Further Electron Microscope Studies on Pyruvate Carboxylase

Using negative staining and electron microscopic tilting techniques in conjunction with modelling experiments, the fine structure of chicken, sheep and rat pyruvate carboxylases has been studied. The overall configuration appears to be a tetrahedron‐like structure consisting of two pairs of subunits in two different planes orthogonal to each other with the opposing pairs of subunits interacting with each other on their convex surfaces. The predominant form of the enzyme particles mounted and stained in the presence of acetylcoenzyme A consisted of a compact, triangular outline enclosing three readily visible intensity maxima. When samples were mounted in the absence of acetyl‐coenzyme A the molecules were more ‘open’ predominantly rhomboid structures. From tilting experiments it is concluded that the rhomboid images found in the absence of acetyl‐coenzyme A represent partly or wholly flattened forms of the tetrahedron‐like molecule. A feature of the enzyme when mounted in the absence of acetyl‐coenzyme A was the existence of a ‘cleft’ along the longitudinal midline of each subunit, suggesting that the subunits may consist of two distinct domains.