Nitropyrrole natural products: isolation, biosynthesis and total synthesis

A review of the isolation, biological activity, biosynthesis and chemical synthesis of nitropyrrole-containing natural products reported to date, including the pyrrolomycins, heronapyrroles and nitropyrrolins.

Structure–mechanical property correlations of hydrogel forming β-sheet peptides

This review discusses about β-sheet peptide structure at the molecular level and the bulk mechanical properties of the corresponding hydrogels.

Substituted Carbazoles – A New Class of Anthelmintic Agent

A series of novel carbazoles were synthesized based on structural modifications to lead carbazole 1 (EC100 = 2.5 μM against Haemonchus contortus in vitro), which was revealed in a small molecule screening program as a potentially promising platform for the development of new anthelmintic drugs. Subsequently, analogues 19, 21, 41, 42 (EC100 = 1.25 μM, all), and 39 (EC100 = 0.625 μM) were demonstrated to exhibit enhanced in vitro anthelmintic activity over the lead structure, with compound 39 also being shown to be active in vivo against Heligmosomoides polygyrus.

Peptide binding to a bacterial signal peptidase visualized by peptide tethering and carrier-driven crystallization

Bacterial type I signal peptidases (SPases) are membrane-anchored serine proteases that process the signal peptides of proteins exported via the Sec and Tat secretion systems. Despite their crucial importance for bacterial virulence and their attractiveness as drug targets, only one such enzyme, LepB from Escherichia coli, has been structurally characterized, and the transient nature of peptide binding has stymied attempts to directly visualize SPase-substrate complexes. Here, the crystal structure of SpsB, the type I signal peptidase from the Gram-positive pathogen Staphylococcus aureus, is reported, and a peptide-tethering strategy that exploits the use of carrier-driven crystallization is described. This enabled the determination of the crystal structures of three SpsB-peptide complexes, both with cleavable substrates and with an inhibitory peptide. SpsB-peptide interactions in these complexes are almost exclusively limited to the canonical signal-peptide motif Ala-X-Ala, for which clear specificity pockets are found. Minimal contacts are made outside this core, with the variable side chains of the peptides accommodated in shallow grooves or exposed faces. These results illustrate how high fidelity is retained despite broad sequence diversity, in a process that is vital for cell survival.

Synthesis and bioactivity of antitubercular peptides and peptidomimetics: an update

This mini-review provides an update on the synthesis and bioactivity of peptides and peptidomimetics that exhibit very potent antitubercular activity.

A new high-capacity metal ion-complexing gel containing cyclen ligands

“Hairy” nano-scale objects cross-linked into a high-capacity metal-binding hydrogel.

A peptide hydrogel derived from a fragment of human cardiac troponin C

The human cardiac troponin C peptide fragment H-V⁹EQLTEEQKNEFKAAFDIFVLGA³¹-OH self assembles into β-sheets fibrils that further entangle to give a hydrogels.

Glicentin-related pancreatic polypeptide inhibits glucose-stimulated insulin secretion from the isolated pancreas of adult male rats

Peptides derived from the glucagon gene Gcg, for example, glucagon and glucagon‐like peptide 1 (GLP‐1), act as physiological regulators of fuel metabolism and are thus of major interest in the pathogenesis of diseases, such as type‐2 diabetes and obesity, and their therapeutic management. Glicentin‐related pancreatic polypeptide (GRPP) is a further, 30 amino acid Gcg‐derived peptide identified in human, mouse, rat, and pig. However, the potential glucoregulatory function of this peptide is largely unknown. Here, we synthesized rat GRPP (rGRPP) and a closely related peptide, rat GRPP‐like peptide (rGRPP‐LP), and investigated their actions in the liver and pancreas of adult male rats by employing isolated‐perfused organ preparations. Rat GRPP and rGRPP‐LP did not affect glucose output from the liver, but both elicited potent inhibition of glucose‐stimulated insulin secretion (GSIS) from the rat pancreas. This action is unlikely to be mediated by glucagon or GLP‐1 receptors, as rGRPP and rGRPP‐LP did not stimulate cyclic adenosine monophosphate (cAMP) production from the glucagon or GLP‐1 receptors, nor did they antagonize glucagon‐ or GLP‐1‐stimulated cAMP‐production at either receptor. GRPP and GRPP‐LP may be novel regulators of insulin secretion, acting through an as‐yet undefined receptor.

Convergent chemo-enzymatic synthesis of mannosylated glycopeptides; targeting of putative vaccine candidates to antigen presenting cells

The combination of solid phase peptide synthesis and endo-β-N-acetylglucosaminidase (ENGase) catalysed glycosylation is a powerful convergent synthetic method allowing access to glycopeptides bearing full-length N-glycan structures. Mannose-terminated N-glycan oligosaccharides, produced by either total or semi-synthesis, were converted into oxazoline donor substrates. A peptide from the human cytomegalovirus (CMV) tegument protein pp65 that incorporates a well-characterised T cell epitope, containing N-acetylglucosamine at specific Asn residues, was accessed by solid phase peptide synthesis, and used as an acceptor substrate. High-yielding enzymatic glycosylation afforded glycopeptides bearing defined homogeneous high-mannose N-glycan structures. These high-mannose containing glycopeptides were tested for enhanced targeting to human antigen presenting cells (APCs), putatively mediated via the mannose receptor, and for processing by the APCs for presentation to human CD8+ T cells specific for a 9-mer epitope within the peptide. Binding assays showed increased binding of glycopeptides to APCs compared to the non-glycosylated control. Glycopeptides bearing high-mannose N-glycan structures at a single site outside the T cell epitope were processed and presented by the APCs to allow activation of a T cell clone. However, the addition of a second glycan within the T cell epitope resulted in ablation of T cell activation. We conclude that chemo-enzymatic synthesis of mannosylated glycopeptides enhances uptake by human APCs while preserving the immunogenicity of peptide epitopes within the glycopeptides, provided those epitopes are not themselves glycosylated.

NKT cell-dependent glycolipid-peptide vaccines with potent anti-tumour activity

Glycolipid–peptide conjugates designed to release vaccine components within target cells ensuring potent CD1d dependent T cell responses.

It is known that T cells can eliminate tumour cells through recognition of unique or aberrantly expressed antigens presented as peptide epitopes by major histocompatibility complex (MHC) molecules on the tumour cell surface. With recent advances in defining tumour-associated antigens, it should now be possible to devise therapeutic vaccines that expand specific populations of anti-tumour T cells. However there remains a need to develop simpler efficacious synthetic vaccines that possess clinical utility. We present here the synthesis and analysis of vaccines based on conjugation of MHC-binding peptide epitopes to α-galactosylceramide, a glycolipid presented by the nonpolymorphic antigen-presenting molecule CD1d to provoke the stimulatory activity of type I natural killer T (NKT) cells. The chemical design incorporates an enzymatically cleavable linker that effects controlled release of the active components in vivo. Chemical and biological analysis of different linkages with different enzymatic targets enabled selection of a synthetic vaccine construct with potent therapeutic anti-tumour activity in mice, and marked in vitro activity in human blood.

Regulation and Quality Control of Adiponectin Assembly by Endoplasmic Reticulum Chaperone ERp44

Adiponectin, a collagenous hormone secreted abundantly from adipocytes, possesses potent antidiabetic and anti-inflammatory properties. Mediated by the conserved Cys(39) located in the variable region of the N terminus, the trimeric (low molecular weight (LMW)) adiponectin subunit assembles into different higher order complexes, e.g. hexamers (middle molecular weight (MMW)) and 12-18-mers (high molecular weight (HMW)), the latter being mostly responsible for the insulin-sensitizing activity of adiponectin. The endoplasmic reticulum (ER) chaperone ERp44 retains adiponectin in the early secretory compartment and tightly controls the oxidative state of Cys(39) and the oligomerization of adiponectin. Using cellular and in vitro assays, we show that ERp44 specifically recognizes the LMW and MMW forms but not the HMW form. Our binding assays with short peptide mimetics of adiponectin suggest that ERp44 intercepts and converts the pool of fully oxidized LMW and MMW adiponectin, but not the HMW form, into reduced trimeric precursors. These ERp44-bound precursors in the cis-Golgi may be transported back to the ER and released to enhance the population of adiponectin intermediates with appropriate oxidative state for HMW assembly, thereby underpinning the process of ERp44 quality control.

An update on new methods to synthesize cyclotetrapeptides

Cyclotetrapeptides are important bioactive lead drug molecules that display a wide spectrum of pharmacological activities. However, the synthesis of cyclotetrapeptides from their linear precursors is challenging due to the highly constrained conformation required for cyclisation, thus hampering their progress to a clinical setting. This review provides an account of the reported methods used for the synthesis of cyclotetrapeptides.

Diabetes-induced alterations in tissue collagen and carboxymethyllysine in rat kidneys: Association with increased collagen-degrading proteinases and amelioration by Cu(II)-selective chelation

Advanced glycation end-products (AGEs) comprise a group of non-enzymatic post-translational modifications of proteins and are elevated in diabetic tissues. AGE-modification impairs the digestibility of collagen in vitro but little is known about its relation to collagen-degrading proteinases in vivo. N(ε)-carboxymethyllysine (CML) is a stable AGE that forms on lysyl side-chains in the presence of glucose, probably via a transition metal-catalysed mechanism. Here, rats with streptozotocin-induced diabetes and non-diabetic controls were treated for 8weeks with placebo or the Cu(II)-selective chelator, triethylenetetramine (TETA), commencing 8weeks after disease induction. Actions of diabetes and drug treatment were measured on collagen and collagen-degrading proteinases in kidney tissue. The digestibility and CML content of collagen, and corresponding levels of mRNAs and collagen, were related to changes in collagen-degrading-proteinases. Collagen-degrading proteinases, cathepsin L (CTSL) and matrix metalloproteinase-2 (MMP-2) were increased in diabetic rats. CTSL-levels correlated strongly and positively with increased collagen-CML levels and inversely with decreased collagen digestibility in diabetes. The collagen-rich mesangium displayed a strong increase of CTSL in diabetes. TETA treatment normalised kidney collagen content and partially normalised levels of CML and CTSL. These data provide evidence for an adaptive proteinase response in diabetic kidneys, affected by excessive collagen-CML formation and decreased collagen digestibility. The normalisation of collagen and partial normalisation of CML- and CTSL-levels by TETA treatment supports the involvement of Cu(II) in CML formation and altered collagen metabolism in diabetic kidneys. Cu(II)-chelation by TETA may represent a treatment option to rectify collagen metabolism in diabetes independent of alterations in blood glucose levels.

Using mass spectrometry to detect, differentiate, and semiquantitate closely related peptide hormones in complex milieu: measurement of IGF-II and vesiculin

The search for an islet β-cell growth factor has been a key objective in recent diabetes research, because the ability to regenerate and/or protect the functioning β-cell population in patients could result in a great advancement for diabetes treatment. IGF-I and IGF-II are known to play crucial roles in fetal growth and prenatal development, and there is growing evidence that IGF-II increases β-cell proliferation and survival in vitro and in vivo. A search for the source of IGF-II-like immunoreactivity in isolated β-cell secretory granules from the murine cell line βTC6-F7 revealed a novel 2-chain IGF-II-derived peptide, which we named vesiculin and which has been shown to be a full insulin agonist. Here, we present a liquid chromatography-tandem mass spectrometry method that enables selective detection and semiquantitation of the highly related IGF-II and vesiculin molecules. We have used this method to measure these 2 peptides in conditioned media from 2 β-cell lines, produced under increasing glucose concentrations. This technique detected both IGF-II and vesiculin in media conditioned by MIN6 and βTC6-F7 cells at levels in the range of 0 to 6 μM (total insulin, 80-450 μM) and revealed a glucose-stimulated increase in insulin, IGF-II, and vesiculin. IGF-II was detected in adult human and neonatal mouse serum in high levels, but vesiculin was not present. The methodology we present herein has utility for detecting and differentiating active peptides that are highly related and of low abundance.