Design, synthesis, and characterization of novel fluorogenic substrates of the proprotein convertases furin, PC1/3, PC2, PC5/6, and PC7.

Proprotein convertases (PCs) are involved in the pathogenesis of various diseases, making them promising drug targets. Most assays for PCs have been performed with few standard substrates, regardless of differences in cleavage efficiencies. Derived from studies on substrate-analogue inhibitors, 11 novel substrates were synthesized and characterized with five PCs. H-Arg-Arg-Tle-Lys-Arg-AMC is the most efficiently cleaved furin substrate based on its kcat/KM value. Due to its higher kcat value, acetyl-Arg-Arg-Tle-Arg-Arg-AMC was selected for further measurements to demonstrate the benefit of this improved substrate. Compared to our standard conditions, its use allowed a 10-fold reduction of the furin concentration, which enabled Ki value determinations of previously described tight-binding inhibitors under classical conditions. Under these circumstances, a slow-binding behavior was observed for the first time with inhibitor MI-1148. In addition to furin, four additional PCs were used to characterize these substrates. The most efficiently cleaved PC1/3 substrate was acetyl-Arg-Arg-Arg-Tle-Lys-Arg-AMC. The highest kcat/KM values for PC2 and PC7 were found for the N-terminally unprotected analogue of this substrate, although other substrates possess higher kcat values. The highest efficiency for PC5/6A was observed for the substrate acetyl-Arg-Arg-Tle-Lys-Arg-AMC. In summary, we have identified new substrates for furin, PC1/3, PC2, and PC7 suitable for improved enzyme-kinetic measurements.

The Basicity Makes the Difference: Improved Canavanine-Derived Inhibitors of the Proprotein Convertase Furin.

Furin activates numerous viral glycoproteins, and its inhibition prevents virus replication and spread. Through the replacement of arginine by the less basic canavanine, new inhibitors targeting furin in the trans-Golgi network were developed. These inhibitors exert potent antiviral activity in cell culture with much lower toxicity than arginine-derived analogues, most likely due to their reduced protonation in the blood circulation. Thus, despite its important physiological functions, furin might be a suitable antiviral drug target.