title: Crystal structure of a papain-E-64 complex.
abstract: E-64 [1-[N-[(L-3-trans-carboxyoxirane-2-carbonyl)-L-leucyl] amino]-4-guanidinobutane] is an irreversible inhibitor of many cysteine proteases. A papain-E-64 complex was crystallized at pH 6.3 by using the hanging drop method. Three different crystal forms grew in 3-7 days; the form chosen for structure analysis has space group P212121, with a = 42.91(4) A, b = 102.02(6) A, c = 49.73(2) A, and Z = 4. Diffraction data were measured to 2.4-A resolution, giving 9367 unique reflections. The papain structure was solved by use of the molecular replacement method, and then the inhibitor was located from a difference electron density map and fitted with the aid of a PS330 computer graphics system. The structure of the complex was refined to R = 23.3%. Our analysis shows that a covalent link is formed between the sulfur of the active-site cysteine 25 and the C-2 atom of the inhibitor. Contrary to earlier predictions, the E-64 inhibitor clearly interacts with the S subsites on the enzyme rather than the S' subsites, and papain's histidine 159 imidazole group plays a binding rather than a catalytic role in the inactivation process.
authors.affiliation: Division of Biological Sciences, National Research Council of Canada, Ottawa, Ontario.
authors.forename: K I
authors.initials: KI
authors.lastname: Varughese
authors.forename: F R
authors.initials: FR
authors.lastname: Ahmed
authors.forename: P R
authors.initials: PR
authors.lastname: Carey
authors.forename: S
authors.initials: S
authors.lastname: Hasnain
authors.forename: C P
authors.initials: CP
authors.lastname: Huber
authors.forename: A C
authors.initials: AC
authors.lastname: Storer
mesh_terms: D003460:Crystallization; D006860:Hydrogen Bonding; D007930:Leucine / Q000031:analogs & derivatives* / Q000378:metabolism; D010206:Papain / Q000378:metabolism*; D011480:Protease Inhibitors / Q000378:metabolism*; D013050:Spectrometry, Fluorescence
chemical_list: D011480:Protease Inhibitors; D010206:Papain; D007930:Leucine; C024974:E 64