Triple Function of 4-Mercaptophenylacetic Acid Promotes One-Pot Multiple Peptide Ligation

Chemical Synthesis of Bioactive Proteins

Nature has developed a plethora of protein machinery to operate and maintain nearly every task of cellular life. These processes are tightly regulated via post-expression modifications-transformations that modulate intracellular protein synthesis, folding, and activation. Methods to prepare homogeneously and precisely modified proteins are essential to probe their function and design new bioactive modalities. Synthetic chemistry has contributed remarkably to protein science by allowing the preparation of novel biomacromolecules that are often challenging or impractical to prepare via common biological means. The ability to chemically build and precisely modify proteins has enabled the production of new molecules with novel physicochemical properties and programmed activity for biomedical research, diagnostic, and therapeutic applications. This minireview summarizes recent developments in chemical protein synthesis to produce bioactive proteins, with emphasis on novel analogs with promising in vitro and in vivo activity.

Intracellular Protein Photoactivation Using Sterically Bulky Caging

Methods for intracellular protein photoactivation have been studied to elucidate the spatial and temporal roles of proteins of interest. In this study, an intracellular protein photoactivation method was developed using sterically bulky caging. The protein of interest was modified with biotin via a photocleavable linker, and then conjugated with streptavidin to sterically block the protein surface for inactivation. The caged protein was transduced into cells and reactivated by light-induced degradation of the conjugates. A cytotoxic protein, saporin, was caged and photoactivated both in vitro and in living cells with this method. This method achieved control of the cytotoxic activity in an off-on manner, introducing cell death selectively at the designed location using light. This simple and versatile photoactivation method is a promising tool for studying spatio-temporal cellular events that are related to intracellular proteins of interest.

Copper-Mediated Selenazolidine Deprotection Enables One-Pot Chemical Synthesis of Challenging Proteins

While chemical protein synthesis has granted access to challenging proteins, the synthesis of longer proteins is often limited by low abundance or non-strategic placement of cysteine residues, which are essential for native chemical ligations, as well as multiple purification and isolation steps. We describe the one-pot total synthesis of human thiosulfate:glutathione sulfurtransferase (TSTD1). WT-TSTD1 was synthesized in a C-to-N synthetic approach involving multiple NCL reactions, Cu^II -mediated deprotection of selenazolidine (Sez), and chemoselective deselenization. The seleno-analog Se-TSTD1, in which the active site Cys is replaced with selenocysteine, was also synthesized with a kinetically controlled ligation with an N-to-C synthetic approach. The catalytic activity of the two proteins indicated that Se-TSTD1 possessed only four-fold lower activity than WT-TSTD1, thus suggesting that selenoproteins can have physiologically comparable sulfutransferase activity to their cysteine counterparts.

Ligation of Soluble but Unreactive Peptide Segments in the Chemical Synthesis of Haemophilus Influenzae DNA Ligase

During the total chemical synthesis of the water-soluble globular Haemophilus Influenzae DNA ligase (Hin-Lig), we observed the surprising phenomenon of a soluble peptide segment that failed to undergo native chemical ligation. Based on dynamic light scattering and transmission electron microscopy experiments, we determined that the peptide formed soluble colloidal particles in a homogeneous solution containing 6 m guanidine hydrochloride. Conventional peptide performance-improving strategies, such as installation of a terminal/side-chain Arg tag or O-acyl isopeptide, failed to enable the reaction, presumably because of their inability to disrupt the formation of soluble colloidal particles. However, a removable backbone modification strategy recently developed for the synthesis of membrane proteins did disrupt the formation of the colloids, and the desired ligation of this soluble but unreactive system was eventually accomplished. This work demonstrates that an appropriate solution dispersion state, in addition to good peptide solubility, is a prerequisite for successful peptide ligation.