The importance of ProB28 and LysB29 on the self-association of insulin was established by systematically truncating the C terminus of the B chain. The relationship between structure and association was further explored by making numerous amino acid replacements at B28 and B29. Association was studied by circular dichroism, size-exclusion chromatography and ultracentrifugation. Our results show that the location of a prolyl residue at B28 is critical for high-affinity self-association. Removal of ProB28 in a series of C-terminal truncated insulins, or amino acid replacement of ProB28, greatly reduced association. The largest disruption to association was achieved by replacing LysB29 with Pro and varying the amino acid at B28. Several of the analogs were predominantly monomers in solutions up to 3 mg/ml. These amino acid substitutions decreased association by primarily disrupting the formation of dimers. Such amino acid substitutions also substantially reduced the Zn-induced insulin hexamer formation. The formation of monomeric insulins through amino acid replacements was accompanied by conformational changes that may be the cause for decreased association. It is demonstrated that self-association of insulin can be drastically altered by substitution of one or two key amino acids.
The Diabetes Control and Complications Trial has emphasized the need for improved control of blood glucose as a means to diminish long-term complications of diabetes. LysPro-insulin is an analog of human insulin whose design was modeled on structural homology with insulin-like growth factor I. An analysis of the structural conformation of insulin suggested that an inversion of amino acids B28 and B29 in the C-terminus of the B chain could yield an insulin analog with a faster onset of biological action. This insulin analog has proved to be virtually identical to human insulin in action, with one important exception. LysPro-insulin has demonstrated an improved time course of action in control of a mealtime glucose elevation. This offers the opportunity for improved convenience and safety for patients with insulin-dependent diabetes mellitus.
Insulin analogs designed to decrease self-association and increase absorption rates from subcutaneous tissue were found to have altered stability. Replacement of HB10 with aspartic acid increased stability while substitutions at B28 and/or B29 were either comparable to insulin or had decreased stability. The principal chemical degradation product of accelerated storage conditions was a disulfide-linked multimer that was formed through a disulfide interchange reaction which resulted from beta-elimination of the disulfides. The maintenance of the native state of insulin was shown to be important in protecting the disulfides from reduction by dithiothreitol and implicitly from the disulfide interchange reaction that occurs during storage. To understand how these amino acid changes alter chemical stability, the intramolecular conformational equilibria of each analog was assessed by equilibrium denaturation. The Gibbs free energy of unfolding was compared with the chemical stability during storage for over 20 analogs. A significant positive correlation (R2 = 0.8 and P less than 0.0005) exists between the conformational stability and chemical stability of these analogs, indicating that the chemical stability of insulin's disulfides is under the thermodynamic control of the conformational equilibria.
To determine the conformational properties of the C-terminal region of the insulin B-chain relative to the helical core of the molecule, we have investigated the fluorescence properties of an insulin analog in which amino acids B28 and B29 have been substituted with a tryptophan and proline residue respectively, ([WB28,PB29]insulin). The biological properties and far-UV circular dichroism (CD) spectrum of the molecule indicate that the conformation is similar to that of native human insulin. Guanidine hydrochloride (GdnHCl)-induced equilibrium denaturation of the analog as monitored by CD intensity at 224 nm indicates a single cooperative transition with a midpoint of 4.9 M GdnHCl. In contrast, when the equilibrium denaturation is observed by steady-state fluorescence emission intensity at 350 nm, two distinct transitions are observed. The first transition accounts for 60% of the observed signal and has a midpoint of 1.5 M GdnHCl. The second transition roughly parallels that observed by CD measurements with an approximate midpoint of 4.5 M GdnHCl. The near-UV CD spectrum, size-exclusion, and ultracentrifugation properties of [WB28,PB29]insulin indicate that this analog does not self-associate in a concentration-dependent manner as does human insulin. Thus, the observed fluorescence changes must be due to specific conformational transitions which occur upon unfolding of the insulin monomer with the product of the first transition representing a stable folding intermediate of this molecule.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.