Kinetic and structural characterization of amyloid-β peptide hydrolysis by human angiotensin-1-converting enzyme

Carriers of Heterozygous Loss-of-Function ACE Mutations Are at Risk for Alzheimer's Disease

We hypothesized that subjects with heterozygous loss-of-function (LoF) ACE mutations are at risk for Alzheimer's disease because amyloid Aβ42, a primary component of the protein aggregates that accumulate in the brains of AD patients, is cleaved by ACE (angiotensin I-converting enzyme). Thus, decreased ACE activity in the brain, either due to genetic mutation or the effects of ACE inhibitors, could be a risk factor for AD. To explore this hypothesis in the current study, existing SNP databases were analyzed for LoF ACE mutations using four predicting tools, including PolyPhen-2, and compared with the topology of known ACE mutations already associated with AD. The combined frequency of >400 of these LoF-damaging ACE mutations in the general population is quite significant-up to 5%-comparable to the frequency of AD in the population > 70 y.o., which indicates that the contribution of low ACE in the development of AD could be under appreciated. Our analysis suggests several mechanisms by which ACE mutations may be associated with Alzheimer's disease. Systematic analysis of blood ACE levels in patients with all ACE mutations is likely to have clinical significance because available sequencing data will help detect persons with increased risk of late-onset Alzheimer's disease. Patients with transport-deficient ACE mutations (about 20% of damaging ACE mutations) may benefit from preventive or therapeutic treatment with a combination of chemical and pharmacological (e.g., centrally acting ACE inhibitors) chaperones and proteosome inhibitors to restore impaired surface ACE expression, as was shown previously by our group for another transport-deficient ACE mutation-Q1069R.

Structural basis for the inhibition of human angiotensin-1 converting enzyme by fosinoprilat

Human angiotensin I-converting enzyme (ACE) has two isoforms, somatic ACE (sACE) and testis ACE (tACE). The functions of sACE are widespread, with its involvement in blood pressure regulation most extensively studied. sACE is composed of an N-domain (nACE) and a C-domain (cACE), both catalytically active but have significant structural differences, resulting in different substrate specificities. Even though ACE inhibitors are used clinically, they need much improvement because of serious side effects seen in patients (~ 25-30%) with long-term treatment due to nonselective inhibition of nACE and cACE. Investigation into the distinguishing structural features of each domain is therefore of vital importance for the development of domain-specific inhibitors with minimal side effects. Here, we report kinetic data and high-resolution crystal structures of both nACE (1.75 Å) and cACE (1.85 Å) in complex with fosinoprilat, a clinically used inhibitor. These structures allowed detailed analysis of the molecular features conferring domain selectivity by fosinoprilat. Particularly, altered hydrophobic interactions were observed to be a contributing factor. These experimental data contribute to improved understanding of the structural features that dictate ACE inhibitor domain selectivity, allowing further progress towards designing novel 2nd-generation domain-specific potent ACE inhibitors suitable for clinical administration, with a variety of potential future therapeutic benefits. DATABASE: The atomic coordinates and structure factors for nACE-fosinoprilat and cACE-fosinoprilat structures have been deposited with codes 7Z6Z and 7Z70, respectively, in the RCSB Protein Data Bank, www.pdb.org.

Cryo-EM reveals mechanisms of angiotensin I-converting enzyme allostery and dimerization

Hypertension (high blood pressure) is a major risk factor for cardiovascular disease, which is the leading cause of death worldwide. The somatic isoform of angiotensin I‐converting enzyme (sACE) plays a critical role in blood pressure regulation, and ACE inhibitors are thus widely used to treat hypertension and cardiovascular disease. Our current understanding of sACE structure, dynamics, function, and inhibition has been limited because truncated, minimally glycosylated forms of sACE are typically used for X‐ray crystallography and molecular dynamics simulations. Here, we report the first cryo‐EM structures of full‐length, glycosylated, soluble sACE (sACE^S1211). Both monomeric and dimeric forms of the highly flexible apo enzyme were reconstructed from a single dataset. The N‐ and C‐terminal domains of monomeric sACE^S1211 were resolved at 3.7 and 4.1 Å, respectively, while the interacting N‐terminal domains responsible for dimer formation were resolved at 3.8 Å. Mechanisms are proposed for intradomain hinging, cooperativity, and homodimerization. Furthermore, the observation that both domains were in the open conformation has implications for the design of sACE modulators.

Principal component analysis of alpha-helix deformations in transmembrane proteins

α-helices are deformable secondary structural components regularly observed in protein folds. The overall flexibility of an α-helix can be resolved into constituent physical deformations such as bending in two orthogonal planes and twisting along the principal axis. We used Principal Component Analysis to identify and quantify the contribution of each of these dominant deformation modes in transmembrane α-helices, extramembrane α-helices, and α-helices in soluble proteins. Using three α-helical samples from Protein Data Bank entries spanning these three cellular contexts, we determined that the relative contributions of these modes towards total deformation are independent of the α-helix's surroundings. This conclusion is supported by the observation that the identities of the top three deformation modes, the scaling behaviours of mode eigenvalues as a function of α-helix length, and the percentage contribution of individual modes on total variance were comparable across all three α-helical samples. These findings highlight that α-helical deformations are independent of cellular location and will prove to be valuable in furthering the development of flexible templates in de novo protein design.

Renin-Angiotensin System and Alzheimer's Disease Pathophysiology: From the Potential Interactions to Therapeutic Perspectives

New roles of the Renin-Angiotensin System (RAS), apart from fluid homeostasis and Blood Pressure (BP) regulation, are being progressively unveiled, since the discoveries of RAS alternative axes and local RAS in different tissues, including the brain. Brain RAS is reported to interact with pathophysiological mechanisms of many neurological and psychiatric diseases, including Alzheimer's Disease (AD). Even though AD is the most common cause of dementia worldwide, its pathophysiology is far from elucidated. Currently, no treatment can halt the disease course. Successive failures of amyloid-targeting drugs have challenged the amyloid hypothesis and increased the interest in the inflammatory and vascular aspects of AD. RAS compounds, both centrally and peripherally, potentially interact with neuroinflammation and cerebrovascular regulation. This narrative review discusses the AD pathophysiology and its possible interaction with RAS, looking forward to potential therapeutic approaches. RAS molecules affect BP, cerebral blood flow, neuroinflammation, and oxidative stress. Angiotensin (Ang) II, via angiotensin type 1 receptors may promote brain tissue damage, while Ang-(1-7) seems to elicit neuroprotection. Several studies dosed RAS molecules in AD patients' biological material, with heterogeneous results. The link between AD and clinical conditions related to classical RAS axis overactivation (hypertension, heart failure, and chronic kidney disease) supports the hypothesized role of this system in AD. Additionally, RAStargeting drugs as Angiotensin Converting Enzyme inhibitors (ACEis) and Angiotensin Receptor Blockers (ARBs) seem to exert beneficial effects on AD. Results of randomized controlled trials testing ACEi or ARBs in AD are awaited to elucidate whether AD-RAS interaction has implications on AD therapeutics.

ACE2 and ACE: structure-based insights into mechanism, regulation and receptor recognition by SARS-CoV

Angiotensin converting enzyme (ACE) is well-known for its role in blood pressure regulation via the renin–angiotensin aldosterone system (RAAS) but also functions in fertility, immunity, haematopoiesis and diseases such as obesity, fibrosis and Alzheimer’s dementia. Like ACE, the human homologue ACE2 is also involved in blood pressure regulation and cleaves a range of substrates involved in different physiological processes. Importantly, it is the functional receptor for severe acute respiratory syndrome (SARS)-coronavirus (CoV)-2 responsible for the 2020, coronavirus infectious disease 2019 (COVID-19) pandemic. Understanding the interaction between SARS-CoV-2 and ACE2 is crucial for the design of therapies to combat this disease. This review provides a comparative analysis of methodologies and findings to describe how structural biology techniques like X-ray crystallography and cryo-electron microscopy have enabled remarkable discoveries into the structure–function relationship of ACE and ACE2. This, in turn, has enabled the development of ACE inhibitors for the treatment of cardiovascular disease and candidate therapies for the treatment of COVID-19. However, despite these advances the function of ACE homologues in non-human organisms is not yet fully understood. ACE homologues have been discovered in the tissues, body fluids and venom of species from diverse lineages and are known to have important functions in fertility, envenoming and insect–host defence mechanisms. We, therefore, further highlight the need for structural insight into insect and venom ACE homologues for the potential development of novel anti-venoms and insecticides.

Angiotensin-converting enzyme open for business: structural insights into the subdomain dynamics

Angiotensin‐1‐converting enzyme (ACE) is a key enzyme in the renin–angiotensin–aldosterone and kinin systems where it cleaves angiotensin I and bradykinin peptides, respectively. However, ACE also participates in numerous other physiological functions, can hydrolyse many peptide substrates and has various exo‐ and endopeptidase activities. ACE achieves this complexity by containing two homologous catalytic domains (N‐ and C‐domains), which exhibit different substrate specificities. Here, we present the first open conformation structures of ACE N‐domain and a unique closed C‐domain structure (2.0 Å) where the C terminus of a symmetry‐related molecule is observed inserted into the active‐site cavity and binding to the zinc ion. The open native N‐domain structure (1.85 Å) enables comparison with ACE2, a homologue previously observed in open and closed states. An open S₂_S′‐mutant N‐domain structure (2.80 Å) includes mutated residues in the S₂ and S′ subsites that effect ligand binding, but are distal to the binding site. Analysis of these structures provides important insights into how structural features of the ACE domains are able to accommodate the wide variety of substrates and allow different peptidase activities.

Database

The atomic coordinates and structure factors for Open nACE, Open S2_S′‐nACE and Native G13‐cACE structures have been deposited with codes 6ZPQ, 6ZPT and 6ZPU, respectively, in the RCSB Protein Data Bank, www.pdb.org

Interacting cogs in the machinery of the renin angiotensin system

Somatic angiotensin converting enzyme (sACE) is well-known for its role in blood pressure regulation and consequently, ACE inhibitors are widely prescribed for the treatment of hypertension. More than 60 years after the discovery of sACE, however, the molecular details of its substrate hydrolysis and inhibition are still poorly understood. Isothermal titration calorimetry, molecular dynamics simulations and fine epitope mapping suggest that substrate or inhibitor binding triggers a hinging motion between the two subdomains of each domain. Ligand binding to one domain further induces a conformational change in sACE to negatively affect the second domain's function and can also cause dimerization between sACE molecules. This has been linked to an increase in sACE expression via intracellular signalling. Inhibitor-induced dimerization could thus decrease the efficacy of hypertension treatment. At present, the only structural information available for sACE are crystal structures of the truncated domains in the closed conformation due to the presence of ligands. These structures do not provide any information regarding the open active site conformation prior to ligand binding, the relative orientation of the two domains in full-length sACE, or the dimerization interface. To guarantee effective therapeutic intervention, further research is required to investigate the hinging, negative cooperativity and dimerization of sACE. This review describes our current understanding of these interactions and proposes how recent advances in cryo-electron microscopy could enable structural elucidation of their mechanisms.

Advances in Structural Biology of ACE and Development of Domain Selective ACE-inhibitors

BACKGROUND

The Angiotensin-I converting enzyme (ACE) is one of the most important components of the renin-angiotensin-aldosterone system controlling blood pressure and renal functions. Inhibitors of ACE are first line therapeutics used in the treatment of hypertension and related cardiovascular diseases. Somatic ACE consists of two homologous catalytic domains, the C- and N-domains. Recent findings have shown that although both domains are highly homologous in structure, they may have different physiological functions. The C-domain is primarily involved in the control of blood pressure, in contrast to the N-domain that is engaged in the regulation of hematopoietic stem cell proliferation. The currently available ACE inhibitors have some adverse effects that can be attributed to the non-selective inhibition of both domains. In addition, specific Ndomain inhibitors have emerged as potential antifibrotic drugs. Therefore, ACE is still an important drug target for the development of novel domain-selective drugs not only for the cardiovascular system but also for other systems.

OBJECTIVE

Detailed structural information about interactions in the protein-ligand complex is crucial for rational drug design. This review highlights the structural information available from crystallographic data which is essential for the development of domain selective inhibitors of ACE.

METHODS

Over eighty crystal complexes of ACE are placed into the Protein Database. An overview of X-ray ACE complexes with various inhibitors in C- and N-domains and an analysis of their binding mode have given mechanistic explanation of the structural determinants of selective ligand binding. In addition, ACE domain selective inhibitors with dual modes of action in complexes with ACE are also discussed.

CONCLUSION

Selectivity of ACE inhibitors for the N- and C-domain is controlled by subtle differences in the amino-acids forming the active site. Reported studies of crystal complexes of inhibitors in the C- and N-domains revealed that most selective inhibitors interact with non-conserved amino-acids between domains and have distinct interactions with the residues in the S2 and S2' subsites of the ACE catalytic site. Moreover, unusual binding of the second molecule of inhibitors in the binding cavity opens new possibilities of exploiting more distant regions of the catalytic center in structure-based design of novel drugs.

Molecular Basis for Multiple Omapatrilat Binding Sites within the ACE C-Domain: Implications for Drug Design

Omapatrilat was designed as a vasopeptidase inhibitor with dual activity against the zinc metallopeptidases angiotensin-1 converting enzyme (ACE) and neprilysin (NEP). ACE has two homologous catalytic domains (nACE and cACE), which exhibit different substrate specificities. Here, we report high-resolution crystal structures of omapatrilat in complex with nACE and cACE and show omapatrilat has subnanomolar affinity for both domains. The structures show nearly identical binding interactions for omapatrilat in each domain, explaining the lack of domain selectivity. The cACE complex structure revealed an omapatrilat dimer occupying the cavity beyond the S₂ subsite, and this dimer had low micromolar inhibition of nACE and cACE. These results highlight residues beyond the S₂ subsite that could be exploited for domain selective inhibition. In addition, it suggests the possibility of either domain specific allosteric inhibitors that bind exclusively to the nonprime cavity or the potential for targeting specific substrates rather than completely inhibiting the enzyme.

Neurotoxic Effects of Aβ6-42 Peptides Mimicking Putative Products Formed by the Angiotensin Converting Enzyme

Angiotensin converting enzyme (ACE) is involved in proteolytic processing of the amyloid-β(Aβ) peptide implicated in the development of Alzheimer's disease (AD) and known products of ACE-based processing of Aβ42 are characterized by reduced aggregability and cytotoxicity. Recently it has been demonstrated that ACE can act as an arginine specific endopeptidase cleaving the N-terminal pentapeptide (Aβ1-5) from synthetic Aβ peptide analogues. In the context of proteolytic processing of full length Aβ42, this suggests possible formation of Aβ6-42 species. The aim of this study was to test a hypothesis that some N-terminally truncated Aβ peptide(s) could retain aggregability and neurotoxic properties typical for Aβ42. We have investigated aggregability of two amyloid-β peptides, Aβ6-42 and isoD7-Aβ6-42, mimicking potential proteolytic products of Aβ42 and isoD7-Aβ42, and evaluated their effects on the repertoire of brain Aβ binding proteins, and cytotoxicity towards neuroblastoma SH-SY5Y cells. Aggregability of isoD7-Aβ6-42 and Aβ6-42 was higher than that of full-length peptides Aβ42 and isoD7-Aβ42, while the repertoire of mouse brain Aβ binding proteins dramatically decreased. Aβ6-42 and isoD7-Aβ6-42 exhibited higher neurotoxicity towards SH-SY5Y cells than Aβ42 and isoD7-Aβ42, respectively. They effectively stimulated production of ROS and NO, and also TNFα secretion by cells. Thus, our results suggest that ACE-dependent processing of full-length Aβs could result in formation of more pathogenic peptides.

N-domain of angiotensin-converting enzyme hydrolyzes human and rat amyloid-β(1-16) peptides as arginine specific endopeptidase potentially enhancing risk of Alzheimer's disease

Alzheimer’s disease (AD) is a multifactorial neurodegenerative disorder. Amyloid-β (Aβ) aggregation is likely to be the major cause of AD. In contrast to humans and other mammals, that share the same Aβ sequence, rats and mice are invulnerable to AD-like neurodegenerative pathologies, and Aβ of these rodents (ratAβ) has three amino acid substitutions in the metal-binding domain 1-16 (MBD). Angiotensin-converting enzyme (ACE) cleaves Aβ-derived peptide substrates, however, there are contradictions concerning the localization of the cleavage sites within Aβ and the roles of each of the two ACE catalytically active domains in the hydrolysis. In the current study by using mass spectrometry and molecular modelling we have tested a set of peptides corresponding to MBDs of Aβ and ratAβ to get insights on the interactions between ACE and these Aβ species. It has been shown that the N-domain of ACE (N-ACE) acts as an arginine specific endopeptidase on the Aβ and ratAβ MBDs with C-amidated termini, thus assuming that full-length Aβ and ratAβ can be hydrolyzed by N-ACE in the same endopeptidase mode. Taken together with the recent data on the molecular mechanism of zinc-dependent oligomerization of Aβ, our results suggest a modulating role of N-ACE in AD pathogenesis.