The human immunodeficiency virus type 1 (HIV-1) protease is a potential target of acquired immune deficiency syndrome (AIDS) therapy. A highly potent, perfectly symmetrical phosphinate inhibitor of this enzyme, SB204144, has been synthesized. It is a competitive inhibitor of HIV-1 protease, with an apparent inhibition constant of 2.8 nM at pH 6.0. The three-dimensional structure of SB204144 bound to the enzyme has been determined at 2.3-A resolution by X-ray diffraction techniques and refined to a crystallographic discrepancy factor, R (= sigma parallel F(o) magnitude to - Fc parallel/sigma magnitude of F(o)), of 0.178. The inhibitor is held in the enzyme active site by a set of hydrophobic and hydrophilic interactions, including an interaction between Arg8 and the center of the terminal benzene rings of the inhibitor. The phosphinate establishes a novel interaction with the two catalytic aspartates; each oxygen of the central phosphinic acid moiety interacts with a single oxygen of one aspartic acid, establishing a very short (2.2-2.4 A) oxygen-oxygen contact. As with the structures of penicillopepsin bound to phosphinate and phosphonate inhibitors [Fraser, M. E., Strynadka, N. C., Bartlett, P. A., Hanson, J. E., & James, M. N. (1992) Biochemistry 31, 5201-14], we interpret this short distance and the stereochemical environment of each pair of oxygens in terms of a hydrogen bond that has a symmetric single-well potential energy curve with the proton located midway between the two atoms.(ABSTRACT TRUNCATED AT 250 WORDS)