Rapid flow-based peptide synthesis.

A flow-based solid-phase peptide synthesis methodology that enables the incorporation of an amino acid residue every 1.8 min under automatic control or every 3 min under manual control is described. This is accomplished by passing a stream of reagent through a heat exchanger into a low volume, low backpressure reaction vessel, and through a UV detector. These features enable continuous delivery of heated solvents and reagents to the solid support at high flow rate, thereby maintaining maximal concentration of reagents in the reaction vessel, quickly exchanging reagents, and eliminating the need to rapidly heat reagents after they have been added to the vessel. The UV detector enables continuous monitoring of the process. To demonstrate the broad applicability and reliability of this method, it was employed in the total synthesis of a small protein, as well as dozens of peptides. The quality of the material obtained with this method is comparable to that for traditional batch methods, and, in all cases, the desired material was readily purifiable by RP-HPLC. The application of this method to the synthesis of the 113-residue Bacillus amyloliquefaciens RNase and the 130-residue DARPin pE59 is described in the accompanying manuscript.

Flow-based enzymatic ligation by sortase A.

Sortase-mediated ligation (sortagging) is a versatile, powerful strategy for protein modification. Because the sortase reaction reaches equilibrium, a large excess of polyglycine nucleophile is often employed to drive the reaction forward and suppress sortase-mediated side reactions. A flow-based sortagging platform employing immobilized sortase A within a microreactor was developed that permits efficient sortagging at low nucleophile concentrations. The platform was tested with several reaction partners and used to generate a protein bioconjugate inaccessible by solution-phase batch sortagging.

Protein thioester synthesis enabled by sortase.

Proteins containing a C-terminal thioester are important intermediates in semisynthesis. Currently there is one main method for the synthesis of protein thioesters that relies upon the use of engineered inteins. Here we report a simple strategy, utilizing sortase A, for routine preparation of recombinant proteins containing a C-terminal (α)thioester. We used our method to prepare two different anthrax toxin cargo proteins: one containing an (α)thioester and another containing a D-polypeptide segment situated between two protein domains. We show that both variants can translocate through protective antigen pore. This new method to synthesize a protein thioester allows for interfacing of sortase-mediated ligation and native chemical ligation.

Asymmetric Total Synthesis of C9'-epi-Sinefungin.

The natural nucleoside (+)-sinefungin, structurally similar to cofactor S-adenosyl-l-methionine, inhibits various SAM-dependent methyltransferases (MTs). Access to sinefungin analogues could serve as the basis for the rational design of small molecule methyltransferase inhibitors. We developed a route to the unnatural C9' epimer of sinefungin that employed a diastereoselective Overman rearrangement to install the key C6' amino stereocenter. The ability for late-stage modification is highlighted, opening an avenue for the discovery of new MT inhibitors.

High-Affinity Alkynyl Bisubstrate Inhibitors of Nicotinamide N-Methyltransferase (NNMT).

Nicotinamide N-methyltransferase (NNMT) is a metabolic enzyme that methylates nicotinamide (NAM) using cofactor S-adenosylmethionine (SAM). NNMT overexpression has been linked to diabetes, obesity, and various cancers. In this work, structure-based rational design led to the development of potent and selective alkynyl bisubstrate inhibitors of NNMT. The reported nicotinamide-SAM conjugate (named NS1) features an alkyne as a key design element that closely mimics the linear, 180° transition state geometry found in the NNMT-catalyzed SAM → NAM methyl transfer reaction. NS1 was synthesized in 14 steps and found to be a high-affinity, subnanomolar NNMT inhibitor. An X-ray cocrystal structure and SAR study revealed the ability of an alkynyl linker to span the methyl transfer tunnel of NNMT with ideal shape complementarity. The compounds reported in this work represent the most potent and selective NNMT inhibitors reported to date. The rational design principle described herein could potentially be extended to other methyltransferase enzymes.