High-performance liquid chromatography and nuclear magnetic resonance study of linear tetrapeptides and octapeptides containing N-methylated amino acid residues

Nβ-methylation changes the recognition pattern of aza-β3-amino acid containing peptidomimetic substrates by protein kinase A

The protein kinase A (PKA)-catalyzed phosphorylation of peptide substrate RRASVA analogs, containing N^β-Me-aza-β³-amino acid residues in all subsequent positions, was studied. This work follows along the lines of our previous research of the phosphorylation of aza-β³-analogs of RRASVA (the shortest active substrate of PKA) and allows characterizing the influence of N^β-methylation of aza-β³-amino acid residues on substrate recognition by PKA on substrate binding and phosphorylation steps. It was found that the effect of N^β-methylation was dependent upon the position of the structure alteration. Moreover, the presence of a single N^β-methylation site in the substrate changed the recognition pattern of this series of peptidomimetics, strongly affecting the phosphorylation step. Structure modeling of aza-β³- and N^β-Me-aza-β³-containing substrates revealed that N^β-methylation of aza-β³-moieties changed the peptide bond geometry from trans- to cis-configuration in -CO-NMe- fragments, with an exception for the N-terminally methylated N^β-Me-aza-β³-RRRASVA (with the N-terminal amino group not participating in the peptide bond) and RRAS-N^β-Me-aza-β³-VA. As has been shown in literature, this conformational preference of the backbone has a significant influence on the flexibility of the peptide substrate chain. Following our results, this property seems to have significant influence on the recognition of the amino acid side groups by the enzyme binding site, and in the case of PKA this structural modification was decisive for the phosphate transfer step of the catalytic process.

Implications for the active form of human insulin based on the structural convergence of highly active hormone analogues

Insulin is a key protein hormone that regulates blood glucose levels and, thus, has widespread impact on lipid and protein metabolism. Insulin action is manifested through binding of its monomeric form to the Insulin Receptor (IR). At present, however, our knowledge about the structural behavior of insulin is based upon inactive, multimeric, and storage-like states. The active monomeric structure, when in complex with the receptor, must be different as the residues crucial for the interactions are buried within the multimeric forms. Although the exact nature of the insulin's induced-fit is unknown, there is strong evidence that the C-terminal part of the B-chain is a dynamic element in insulin activation and receptor binding. Here, we present the design and analysis of highly active (200-500%) insulin analogues that are truncated at residue 26 of the B-chain (B(26)). They show a structural convergence in the form of a new beta-turn at B(24)-B(26). We propose that the key element in insulin's transition, from an inactive to an active state, may be the formation of the beta-turn at B(24)-B(26) associated with a trans to cis isomerisation at the B(25)-B(26) peptide bond. Here, this turn is achieved with N-methylated L-amino acids adjacent to the trans to cis switch at the B(25)-B(26) peptide bond or by the insertion of certain D-amino acids at B(26). The resultant conformational changes unmask previously buried amino acids that are implicated in IR binding and provide structural details for new approaches in rational design of ligands effective in combating diabetes.