufacturing of insulin (e.g., pH 3, room temperature), biosynthetic human insulin (BHI), and Lispro insulin (Lispro) require a nucleation step to initiate aggregation. However, upon seeding with preformed aggregates, both insulins rapidly aggregate into nonnative fibrils. Far ultraviolet circular dichroism (far‐UV CD) and second derivative Fourier transform infrared (2D‐FTIR) spectroscopic analyses show that the fibrillation process involves a change in protein secondary structure from α‐helical in native insulin to predominantly β‐sheet in the nonnative fibrils. After seeding, Lispro aggregates faster than BHI, likely because of a reduced propensity to reversibly self‐associate. Composition gradient multi‐angle light scattering (CG‐MALS) analyses show that Lispro is more monomeric than BHI, whereas their conformational stabilities measured by denaturant‐induced unfolding are statistically indistinguishable. For both BHI and Lispro, as the protein concentration increases, the apparent first‐order rate constant for soluble protein loss decreases. To explain these phenomena, we propose an aggregation model that assumes fibril growth through monomer addition with competitive inhibition by insulin dimers. Biotechnol. Bioeng. 2011;108: 2359–2370. © 2011 Wiley Periodicals, Inc.     

Toward understanding the role of insulin alpha-helix II in the growth-promoting activity of insulin.

For further understanding the contribution of the alpha-helix II (alphaII) in the growth-promoting activity of insulin, the residues A2Ile, A5Gln, and A8Thr located in alphaII were mutated to Leu, Glu, and Tyr, respectively. Three mutant insulins, [A2Leu]human insulin, [A5Glu]human insulin, and [A8Tyr]human insulin, were prepared by means of site-directed mutagenesis. The in vitro growth-promoting activities of the three mutant insulins, measured using GR2H6 cells, were 7.5%, 291%, and 250% of that of native insulin, respectively. Their receptor-binding activities to the insulin receptor were 2.3%, 46.7%, and 138.7%, respectively, compared with native insulin. Both the growth-promoting and receptor-binding activities of [A2Leu]human insulin and [A3Leu]insulin (Shi et al., 1997) were parallel and greatly decreased compared with native insulin. The results demonstrate that the residues A2Ile and A3Val in the alphaII are essential for the growth-promoting activity of insulin, and the growth-promoting function of insulin might be performed through, or mainly through, binding to the insulin receptor. The growth-promoting activities of [A5Glu]human insulin and [A8Tyr]human insulin were increased 6-fold and 2-fold, respectively, compared with native insulin, indicating that their growth-promoting activities might be expressed by, or mainly by, binding to the IGF-1 receptor.     

Binding of mutant insulins to a mutated insulin receptor.

We studied the binding of mutant insulins to both the normal human insulin receptor and an insulin receptor in which the sequence 240-250 of the receptor alpha subunit was mutated to provide an additional net positive charge. One mutant insulin (AspB10), which has an additional negative charge, bound to both types of receptors with a higher affinity than native insulin. Moreover, this mutant insulin was more effective in activating the tyrosine kinase activity of both types of receptors. This study suggests, therefore, that charge interactions between insulin and its receptor may play a role in insulin receptor binding and action.     

pH-jump titration of the tyrosyl groups of bovine plasma albumin

Ionization properties of the tyrosyl groups of bovine plasma albumin in various conformational states—the native state (N), the two acid states (F and E), and the state (B) stable at slightly alkaline pH—were studied by means of a stopped-flow-pH-jump technique. The technique allows us to obtain the tyrosyl titration curve in a conformational state that is unstable in the pH region of the titration. The pH jumps from the N and B states to various alkaline pH's, where the tyrosines ionize to bring about a time-dependent increase in absorption at 296 nm, indicating that a number of the tyrosines buried initially become susceptible to ionization as a result of the alkaline transition occurring above pH 10.8. Extrapolation of the observed absorption change to zero time gives a spectrophotometric titration curve in the initial conformational state. Only 30–401% of the 19 tyrosines of the protein can ionize both in the N and the B state at pH 12. The pH jumps from the F and E states, on the other hand, give a decrease in absorption between pH 9 and 11.7, indicating that the tyrosyl groups initially exposed are remarked by refolding after the pH jumps, and the zero-time titration curves show that essentially all the tyrosyl groups ionize normally in these acid states. The results are discussed in relation to the known results of the tyrosyl exposure of the protein measured by other techniques, and the consistency among them demonstrates the effectiveness of the pH-jump titration method. Hydrogen bonding between the abnormal tyrosyl and carboxylate groups as a mechanism to stabilize native albumin is suggested from characteristics of the alkaline transition, which also involves the exposure of the tyrosyl groups, and from the tyrosyl titration curves in the native and acid states.     

Ionization behaviour of native apolipoproteins and of their complexes with lecithin. 1. Calorimetric and potentiometric titration of the native apoA-I protein and of the apoA-I protein-dimyristoyl lecithin complex.

The ionization behaviour of native apoA-I protein is compare to that of its complex with synthetic dimyristoyl lecithin in studies using calorimetric, potentiometric and spectrophotometric titration. In the presence of phospholipids, 10 out of 21 lysines together with 22 acidic residues are masked in the complex. All tyrosines remain accessible to titration below pH 13. The apparent ionization enthalpy of the 11 lysine residues is not affected by the presence of phospholipids. These data are consistent with discrete binding sites located in the apoprotein helical segments as suggested by the model of Segrest et al. [FEBS Lett. 38, 247-253 (1974)]. A tentative localisation of lysine, arginine, aspartic acid and glutamic acid residues directly involved in phospholipid binding is suggested, assuming that such helical regions are involved in apoprotein-phospholipid association.     

Anomalies in the ionic properties of serum albumin

The documented difference in the isoionic points of native and unfolded serum albumins is revisited with computational methods. Good agreement between calculated and measured DeltapIs is found, with the molecular origin appearing to reside in a diverse set of carboxyl group titrations. Although histidine ionization plays only a minor role in bringing about the DeltapI, the identification of three histidine residues with low computed pK(a)s (around pH 5) suggests an explanation for the mismatch between experimentally derived group pK(a)s and the pI of the native protein. Computed electrostatic properties (including pI) of native serum albumins are compared with a set of 178 nonenzyme proteins. We find that the degree of interdigitation of positive and negative charges is extreme for serum albumin, and discuss the relationship to protein solubility and pH-dependent structural transitions.     

Insulins with built-in glucose sensors for glucose responsive insulin release.

Derivatization of insulin with phenylboronic acids is described, thereby equipping insulin with novel glucose sensing ability. It is furthermore demonstrated that such insulins are useful in glucose‐responsive polymer‐based release systems. The preferred phenylboronic acids are sulfonamide derivatives, which, contrary to naïve boronic acids, ensure glucose binding at physiological pH, and simultaneously operate as handles for insulin derivatization at LysB29. The glucose affinities of the novel insulins were evaluated by glucose titration in a competitive assay with alizarin. The affinities were in the range 15–31 mM (K_d), which match physiological glucose fluctuations. The dose‐responsive glucose‐mediated release of the novel insulins was demonstrated using glucamine‐derived polyethylene glycol polyacrylamide (PEGA) as a model, and it was shown that Zn(II) hexamer formulation of the boronated insulins resulted in steeper glucose sensitivity relative to monomeric insulin formulation. Notably, two of the boronated insulins displayed enhanced insulin receptor affinity relative to native insulin (113%–122%) which is unusual for insulin LysB29 derivatives. Copyright © 2004 European Peptide Society and John Wiley & Sons, Ltd.     

Potentiometric titration curves of oxidized and reduced horse heart cytochrome c

Potentiometric titration curves of oxidized and reduced horse heart cytochrome c in 0.15M KCl at 20°C have been obtained by timed titration (0.125–0.500 μmol/sec) from the isoionic points (pH 10.2–10.4) to pH 3 and back to the isoionic point. Computer-assisted (PROPHET) data acquisition and blank corrections give curves with good precision with a maximum standard deviation of 0.3 groups for an average error of 1%. The potentiometric titration curve of reduced cytochrome c is reversible within the precision of the method and for the pH range studied. The potentiometric curves for oxidized cytochrome c titrated upscale (pH 3–10) and downscale (pH 10–3) are not reversible. However, they show the same ionization behavior after the initial downscale titration. This is probably the result of a conformational change. Comparison of the data herein reported with the titration curves of oxidized cytochrome c already published by others indicates good agreement on the basis of a normalization of the concentration of protein or on the basis of 25 titrable groups between the acid end point and the isoionic pH. Titration of the 2 μmol imidazole in the upscale or downscale direction gives the correct analytical concentration and pK′ after correction for the solvent titration. Titration of reduced cytochrome c in the presence and absence of an additional equivalent of imidazole gave a difference titration curve, which indicates that a group on the protein shifts from pK′ 5.8 to pK′ 5.3 in the presence of imidazole. The pK′ of imidazole, in the presence of the protein, remains at a nearly normal value of 7.34.     

Hydrogen ion interactions of horse spleen ferritin and apoferritin.

The interactions of horse spleen ferritin and its derivative apoferritin with H+ ions were studied by potentiometric and spectrophotometric titration; to aid in data analysis, heats of ionization over a limited pH range and amide content were also determined. Per apoferritin subunit, all tyrosine and cysteine side chains, two of the nine lysine side chains and at least three of the six histidine side chains were found not to titrate; a preliminary but self-consistent analysis of the titration data is proposed. The titration curve of ferritin was identical with that of apoferritin in the pH range 5.5 to 3. In addition, under the conditions used, the reactivities of ferritin histidines to bromoacetate and of ferritin lysines to formaldehyde were identical with those in apoferritin. Above pH 8, a time-dependent titration of the ferritin core occurs which prevents comparison of the titration curves of the two proteins in this region. However, in the pH regions 5.5 to 7.5, two extra groups per subunit titrate reversibly in ferritin relative to apoferritin. Moreover, although the isoionic points of ferritin and apoferritin are identical in water, the isoionic point of ferritin is 0.5 pH unit lower than that of apoferritin in 0.16 to 1 M KCl. The different effects of KCl and NaCl on the two proteins indicate the presence of cation binding sites in ferritin that are absent in apoferritin and possibly also the presence of anion binding sites in apoferritin that are occupied in ferritin by anions of the core. The difference between the isoionic points of the two proteins in KCl has been interpreted to indicate the presence of approximately 2 phosphate residues per ferritin subunit which serve as cation binding sites and which are negatively charged at the isoionic point in KCl. These phosphates may also represent the additional residues that titrate in ferritin between pH 5.5 and 7.5, or may interact with positively charged residues on the inner surface of the ferritin shell, or both.     

Ionization behavior of acidic residues in calbindin D(9k).

The ionization state of seven glutamate residues, one aspartate, and the C-terminal alpha-COOH group in bovine apo calbindin D(9k) has been studied by measurement and modeling of the pH titration curves and apparent pK(a) values. The observed pK(a) ranged from 3.0 to 6.5. Most of the observed acidic groups were half-ionized at lower pH values than those in unstructured proteins. As a rule, the ionization equilibria extended over a wider pH range than in the case of unperturbed single titrations, indicating a complex influence of protein charges on the charge state of each individual residue. Glu17, which is a backbone Ca(2+)-ligand in the N-terminal binding loop of calbindin D(9k), was half-protonated at pH 3.6 but manifested biphasic titration with apparent pK(a) values of 3.2 and 6.5. Complementary Monte Carlo simulations of the titration process and pK(a) values of the acidic groups in calbindin D(9k) reproduce the experimentally observed titration features, except for the pronounced double titration of Glu17. Discrepancies between the results from direct measurement and from modeling may be partly caused by changes in the protein structure when the net charge changes from -8 to +11 over the isoelectric point at pH 5. Proteins 1999;37:106-115.     

1H NMR spectrum of the native human insulin monomer. Evidence for conformational differences between the monomer and aggregated forms

The effects of high dilution on the 1H Fourier transform NMR spectrum of native human insulin at pH* 8.0 and 9.3 have been examined at 500 MHz resolution. The dependence of the spectrum on concentration and comparison with the spectrum of a biologically highly potent monomeric insulin mutant (SerB9----Asp) establish that at 36 microM (pH* 9.3) or 18 microM (pH* 8) and no added buffer or salts, human insulin is monomeric. Under these conditions of dilution, ionic strength, and pH*, human insulin and the SerB9----Asp mutant exhibit nearly identical 1H NMR spectra. At higher concentrations (i.e. greater than 36 microM to 0.91 mM), native human insulin dimerizes, and this aggregation causes a change in insulin conformation. Although there are many changes in the spectrum, the TyrB26 ring H3,5 proton signals located at 6.63 ppm and the methyl signal located at 0.105 ppm (characteristics of monomeric insulin) are particularly distinct signatures of the conformation change that accompanies dimerization. Magnetization transfer experiments show that the 0.105 ppm methyl signal shifts downfield to a new position at 0.45 ppm. We conclude that the 0.105 ppm methyl signal is due to a conformation in which a Leu methyl group is centered over and in van der Waals contact with the ring of an aromatic side chain. Dimerization causes a conformation change that alters this interaction, thereby causing the downfield shift. Nuclear Overhauser studies indicate that the methyl group involved is located within a cluster of aromatic side chains and that the closest ring-methyl group interaction is with the ring of PheB24.     

Diabetes mellitus caused by mutations in human insulin: analysis of impaired receptor binding of insulins Wakayama,Los Angeles and Chicago using pharmacoinformatics

Several naturally occuring mutations in the human insulin gene are associated with diabetes mellitus. The three known mutant molecules, Wakayama, Los Angeles and Chicago were evaluated using molecular docking and molecular dynamics (MD) to analyse mechanisms of deprived binding affinity for insulin receptor (IR). Insulin Wakayama, is a variant in which valine at position A3 is substituted by leucine, while in insulin Los Angeles and Chicago, phenylalanine at positions B24 and B25 is replaced by serine and leucine, respectively. These mutations show radical changes in binding affinity for IR. The ZDOCK server was used for molecular docking, while AMBER 14 was used for the MD study. The published crystal structure of IR bound to natural insulin was also used for MD. The binding interactions and MD trajectories clearly explained the critical factors for deprived binding to the IR. The surface area around position A3 was increased when valine was substituted by leucine, while at positions B24 and B25 aromatic amino acid phenylalanine replaced by non-aromatic serine and leucine might be responsible for fewer binding interactions at the binding site of IR that leads to instability of the complex. In the MD simulation, the normal mode analysis, rmsd trajectories and prediction of fluctuation indicated instability of complexes with mutant insulin in order of insulin native insulin < insulin Chicago < insulin Los Angeles < insulin Wakayama molecules which corresponds to the biological evidence of the differing affinities of the mutant insulins for the IR.     

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.     

Human insulin

The two human insulins of clinical importance are (a) semisynthetic human insulin prepared from pork pancreas by enzymatically substituting threonine for alanine-the last amino acid in the beta chain-thereby transforming pork insulin in vitro to human insulin; and (b) biosynthetic human insulin synthesized biotechnologically in Escherichia coli-K12. Using this latter technique, it is possible to produce mass quantities of highly purified insulin for the treatment of insulin-dependent diabetics, avoiding the problems inherent in supplies of insulin produced from animal pancreas. It has been suggested that to avoid confusion the two human insulins should be called semisynthetic human insulin of pork origin and biosynthetic human insulin of E. coli origin, respectively. These insulins have four advantages over highly purified animal insulins: (a) they induce lower titers of circulating insulin antibodies; (b) their subcutaneous injection is associated with fewer skin reactions; (c) they are absorbed more rapidly from the injection site; and (d) less degradation occurs at the site of injection. These data indicate that newly diagnosed insulin-dependent diabetes, particularly in children, should be treated with either of the two human insulins. The warranty against inadequate supplies of insulin offered by biosynthetic human insulin makes the use of pork insulins unnecessary and beef insulins totally useless.     

Ionization behaviour of native apolipoproteins and of their complexes with lecithin. 2. Potentiometric titration of the native apo-A-II, apoC-I, apoC-III proteins and of their complexes with dimyristoyl lecithin.

A comparison of the ionization behaviour of the human apoA-II, apoC-I, apoC-III proteins and of their complexes with dimyristoyl lecithin is based on potentiometric titration of the basic and acidic residues and spectrophotometric titration of the phenolic groups. Experimental data suggest that a number of lysine, arginine, aspartic acid and glutamic acid residues are masked in the complexes. For each of these amino acids and in all three proteins the number of masked residues is consistent with the content of those regions predicted to be involved in lipid binding by the model of Segrest et al. [FEBS Lett. 38, 247-253 (1974)]. These data taken together with the results of calorimetric and titration experiments with the apoA-I protein reported in the accompanying article [Rosseneu et al. (1977) Eur. J. Biochem. 79, 251-257] strongly support the general nature of the proposed model and further suggest that ionic interactions have some role in the formation of the dimyristoyl lecithin/apolipoprotein complexes.     

Enzymatic and hyperglycemia stability of chemically modified insulins with hydrophobic acyl groups

The acylated insulins were synthesized by the reaction of insulin protected by p-methoxybenzoxy carbonyl azide at Gly-A1 site with N-hydroxysuccinimide ester of caproic acid or benzoic acid (Cap-insulin and Bz-insulin). The noteworthy aspects are as follows: (a) the acylated insulins were more stable to the decomposition by various digestive enzymes as compared with native insulin in vitro. (b) Animal experiments using normal rats in vivo revealed that the Bz-insulin had an effective hypoglycemia activity almost similar to that of native insulin.     

猪B_0L-丙氨酰胰岛素和猪B_1L-亮氨酰胰岛素的制备及性质研究

 猪胰岛素经与MSC·ONsu选择性反应,得到[MSC]A_(21)B_(29)胰岛素,再与BOC-L-Ala·TTT缩合,去保护后得到[L-Ala]B_0胰岛素,将得到的[MSC]A_(21)B_(29)胰岛素经Edman降解,再与BOC-L-Leu·TTT缩合、去保护后得到[L-Leu]B_1胰岛素。样品经N-末端分析,醋酸纤维素薄膜电泳、氨基酸组成分析和紫外吸收光谱鉴定,确定它们分别为均一的[L-Ala]B_0胰岛素和[L-Leu]B_1胰岛素。放射免疫法分析[L-Al_a]B_0肽岛素和[L-Leu]B_1胰岛素的免疫活性分别相当于天然胰岛素的30％和47％,小鼠惊厥法分析表明[L-Ala]B_6胰岛素与[L-Leu]B_1胰岛素的生物活力为19.7国际单位/mg和19.1国际单位/mg,相当于天然胰岛素的76％和73％。     

The relation of conformation and association of insulin to receptor binding; x-ray and circular-dichroism studies on bovine and hystricomorph insulins.

Crystal and solution structure studies on insulins of different sequences and of widely different receptor binding affinities are reported. Bovine insulin, studied as a control, has a circular dichroism spectrum which is dependent both on protein concentration and zinc concentration. The spectrum appears to be related to the level of association of the insulin molecules. This implies that when using circular dichroism to compare solution structures of insulin derivatives or species variants one must make the comparison at equivalent levels of association and not merely at the same concentration. Changes in circular dichroism are related to the known crystal structure of zinc insulin hexamers. The chinchilla insulin spectrum shows a reduced zinc dependence in low-salt conditions which correlates with the inability to form crystals in similar conditions. This is attributed to an amino acid substitution at position B4. Crystals are obtained in high-salt conditions and X-ray diffraction patterns show these to be isomorphous with bovine 4Zn insulin crystals. Guinea pig insulin failed to crystallise under conditions which are normally conducive to the formation of crystals of zinc insulin hexamers and the circular dichroism showed no zinc dependence. This is consistent with a monomeric structure. The significance of the association behaviour of chinchilla and guinea pig insulins may be in the storage of the hormone in vivo. Whereas the monomeric form of chinchilla insulin has a structure closely related to bovine insulin, the circular dichroism indicates a gross structural difference for guinea pig insulin. This may be similar to that in des-A21, des-B30-insulin, as both lack the Arg-B22--Asn-A21 carboxylate ion pair. The similarity of structure of chinchilla and bovine insulins is reflected in their receptor binding whereas the low receptor binding of guinea pig insulin probably results from the changes in its conformation rather than an alteration in residues of a receptor binding region.