The Conformation and Assignment of the Proton NMR Spectrum in Water of DX600, a Bioactive Peptide with a Random Coil Conformation

DX600, a small peptide with 26 residues, is a potent, highly selective inhibitor of angiotensin converting enzyme 2 (ACE2). A range of NMR methods including TOCSY and ROESY yield an assignment of its proton spectrum in water and constraints on its conformation. Constrained molecular dynamics simulations of solvated DX600 show that the peptide's most abundant conformer adopts a predominantly random coil conformation. Constrained by the disulfide bond, its backbone defines an overhand knot with frayed ends. 1. Introduction Angiotensin converting enzyme (ACE), a dipeptidase, is a key component of the renin-angiotensin system that regulates blood pressure [1, 2]. It catalyzes the conversion of inactive angiotensin I to angiotensin II, a potent vasoconstrictor, and the hydrolysis of bradykinin, a vasodilator. Consequently the treatment of hypertension makes extensive use of potent, selective inhibitors of ACE such as captopril and trandolapril [3]. Given the medical importance of the inhibition of ACE, the drug-design community has produced several X-ray structures of ACE-inhibitor complexes [4] and NMR solution-phase structures of the free inhibitors [5, 6]. ACE2 is a closely related enzyme which exhibits considerable sequence and structural homology with ACE [7, 8]. Both are peptidases with zinc at the active center. However, there are differences. Whereas ACE is widely distributed in the body, ACE2 is found principally in the heart, kidney, and testis. It cleaves the terminal leucine residue from the decapeptide angiotensin I and does not play a major role in the control of blood pressure. Knockout studies with mice indicate that ACE2 affects cardiac function [9]. To date, no inhibitors of ACE2 have entered clinical practice [3]. However, inhibitors of ACE2 have been developed as tools for exploring and modulating its biological function [10, 11]. Huang et al. screened peptide libraries displayed on phage and identified six highly potent inhibitors of ACE2 which do not inhibit ACE [12]. DX600, the most potent in the set with (the reciprocal of the equilibrium constant for the binding of the inhibitor, e.g., DX600, to the enzyme) equal to 2.8？nM, has the sequence Gly1-Asp2-Tyr3-Ser4-His5-Cys6-Ser7-Pro8-Leu9-Arg10-Tyr11-Tyr12-Pro13-Trp14-Trp15-Lys16-Cys17-Thr18-Tyr19-Pro20-Asp21-Pro22-Glu23-Gly24-Gly25-Gly26. It is acetylated at the amino terminus and amidated at the carboxyl terminus and a disulfide bond links the two cysteine residues, Cys6 and Cys17. DX600’s properties as an inhibitor mark it as a candidate for an NMR study and its unusual composition