Detailed structures of a protein that helps regulate blood pressure, known as angiotensin-converting enzyme (ACE), have been generated using cryo-electron microscopy (cryo-EM). The structures, which provide the most holistic view of ACE to date, will help improve drug design for heart disease.

The work was performed by researchers from the University of Cape Town (UCT) in collaboration with the electron Bio-Imaging Centre (eBIC) at the U.K.’s national synchroton, Diamond Light Source, as part of START. The researchers published their results in The EMBO Journal (“Cryo-EM reveals mechanisms of angiotensin I-converting enzyme allostery and dimerization”).

ACE produces the hormone angiotensin II, which constricts blood vessels and raises blood pressure. Elevated blood pressure, or hypertension, is a major risk factor for heart disease and stroke.

Cryo-EM allowed the researchers to visualize ACE in a more functionally relevant state than was possible with previous methods. Their work provided critical insight into its biological function and potential drug-binding properties.

One copy of the ACE protein (i.e., the monomeric form) is composed of two structurally similar but functionally distinct domains that are linked together. Dimerization (i.e., the interaction of two ACE monomers) occurs near a small surface cavity and changes the conformation of core amino acids that are critical for ACE function.

The researchers proposed that this dimerization could be like an “off switch” that triggers changes in the core of the protein and potentially inhibits it. If a drug-like molecule could be designed to bind in the cavity and elicit the same effect, it could provide a novel means to inactivate the enzyme.

Currently, many ACE inhibitors are clinically available to treat hypertension. “But [these inhibitors] non-selectively target both ACE domains and thereby trigger side effects in some patients,” explained Edward Sturrock, PhD, a professor at the University of Cape Town and the principal investigator on the study. “It is really important to understand the structure and dynamics of these newly seen forms of ACE because this could help identify novel sites for the design of domain-selective inhibitors that avoid such side effects.”

The ACE protein was produced in Sturrock’s laboratory, prepared for imaging at UCT’s Electron Microscope Unit (EMU), and then transported to the eBIC for cryo-EM imaging on a Titan Krios. Image processing took place at South Africa’s CSIR Centre for High Performance Computing (CHPC) and the EMU.

“Even with high-resolution imaging, the unique shape, small size, and dynamic nature of ACE posed many challenges,” said Jeremy Woodward, PhD, one of the study’s co-authors.

“Recently developed cryo-EM image processing methods were crucial to solving the structures,” Lizelle Lubbe, PhD, the first author on the study, explained. “We had to separate the images computationally through extensive classification, amounting to ‘digital purification’ because biochemical methods failed to separate the monomeric and dimeric forms of ACE. We could then solve both ACE structures by focusing the 3D refinement on different parts of the structure in turn.”

The study’s findings uniquely reveal ACE’s highly dynamic nature and the mechanisms by which dimerization and communication occur between its different domains—which could lead to the discovery of new drugs for heart disease.

“We are delighted with the findings of this study achieved by a brilliant team of scientists in Africa, using eBIC’s advanced cryo-EM,” said Chris Nicklin, PhD, group leader at Diamond. “The world urgently needs sustainable solutions for killer heart diseases and other chronic health conditions. We are very excited that the study’s structural insights could pave the way for improved antihypertensive drug design.”