RESUMO
The structure and function of hydroxynitrile lyase from Manihot esculenta (MeHNL) have been analyzed by X-ray crystallography and site-directed mutagenesis. The crystal structure of the MeHNL-S80A mutant enzyme has been refined to an R-factor of 18.0% against diffraction data to 2.1-A resolution. The three-dimensional structure of the MeHNL-S80A-acetone cyanohydrin complex was determined at 2.2-A resolution and refined to an R-factor of 18.7%. Thr11 and Cys81 involved in substrate binding have been substituted by Ala in site-directed mutagenesis. The kinetic measurements of these mutant enzymes are presented. Combined with structural data, the results support a mechanism for cyanogenesis in which His236 as a general base abstracts a proton from Ser80, thereby allowing proton transfer from the hydroxyl group of acetone cyanohydrin to Ser80. The His236 imidazolium cation then facilitates the leaving of the nitrile group by proton donating.
Assuntos
Aldeído Liases/química , Aldeído Liases/metabolismo , Substituição de Aminoácidos/genética , Manihot/enzimologia , Nitrilas/química , Nitrilas/metabolismo , Aldeído Liases/genética , Sítios de Ligação/genética , Catálise , Domínio Catalítico/genética , Cristalografia por Raios X , Substâncias Perigosas/metabolismo , Cinética , Manihot/genética , Modelos Moleculares , Mutação/genética , Ligação Proteica , Conformação Proteica , Relação Estrutura-AtividadeRESUMO
The crystal structures of hydroxynitrile lyase from Manihot esculenta (MeHNL) complexed with the native substrate acetone and substrate analogue chloroacetone have been determined and refined at 2.2 A resolution. The substrates are positioned in the active site by hydrogen-bond interactions of the carbonyl O atom with Thr11 OG, Ser80 OG and, to a lesser extent, Cys81 SG. These studies support a mechanism for cyanogenesis as well as for the stereospecific MeHNL-catalyzed formation of (S)-cyanohydrins, which closely resembles the base-catalyzed chemical reaction of HCN with carbonyl compounds.