title: Localisation of the active site of pyruvate carboxylase by electron microscopic examination of avidin-enzyme complexes.
abstract: Negatively stained enzyme-avidin complexes, seen at different stages of the titration of the biotin-binding sites on avidin with chicken liver pyruvate carboxylase, have been studied using electron microscopy. Formation of linear, unbranched polymers of the enzyme-avidin complex is seen to occur when the ratio of avidin to enzyme is between 2:1 and 1:2; beyond these limits only single enzyme tetramers are visible. The single avidin molecules observed seem to have a cuboid structure. The orientation and dimensions of the enzyme tetramers within the polymers indicate that the tetrahedron-like structure, observed in the single molecules, has been preserved. From the structure of the polymers and the observation of single enzyme-avidin complexes, we deduce that the biotin groups on the enzyme are located on the external faces of each subunit close to, and probably within 3 nm of, the intersubunit junction.
mesh_terms: D000818:Animals; D001360:Avidin / Q000302:isolation & purification*; D001665:Binding Sites; D001704:Biopolymers; D001710:Biotin / Q000378:metabolism; D055598:Chemical Phenomena; D002621:Chemistry; D002645:Chickens; D008099:Liver / Q000201:enzymology; D008854:Microscopy, Electron; D010047:Ovalbumin / Q000031:analogs & derivatives*; D011766:Pyruvate Carboxylase / Q000302:isolation & purification*; D013194:Staining and Labeling
chemical_list: D001704:Biopolymers; D001360:Avidin; D001710:Biotin; D010047:Ovalbumin; D011766:Pyruvate Carboxylase