Chemical Synthesis of the Sec-To-Cys Homologue of Human Selenoprotein F and Elucidation of Its Disulfide-pairing Mode

Insights into the interaction between UGGT, the gatekeeper of folding in the ER, and its partner, the selenoprotein SEP15

The enzyme UDP-glucose: glycoprotein glucosyltransferase (UGGT) is the gatekeeper of protein folding within the endoplasmic reticulum (ER). One-third of the human proteome traverses the ER where folding and maturation are facilitated by a complex protein homeostasis network. Both glycan modifications and disulfide bonds are of key importance in the maturation of these ER proteins. The actions of UGGT are intimately linked to the glycan code for folding and maturation of secretory proteins in the ER. UGGT selectively glucosylates the N-linked glycan of misfolded proteins so that they can reenter the lectin-folding chaperone cycle and be retained within the ER for further attempts at folding. An intriguing aspect of UGGT function is its interaction with its poorly understood cochaperone, the 15 kDa selenoprotein known as SELENOF or SEP15. This small protein contains a rare selenocysteine residue proposed to act as an oxidoreductase toward UGGT substrates. AlphaFold2 predictions of the UGGT1/SEP15 complex provide insight into this complex at a structural level. The predicted UGGT1/SEP15 interaction interface was validated by mutagenesis and coimmunoprecipitation experiments. These results serve as a springboard for models of the integrated action of UGGT1 and SEP15.

Azole reagents enabled ligation of peptide acyl pyrazoles for chemical protein synthesis

Native chemical ligation (NCL) has been playing an increasingly important role in chemical protein synthesis (CPS). More efficient ligation methods that circumvent the requirement of a peptidyl thioester and thiol additive-which allow the following desulfurization or refolding in one pot-are urgently needed for the synthesis of more complex protein targets and in large quantities. Herein, we discover that the weak acyl donor peptidyl N-acyl pyrazole can be activated by azole reagents like 3-methylpyrazole or imidazole to facilitate its ligation directly with an N-terminal cysteine peptide. As it requires no thioester or thiol additive, this ligation strategy can be conveniently combined with metal-free desulfurization (MFD) or oxidative protein folding to allow various one-pot protocols. The utility and generality of the strategy are showcased by the total synthesis of ubiquitin via an N-to-C sequential ligation-MFD strategy, the semi-synthesis of the copper protein azurin, and the efficient assembly of a sulfated hirudin variant and the cyclotide kalata B1, all in a one-pot fashion.

Functional expression of a thrombin exosite I inhibitor triabin in Escherichia coli and elucidation of the role of key residues in its inhibitory activity

Triabin, a lipocalin-like thrombin inhibitor from the saliva of the blood-sucking triatomine bug Triatoma pallidipennis, exhibits effective inhibition comparable to hirudin despite binding exclusively at exosite I. Interestingly, it was reported that higher triabin doses would not inhibit thrombin completely, which makes it a promising antithrombotic candidate agent with a larger therapeutic window. However, few structural and functional studies about triabin have been reported in the past three decades, mostly due to the lack of a reliable and practicable recombinant expression technology for this seemingly small protein. In this work, we have adopted the SUMO fusion technology for the expression of triabin in E. coli cells-with facile refolding and purification procedures-and the bioactive triabin was produced in ∼12 mg/L culture medium. Subsequently, the structure-function studies through extensive site-directed mutagenesis reveal that triabin's Phe-106 involved in the hydrophobic contacts plays a surprisingly important role in the thrombin inhibition, in contrast to the negatively charged residues Asp-135 or Glu-128 involved in the salt-bridge interaction. As such, this study complements our understanding of the interaction mechanism of natural thrombin inhibitors, which should facilitate the development of anticoagulant drugs with a novel mode of action against thrombin.

Identification and Immunomodulatory Effect on Immunosuppressed Mice of Selenium-Enriched Peptides of Egg White

Selenium-enriched egg white peptides (Se-EWP) were prepared by pre-heat treatment and enzymatic hydrolysis in this study. In addition, their selenopeptide sequence identification and immunomodulatory effect were investigated. Results showed that the yield of Se-EWP obtained from alkaline-neutral protease treatment reached 76.90%, and peptides with a molecular weight of 200-1000 Da accounted for 98.33%. Four characteristic selenopeptides, including SeCys-Trp-Leu-Glu, Trp-Ser-SeCys, SeMet-Ala-Pro, and SeMet-Leu, were identified by HPLC-ESI-MS/MS, which were rich in hydrophobic and branched-chain amino acids. Se-EWP (750 mg/kg/d) could effectively retard the decrease of immune organ index in immunosuppressed mice induced by cyclophosphamide. Moreover, supplementation of Se-EWP could promote a higher content of Se in liver, the number of white blood cells, and the levels of serum cytokines (IL-6, IL-2, and TNF-α) as compared with EWP groups, indicating that Se-EWP could effectively alleviate immunosuppression induced by cyclophosphamide. These findings suggested that Se-EWP exhibited great potential as functional foods for immunomodulatory effect.

A Shelf Stable Fmoc Hydrazine Resin for the Synthesis of Peptide Hydrazides

C-terminal hydrazides are an important class of synthetic peptides with an ever expanding scope of applications, but their widespread application for chemical protein synthesis has been hampered due to the lack of stable resin linkers for synthesis of longer and more challenging peptide hydrazide fragments. We present a practical method for the regeneration, loading, and storage of trityl-chloride resins for the production of hydrazide containing peptides, leveraging 9-fluorenylmethyl carbazate. We show that these resins are extremely stable under several common resin storage conditions. The application of these resins to solid phase peptide synthesis (SPPS) is demonstrated through the synthesis of the 40-mer GLP-1R agonist peptide "P5". These studies support the broad utility of Fmoc-NHNH-Trt resins for SPPS of C-terminal hydrazide peptides.

Chemical synthesis of human selenoprotein F and elucidation of its thiol-disulfide oxidoreductase activity

Selenoprotein F (SelF) is an endoplasmic reticulum-residing eukaryotic protein that contains a selenocysteine (Sec) residue. It has been suggested to be involved in a number of physiological processes by acting as a thiol-disulfide oxidoreductase, but the exact role has remained unclear due to the lack of a reliable production method. We document herein a robust synthesis of the human SelF through a three-segment two-ligation semisynthesis strategy. Highlighted in this synthetic route are the use of a mild desulfurization process to protect the side-chain of the Sec residue from being affected and the simultaneous removal of acetamidomethyl and p-methoxybenzyl protection groups by PdCl2, thus facilitating the synthesis of multi-milligrams of homogenous SelF. The reduction potential of SelF was determined and the thiol-disulfide oxidoreductase activity was further supported by its ability to catalyze the reduction and isomerization of disulfide bonds.

One-Pot Chemical Protein Synthesis Utilizing Fmoc-Masked Selenazolidine to Address the Redox Functionality of Human Selenoprotein F

Human SELENOF is an endoplasmic reticulum (ER) selenoprotein that contains the redox active motif CXU (C is cysteine and U is selenocysteine), resembling the redox motif of thiol-disulfide oxidoreductases (CXXC). Like other selenoproteins, the challenge in accessing SELENOF has somewhat limited its full biological characterization thus far. Here we present the one-pot chemical synthesis of the thioredoxin-like domain of SELENOF, highlighted by the use of Fmoc-protected selenazolidine, native chemical ligations and deselenization reactions. The redox potential of the CXU motif, together with insulin turbidimetric assay suggested that SELENOF may catalyze the reduction of disulfides in misfolded proteins. Furthermore, we demonstrate that SELENOF is not a protein disulfide isomerase (PDI)-like enzyme, as it did not enhance the folding of the two protein models; bovine pancreatic trypsin inhibitor and hirudin. These studies suggest that SELENOF may be responsible for reducing the non-native disulfide bonds of misfolded glycoproteins as part of the quality control system in the ER.