ACE2, the Receptor that Enables Infection by SARS-CoV-2: Biochemistry, Structure, Allostery and Evaluation of the Potential Development of ACE2 Modulators

Abstract

Angiotensin converting enzyme 2 (ACE2) is the human receptor that interacts with the spike protein of coronaviruses, including the one that produced the 2020 coronavirus pandemic (COVID-19). Thus, ACE2 is a potential target for drugs that disrupt the interaction of human cells with SARS-CoV-2 to abolish infection. There is also interest in drugs that inhibit or activate ACE2, that is, for cardiovascular disorders or colitis. Compounds binding at alternative sites could allosterically affect the interaction with the spike protein. Herein, we review biochemical, chemical biology, and structural information on ACE2, including the recent cryoEM structures of full-length ACE2. We conclude that ACE2 is very dynamic and that allosteric drugs could be developed to target ACE2. At the time of the 2020 pandemic, we suggest that available ACE2 inhibitors or activators in advanced development should be tested for their ability to allosterically displace the interaction between ACE2 and the spike protein.

Keywords: ACE2; allostery; coronavirus; drug development; protein dynamics.

Figure 1

Simplified scheme of the role of ACE2 in the renin–angiotensin system. The cleaving of angiotensinogen by the enzyme renin results in the decapeptide angiotensin I (1–10). ACE1 cleaves angiotensin I to angiotensin II (1–8). Angiotensin II is a potent vasoconstrictor that binds to the type 1 angiotensin II receptor (AT₁R) to set off actions that result in higher blood pressure and inflammation. ACE2 cleaves angiotensin II to produce angiotensin 1–7, which binds to the Mas receptor (MasR) producing vasodilation and other cardioprotective actions. ACE2 is cleaved by ADAM17, which releases the active ACE2 protease catalytic domain to the circulation.

Figure 2

Structure and conformations of ACE2. A) Scheme of motifs and domains of ACE2. ACE2 has an N‐terminal protease catalytic domain PD (blue) and a C‐terminal collectrin‐like‐domain CLD (cyan). The first 17 amino acids correspond to the signal peptide that is cleaved during the maturation of the protein (not shown). The CLD consists of an extracellular neck domain, a linker, a single transmembrane (TM) helix and an intracellular tail of 43 amino acids. The sites of cleavage by proteases that release soluble ACE2 to the circulation are indicated. B) Structure of full‐length ACE2 (tight dimer) in complex with B°AT1 (PDB ID: 6 M17). ACE2 is represented as cylindrical helices and loops with surface; the surface of B°AT1 is presented in grey. For simplification, the RBDs present in this structure are not shown. The monomers of ACE2 are coloured in blue and pink (following the colours of A). The different regions of ACE2 and the four key regulatory sites (1: active site, 2: hinge, 3: claw‐like or spike (RBD)‐binding site, 4: PD dimerization interface) are indicated. C) Schematic representation of the open–close hinge movement of the PD. The active site of the PD can adopt an open, intermediate (not shown) or closed conformation. The hinge pocket is disassembled in the closed structure. D) MLN‐4760 binds at the active site, stabilizes the closed structure and does not affect the interaction with spike. E) Small compounds designed to bind at the hinge region, i. e., diminazene, increase the activity of ACE2. F) Small compounds designed to bind at the active site in the closed structure of PD (NAAE) displace interaction with spike protein. G) Schematic representation of the structure of full‐length ACE2 dimers in two conformations identified by cryoEM in complex with B°AT1. In the absence of the spike protein RBD, the two conformations are found in a 3 : 1 proportion. The tight dimer (left) is a scheme representing the structure shown in B. In both dimer conformations, the neck domains form tight interactions. In the loose dimer (right), the PDs rotate with respect to the neck domain and the PD–PD interaction is lost. In the presence of spike protein RBD, only the tight dimer ACE2 structure is present. In the loose dimer, the conformation of the spike protein binding site in the PD appears modified (detailed in Figure 3). H) Chemical structures: 1: XNT, 2: MLN‐4760, 3: resorcinolnaphthalein, 4: NAAE, 5: diminazene. The mechanisms of action for XNT, resorcinolnaphthalein, NAAE and diminazene (E) and (F) are deduced from biochemical work and not validated structurally.

Figure 3

The rotated and twisted conformations of the full‐length ACE2 loose dimer. The images are obtained by alignment of the tight (blue) and loose (pink) dimers. A) Rotation of the PD in relation to the CLD. The rotation is shown upon alignment of the CLD. B) The structure of the PD in the rotated‐twisted loose dimer. The rotated PD is modified at the RBD‐binding site. The image is produced by alignment of PD. The top region, which interacts with the RBD of the spike protein, undergoes changes, particularly in the α1 helix. The zoom depicts the RBD binding site of ACE2 upon aligning the last C‐terminal portion of the α1 helix. In the loose dimer, some of the helix α1 residues that interact with the spike protein RBD move about 4.5 Å (measuring from the Cα). The table indicates the relative movement between the tight and loose dimers of Cα of relevant residues that interact with the spike protein RBD.