Limited tendency of α-helical residues to form disulfide bridges: a structural explanation

Disulfide engineering on temporin-SHf: Stabilizing the bioactive conformation of an ultra-short antimicrobial peptide

In Silico searching for short antimicrobial peptides has revealed temporin-SHf as the short (8AA), hydrophobic, broad spectrum, and natural antimicrobial peptide. Important drawback associated with temporin-SHf is the susceptibility of its bioactive conformation for denaturation and proteolytic degradation. In the current report, disulfide engineering strategy has been adopted to improve the stability of bioactive conformation of temporin-SHf. The functionally non-critical Leu4 and Ile7 residues at i and i + 3 position of helical conformation of temporin-SHf were mutated with cysteine disulfide. Designed [L4C, I7C]temporin-SHf was synthesized, characterized using NMR spectroscopy, and accessed for antimicrobial activity. [L4C, I7C]Temporin-SHf adopts helical conformation from Phe3 to Phe8 in the absence of membrane-mimetic environment and retains broad spectrum antimicrobial activity. The reduction potential of cysteine disulfide of [L4C, I7C]temporin-SHf is -289 mV. Trypsin-induced digestion and serum-induced digestion have confirmed the advantage of cysteine disulfide in imparting proteolytic stability to temporin-SHf. Disulfide-stabilized temporin-SHf may serve as a good model for the rational design of temporin-SHf based antibiotics for treatment of infectious diseases.

Unraveling the Structural Puzzle of the Giant Glutenin Polymer-An Interplay between Protein Polymerization, Nanomorphology, and Functional Properties in Bioplastic Films

A combination of genotype, cultivation environment, and protein separation procedure was used to modify the nanoscale morphology, polymerization, and chemical structure of glutenin proteins from wheat. A low-polymerized glutenin starting material was the key to protein-protein interactions mainly via SS cross-links during film formation, resulting in extended β-sheet structures and propensity toward the formation of nanoscale morphologies at molecular level. The properties of glutenin bioplastic films were enhanced by the selection of a genotype with a high number of cysteine residues in its chemical structure and cultivation environment with a short grain maturation period, both contributing positively to gluten strength. Thus, a combination of factors affected the structure of glutenins in bioplastic films by forming crystalline β-sheets and propensity toward the ordered nanostructures, thereby resulting in functional properties with high strength, stiffness, and extensibility.

Computer simulations reveal changes in the conformational space of the transcriptional regulator MosR upon the formation of a disulphide bond and in the collective motions that regulate its DNA-binding affinity

M. tuberculosis oxidation sense Regulator (MosR) is a transcriptional regulator from Mycobacterium tuberculosis. It senses the environment oxidation and regulates the expression of a secreted oxidoreductase, thus defending the bacilli against oxidative stress from the phagosome. While most of the members of the Multiple antibiotics resistance Regulator (MarR) family are ligand-responsive, MosR may dissociate from its DNA site upon formation of an intrachain disulphide bond. However, the structure of MosR in its oxidized state is not known, and it is not clear how the formation of this disulphide bond would lead to the conformational changes required for dissociation of the DNA. Nonetheless, MosR presents two crystallographically resolved conformations in its reduced state: bound and unbound to DNA. We managed to simulate MosR unbound to the DNA, both in the presence and in the absence of the disulphide bond. Our results indicate that this disulphide bond precludes the N-terminal residues from adopting a conformation that stands in-between the helix α1 and the DNA binding domain (DBD) from the other chain. Once this conformation is achieved in the reduced state, this DBD detaches from the dimerization domain and becomes more flexible, being able to perform motions with higher amplitude and higher degree of collectivity. Only then, MosR may achieve a conformation where its recognition helices fit into the major grooves of its DNA site. The analysis of the collective motions performed by MosR, during the different situations sampled by the molecular dynamics (MDs), was only possible by the method of filtering harmonic modes with specific frequencies. The frequency of the collective motions performed by the DBD of MosR in the reduced state to achieve a DNA-binding conformation is in the range of 20 to 50 MHz, but it may be associated to more sporadic events since it requires the combination of a suitable conformation of the N-terminal residues.

Polyproline and triple helix motifs in host-pathogen recognition

Secondary structure elements often mediate protein-protein interactions. Despite their low abundance in folded proteins, polyproline II (PPII) and its variant, the triple helix, are frequently involved in protein-protein interactions, likely due to their peculiar propensity to be solvent-exposed. We here review the role of PPII and triple helix in mediating host-pathogen interactions, with a particular emphasis to the structural aspects of these processes. After a brief description of the basic structural features of these elements, examples of host-pathogen interactions involving these motifs are illustrated. Literature data suggest that the role played by PPII motif in these processes is twofold. Indeed, PPII regions may directly mediate interactions between proteins of the host and the pathogen. Alternatively, PPII may act as structural spacers needed for the correct positioning of the elements needed for adhesion and infectivity.

Recent investigations have highlighted that collagen triple helix is also a common target for bacterial adhesins. Although structural data on complexes between adhesins and collagen models are rather limited, experimental and theoretical studies have unveiled some interesting clues of the recognition process. Interestingly, very recent data show that not only is the triple helix used by pathogens as a target in the host-pathogen interaction but it may also act as a bait in these processes since bacterial proteins containing triple helix regions have been shown to interact with host proteins.

As both PPII and triple helix expose several main chain non-satisfied hydrogen bond acceptors and donors, both elements are highly solvated. The preservation of the solvation state of both PPII and triple helix upon protein-protein interaction is an emerging aspect that will be here thoroughly discussed.

Crystallization and preliminary X-ray crystallographic analysis of two dimeric hyperthermostable thioredoxins isolated from Sulfolobus solfataricus

The thioredoxin system of the archaeon Sulfolobus solfataricus involves a number of different proteins: two thioredoxin reductases (SsTrxRB2 and SsTrxRB3), two distinct thioredoxins (SsTrxA1 and SsTrxA2) and a disulfide oxidoreductase (SsPDO). Here, the crystallization and preliminary crystallographic analyses of SsTrxA1 and SsTrxA2, two dimeric proteins endowed with extraordinary thermal stability, are reported. In addition to the functional thioredoxin domain, both SsTrxA1 and SsTrxA2 present an extra N-terminal fragment of approximately 30 residues. Although crystallization trials have been conducted on both forms of the proteins, crystals that were suitable for X-ray crystallographic analyses have only been obtained for their truncated variants. The crystals of SsTrxA2 belonged to space group P2, with unit-cell parameters a = 28.27, b = 27.88, c = 62.06 A, beta = 92.34 degrees , and diffracted to 1.83 A resolution, whereas the crystals of SsTrxA1 belonged to space group P2(1), with unit-cell parameters a = 51.76, b = 75.09, c = 55.35 A, beta = 112.64 degrees , and diffracted to 1.90 A resolution. The structures of the two proteins have been solved by molecular replacement.

Structure and stability of a thioredoxin reductase from Sulfolobus solfataricus: a thermostable protein with two functions

Recent investigations have demonstrated that disulfide bridges may play a crucial role in the stabilization of proteins in hyperthermophilic organisms. A major role in the process of disulfide formation is played by ubiquitous proteins belonging to the thioredoxin superfamily, which includes thioredoxins (Trx), thioredoxin reductases (TrxR), and disulfide oxidases/isomerases (PDO/PDI). Here we report a characterization of the structure and stability of the TrxR (SsTrxRB3) isolated from the archaeon Sulfolobus solfataricus. This protein is particularly interesting since it is able to process different substrates (Trxs and PDO) and it is endowed with an additional NADH oxidase activity. The crystal structure of the wild-type enzyme, of its complex with NADP and of the C147A mutant provides interesting clues on the enzyme function. In contrast to what is observed for class II TrxRs, in the structure of the oxidized enzyme, the FAD binding site is occupied by a partially disordered NAD molecule. In the active site of the C147A mutant, which exhibits a marginal NADH oxidase activity, the FAD is canonically bound to the enzyme. Molecular modeling indicates that a FAD molecule can be accommodated in the site of the reduced SsTrxRB3. Depending on the oxidation state, SsTrxRB3 can bind a different cofactor in its active site. This peculiar feature has been related to its dual activity. Denaturation experiments followed by circular dichroism indicate that electrostatic interactions play an important role in the stabilization of this thermostable protein. The analysis of the enzyme 3D-structure has also provided insights into the bases of SsTrxRB3 stability.