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Angiotensin-converting enzyme (ACE) and its homologue angiotensin-converting enzyme 2 (ACE2) are critical counter-regulatory enzymes of the renin–angiotensin system, and have been implicated in cardiac function, renal disease, diabetes, atherosclerosis and acute lung injury. | ỊFEBS Journal Residues affecting the chloride regulation and substrate selectivity of the angiotensin-converting enzymes ACE and ACE2 identified by site-directed mutagenesis Christopher A. Rushworth Jodie L. Guy and Anthony J. Turner Institute of Molecular and Cellular Biology Faculty of BiologicalSciences University of Leeds UK Keywords angiotensin carboxypeptidase chloride metalloprotease zinc Correspondence A. J. Turner Institute of Molecular and Cellular Biology Faculty of Biological Sciences University of Leeds Leeds LS2 9JT UK Fax 44 113 343 3157 Tel 44 113 343 3131 E-mail a.j.turner@leeds.ac.uk Received 19 August 2008 revised 4 October 2008 accepted 7 October 2008 doi 10.1111 j.1742-4658.2008.06733.x Angiotensin-converting enzyme ACE and its homologue angiotensinconverting enzyme 2 ACE2 are critical counter-regulatory enzymes of the renin-angiotensin system and have been implicated in cardiac function renal disease diabetes atherosclerosis and acute lung injury. Both ACE and ACE2 have catalytic activity that is chloride sensitive and is caused by the presence of the CL1 and CL2 chloride-binding sites in ACE and the CL1 site in ACE2. The chloride regulation of activity is also substrate dependent. Site-directed mutagenesis was employed to elucidate which of the CL1 and CL2 site residues are responsible for chloride sensitivity. The CL1 site residues Arg186 Trp279 and Arg489 of testicular ACE and the equivalent ACE2 residues Arg169 Trp271 and Lys481 were found to be critical to chloride sensitivity. Arg522 of testicular ACE was also confirmed to be vital to the chloride regulation mediated by the CL2 site. In addition Arg514 of ACE2 was identified as a residue critical to substrate selectivity with the R514Q mutant relative to the wild-type possessing a fourfold greater selectivity for the formation of the vasodilator angiotensin- 1-7 from the vasoconstrictor angiotensin II. The enhancement of angiotensin II cleavage by R514Q ACE2 was a result of a 2.5-fold .