Angiotensin-Converting Enzyme: Difference between revisions

==Biological Role==

ACE is a Zn and Chloride dependent type-1 membrane protein (N-terminal regions are outside the cell). Two types of Angiotensin-converting enzyme exist, ACE1 and ACE2, although the most focus has been on ACE1 which has been attributed with receptor-mediated effects like vasoconstriction, inflammation and cell growth/proliferation. <ref name="Ferrario">PMID:17083068</ref> The Renin-Angiotensin System (RAS) is a major regulator of blood pressure in the human body. [[Renin]] is an enzyme produced by the liver which cleaves Angiotensinogen into Angiotensin I Angiotensin Ihas the sequence, DRVTIHPFHL, and does not appear to have any biological activity. Angiotensin 1 (See:[[1n9u]]) is converted into Angiotensin II (See:[[1n9v]]) via the removal of the two C-terminal residues by ACE, yielding the active peptide: DRVTIHPF. <ref>PMID:12752436</ref>  

Angiotensin II interacts with two receptor subtypes, AT1 and AT2, which are widely distributed throughout the body. <ref name="Brew">PMID:12915047</ref> Binding of Angiotensin II to ATI leads to vasoconstriction by vascular smooth muscle cells, resulting in increased blood pressure, as well as the release of fluid and electrolyte homeostasis regulator, aldosterone, by the adrenal glands. Further, Angiotensin II binds to kidney AT1 receptors resulting in sodium ion reabsorption, leading to increased water retention in the blood and subsequent increased blood pressure. <ref name="Sturrock">PMID:15549168</ref>

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Additionally, Bradykinin, which is inactivated by ACE1, has vasodilatory and cardioprotective properties by promoting the formation of nitric oxide by the [http://en.wikipedia.org/wiki/Endothelium endothelium]. <ref>PMID:12767053</ref> The essential role ACE1 plays in blood pressure homeostasis is further supported by knockout mice created by Cole et. al. ACE1 knockout mice exhibited an approximate 35% reduction in blood pressure, resulting in hypotension and subsequent organ damage. Thus despite the many systems contributing to blood pressure in mammals, i.e. nitric oxide, endothelin and andregenic stimulation etc. these redundant systems are not enough to overcome a disruption of the RAAS. <ref>PMID:11967804</ref> It should be noted that AT2 binding of Angiotensin II results in many processes that counterbalance the binding of AT1. See the schematic image of the Renin-Angiotensin-Aldosterone System at the left for a visual description and the table below for selected Angiotensin receptor-mediated effects of binding Angiotensin II.

==Structural Analysis, Mechanism, & Activation==

===Zinc Coordinated Substrate Binding and Catalytic Mechanism===

[[Image: Binding_site.png|350px|left|thumb| Binding Site of ACE with Zn Ion in Grey and Cl Ion in Green [[1o86]]]]

Zinc is a critical component of the ACE1 catalytic binding site. Helix 13 contains <scene name='Angiotensin-Converting_Enzyme/Zinc_binding_site/2'>the canonical HEXXH zinc-binding motif</scene>, utilizing His 383 and His 387 along with Glu 411 on helix 14. The active site of ACE1 incorporates the <scene name='Angiotensin-Converting_Enzyme/Active_site/1'>bound zinc along with a number of other stabilizing residues</scene>.<ref name="Natesh"/> Incoming substrate binds zinc by displacing the zinc bound water molecule. The water molecule subsequently binds the nearby Glu 384 resulting in polarization between the negative glutamate carboxylate group and the positive zinc ion. <ref>PMID:6525336</ref>This enhances the nucleophilicity of the water oxygen, promoting attack on the substrate peptide carbonyl carbon. The proton accepted by the active site glutamate is shuttled to the nitrogen, possibly forming a tetrahedral gem-diolate intermediate with the help of Tyr 523. The dipeptide product formed from the cleavage of the C-N bond is released in the protonated form. <ref name="Natesh"/>The remaining peptide substrate is stabilized via hydrogen bond interactions between Ala 354 and the new terminal amide, His 353 & His 513 with the secondary carbonyl group, and Tyr 520 and Lys 511 and the terminal carboxylate. <ref name="Sturrock"/>

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===Medical Implications and Inhibitor Binding===

[[Image: Lisinopril.png|320px|left|thumb| The ACE Inhibitor, Lisinopril]]

Several studies have validated a pathological role for Angiotensin II in cardiac, renal and vascular diseases like hypertension and diabetic renal failure. <ref name="Ferrario"/> The increased blood pressure and oxidative stress associated with elevated levels of Angiotensin II can result in endothelial dysfunction and microvascular damage, ultimately leading to heart failure, stroke and kidney disease among other clinical manifestations. <ref name="Weir">PMID:18035185</ref> Bradykinin, a small peptide that counterbalance the effects of Angiotensin II by acting as a strong vasodilator upon binding AT2, is degraded by the same ACE1 enzymes which create Angiotensin II from Angiotensin I. Since ACE1 is the primary producer of Angiotensin II and primary degrader of Bradykinins, the development of ACE1 inhibitors has been a major focus for drug developers looking to fight these cardiovascular and renal conditions. <ref name="Weir"/> ACE1 inhibitors like [http://en.wikipedia.org/wiki/Captopril Captopril] ([[1uzf]], [[Capoten]]), Ramipril ([[Altace]]), Lisinopril, ([[1o86]], [[Prinivil]]), and Benazepril ([[Lotensin]]) have proven to be effective at reducing Angiotensin II based pathologies. Sale of ACE1 inhibitors topped $5 billion in 2009 with over 150 million prescriptions filled.<ref name="Inhibit">http://www.yourlawyer.com/topics/overview/ace_inhibitors</ref>  

During the SARS scare of 2002-2003, extensive research was focused on the interactions between the SARS virus and its host cells. It was determined that the severe acute respiratory syndrome conavirus (SARS-CoV) enters cells through the activities of a spike shaped protein on its outer envelope. <ref name="SARS">PMID:18448527</ref> The Receptor Binding Domain (RBD) of SARS-CoV binds to ACE2, on the surface of the cell. It was determined that by changing a few selected residues on either the SARS-CoV RBD or the ACE2 binding site, the virus becomes significantly more infectious. <scene name='Angiotensin-Converting_Enzyme/Sars/2'>It is believed that these mutations</scene> ([[3d0g]]), namely at residues 31, 35, 38, & 353 in ACE2 or residues 479 and 487 in the SARS-CoV RBD, are what allowed for SARS transmission from [http://en.wikipedia.org/wiki/Civet Civets] to Humans. In fact, in those SARS strains which were determined to be most infectious, the unfavorable electrostatic interactions at the binding interface were removed via mutations at the critical residues 479 and 487. <ref name="SARS"/>

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==3D Structures of Angiotensin-Converting Enzyme==