Imidazole Nitrogens of Two Histidine Residues Participating in N–H···N Hydrogen Bonds in Protein Structures: Structural Bioinformatics Approach Combined with Quantum Chemical Calculations

Protein structures are stabilized by different types of hydrogen bonds. However, unlike the DNA double helical structure, the N–H···N type of hydrogen bonds is relatively rare in proteins. N–H···N hydrogen bonds formed by imidazole groups of two histidine residues have not been investigated. We have systematically analyzed 5333 high-resolution protein structures with resolution 1.8 Å or better and identified 285 histidine pairs in which the nitrogen atoms of the imidazole side chains can potentially participate in N–H···N hydrogen bonds. The histidine pairs were further divided into two groups, neutral–neutral and protonated–neutral, depending on the protonation state of the donor histidine. Quantum chemical calculations were performed on imidazole groups adopting the same geometry observed in the protein structures. Average interaction energies between the interacting imidazole groups are −6.45 and −22.5 kcal/mol for neutral–neutral and protonated–neutral, respectively. Hydrogen bond interaction between the imidazole moieties is further confirmed by natural bond orbital analyses of the model compounds. Histidine residues involved in N–H···N hydrogen bonds are relatively more buried and have low <i>B</i>-factor values in the protein structures. N–H···N hydrogen bond formed by a pair of buried histidine residues can significantly contribute to the structural stability of proteins

Controlling in Vitro Insulin Amyloidosis with Stable Peptide Conjugates: A Combined Experimental and Computational Study

Insulin aggregation, to afford amyloidogenic polypeptide fibrils, is an energetically driven, well-studied phenomenon, which presents interesting biological ramifications. These aggregates are also known to form around insulin injection sites and in diabetic patients suffering from Parkinson’s disease. Such occurrences force considerable reduction in hormone activity and are often responsible for necrotic deposits in diabetic patients. Changes in physicochemical environment, such as pH, temperature, ionic strength, and mechanical agitation, affect insulin fibrillation, which also presents intrigue from the structural viewpoint. Several reports have tried to unravel underlying mechanisms concerning the aggregation process taking into account a three aromatic amino acid patch Phe^B24-Phe^B25-Tyr^B26 located in the C-terminal part of the B chain, identified as a key site for human insulin–receptor interaction. The present study describes design and inhibitory effects of novel peptide conjugates toward fibrillation of insulin as investigated by thioflavin T assay, circular dichroism, and AFM. Possible interaction of insulin with peptide-based fibrillation inhibitors reveals an important role of hydrophobic interactions in the inhibition process. Molecular dynamics simulation studies demonstrate that inhibitor <b>D4</b> interacts with insulin residues from the helix and the C-terminal extended segment of chain B. These studies present a novel approach for the discovery of stable, peptide-based ligands as novel antiamyloidogenic agents for insulin aggregation

Calmidazolium Chloride and Its Complex with Serum Albumin Prevent Huntingtin Exon1 Aggregation

Huntington’s disease (HD) is a genetic disorder caused by a CAG expansion mutation in <i>Huntingtin</i> gene leading to polyglutamine (polyQ) expansion in the N-terminus side of Huntingtin (Httex1) protein. Neurodegeneration in HD is linked to aggregates formed by Httex1 bearing an expanded polyQ. Initiation and elongation steps of Httex1 aggregation are potential target steps for the discovery of therapeutic molecules for HD, which is currently untreatable. Here we report Httex1 aggregation inhibition by calmidazolium chloride (CLC) by acting on the initial aggregation event. Because it is hydrophobic, CLC was adsorbed to the vial surface and could not sustain an inhibition effect for a longer duration. The use of bovine serum albumin (BSA) prevented CLC adsorption by forming a BSA–CLC complex. This complex showed improved Httex1 aggregation inhibition by interacting with the aggregation initiator, the NT₁₇ part of Httex1. Furthermore, biocompatible CLC-loaded BSA nanoparticles were made which reduced the polyQ aggregates in HD-150Q cells