title: Crystal structures of two engineered thiol trypsins.
abstract: We have determined the three-dimensional structures of engineered rat trypsins which mimic the active sites of two classes of cysteine proteases. The catalytic serine was replaced with cysteine (S195C) to test the ability of sulfur to function as a nucleophile in a serine protease environment. This variant mimics the cysteine trypsin class of thiol proteases. An additional mutation of the active site aspartate to an asparagine (D102N) created the catalytic triad of the papain-type cysteine proteases. Rat trypsins S195C and D102N,S195C were solved to 2.5 and 2.0 A, respectively. The refined structures were analyzed to determine the structural basis for the 10(6)-fold loss of activity of trypsin S195C and the 10(8)-fold loss of activity of trypsin D102N,S195C, relative to rat trypsin. The active site thiols were found in a reduced state in contrast to the oxidized thiols found in previous thiol protease structures. These are the first reported structures of serine proteases with the catalytic centers of sulfhydryl proteases. Structure analysis revealed only subtle global changes in enzyme conformation. The substrate binding pocket is unaltered, and active site amino acid 102 forms hydrogen bonds to H57 and S214 as well as to the backbone amides of A56 and H57. In trypsin S195C, D102 is a hydrogen-bond acceptor for H57 which allows the other imidazole nitrogen to function as a base during catalysis. In trypsin D102N,S195C, the asparagine at position 102 is a hydrogen-bond donor to H57 which places a proton on the imidazole nitrogen proximal to the nucleophile. This tautomer of H57 is unable to act as a base in catalysis.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biochemistry, University of California, San Francisco 94143-0448.
authors.forename: M E
authors.initials: ME
authors.lastname: McGrath
authors.forename: M E
authors.initials: ME
authors.lastname: Wilke
authors.forename: J N
authors.initials: JN
authors.lastname: Higaki
authors.forename: C S
authors.initials: CS
authors.lastname: Craik
authors.forename: R J
authors.initials: RJ
authors.lastname: Fletterick
mesh_terms: D000818:Animals; D001665:Binding Sites; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003196:Computer Graphics; D003460:Crystallization; D003545:Cysteine / Q000378:metabolism*; D005583:Fourier Analysis; D006860:Hydrogen Bonding; D015394:Molecular Structure; D009154:Mutation; D010206:Papain / Q000378:metabolism; D051381:Rats; D012697:Serine Endopeptidases / Q000378:metabolism*; D013438:Sulfhydryl Compounds; D014357:Trypsin; D014961:X-Ray Diffraction
chemical_list: D013438:Sulfhydryl Compounds; D012697:Serine Endopeptidases; D014357:Trypsin; D010206:Papain; D003545:Cysteine