Structure-function relationships of shortened [LeuB25]insulins, semisynthetic analogues of a mutant human insulin

Structure-function relationships of des-(B26-B30)-insulin

In order to study the role of the amino acid in position B25 and its environment in shortened insulins, a series of analogues was prepared with the following modifications: 1, Stepwise shortening of the B-chain including replacements of TyrB26 and ThrB27 by glycine; 2, substitutions at the carboxamide nitrogen of des-(B26-B30)-insulin-B25-amide by apolar, polar or charged residues of various chain lengths; 3, replacement of PheB25 by asparagine-amide, phenylalaninol or a series of alkyl and aralkyl residues. Trypsin-catalyzed semisyntheses were performed with Boc-protected or unprotected des-octapeptide-(B23-B30)-insulin and synthetic peptides. Relative receptor binding and in vitro bioactivity of [AsnB25]-des-(B26-B30)-insulin-B25-amide was 227 and 292% (on insulin), other activities ranged between 1 and ca. 200%. We make the following conclusions. An L-amino acid is essential in position B25. The B25-carbonyl and NH groups favour high binding and "superpotency", but are not indispensible for receptor contacts. For high affinity receptor interaction, the planarity at the C gamma-atom and the distance of B25-side-chain branching in position B25 are important, but an aromatic ring is not necessary.

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.

High-performance liquid chromatography of insulin. Accessibility and flexibility

Current ideas suggest that a conformational change in the insulin monomer may play an important part in its interaction with the insulin receptor. An investigation is reported in which analytical reversed-phase high-performance liquid chromatography of insulin analogues was used to investigate the solution conformation of the insulin monomer. The results are interpreted in terms of elution coefficients modified by the calculated surface accessibilities of individual residues. The results suggest a partial unfolding of the insulin monomer under the experimental conditions used, which is consistent with current ideas on the biologically active conformation of insulin.