Ramipril is a pro-drug. The molecule must be hydrolyzed by the esterase at the OCH2CH3 and form a carboxylate. This carboxylate then interacts with the positive Zn+2 to inhibit the ACE enzyme. The COOH helps orient it with the enzyme. Ramipril is similar in structure to another ACE Inhibitor, trandolapril, but it has a  second cyclopentane ring instead of a cyclohexane ring.