The use of the ultracentrifuge to determine the catalytically competent forms of enzymes with more than one oligomeric structure. Multiple reacting forms of pyruvate carboxylase from chicken and rat liver

Protein engineering of pyruvate carboxylase: investigation on the function of acetyl-CoA and the quaternary structure

Pyruvate carboxylase (PC) from Bacillus thermodenitrificans was engineered in such a way that the polypeptide chain was divided into two, between the biotin carboxylase (BC) and carboxyl transferase (CT) domains. The two proteins thus formed, PC-(BC) and PC-(CT+BCCP), retained their catalytic activity as assayed by biotin-dependent ATPase and oxamate-dependent oxalacetate decarboxylation, for the former and the latter, respectively. Neither activity was dependent on acetyl-CoA, in sharp contrast to the complete reaction of intact PC. When assessed by gel filtration chromatography, PC-(BC) was found to exist either in dimers or monomers, depending on the protein concentration, while PC-(CT + BCCP) occurred in dimers for the most part. The two proteins do not associate spontaneously or in the presence of acetyl-CoA. Based on these observations, this paper discusses how the tetrameric structure of PC is built up and how acetyl-CoA modulates the protein structure.

Studies on dilution inactivation of sheep liver pyruvate carboxylase

When sheep liver pyruvate carboxylase was diluted below 4 EU/ml, it underwent inactivation involving two kinetically distinct processes, i.e., a rapid initial burst followed by a slower second phase. The catalytic activity of the diluted enzyme eventually approached zero, suggesting the occurrence of an irreversible process. Analysis of the quaternary structure of the enzyme by gel filtration chromatography and electron microscopy showed that most of the enzyme molecules occur as tetramers at high enzyme concentrations. However, dilution of the enzyme below 4 EU/ml led to the appearance of dimers and monomers which were essentially inactive under the conditions of the assay system used. The presence of acetyl-CoA during dilution prevented inactivation from occurring and preserved the tetrameric structure. When added after dilution, acetyl-CoA prevented further inactivation from occurring but did not reactivate the enzyme. However, acetyl-CoA did cause a relatively rapid reassociation of the inactive monomers and dimers to form inactive tetramers.

Pyruvate carboxylase from Saccharomyces cerevisiae. Quaternary structure, effects of allosteric ligands and binding of avidin

The quaternary structure of pyruvate carboxylase purified from Saccharomyces cerevisiae was investigated by electron microscopic examination of negatively stained samples. In the most frequently observed projection form four intensity maxima were arranged at the corners of a rhombus; a cleft along the longitudinal axis of individual protomers could often be discerned. The observation of occasional triangular and dual-intensity projections and the interconversion of all three projection forms in tilting studies indicates that this tetrameric enzyme has a structure very similar to the tetrahedron-like configuration previously proposed for pyruvate carboxylases from vertebrate sources [Mayer, F., Wallace, J. C. and Keech, D. B. (1980) Eur. J. Biochem. 112, 265-272] and Aspergillus nidulans [Osmani, S. A., Mayer, F., Marston, F. A. O., Selmes, I. P. and Scrutton, M. C. (1984) Eur. J. Biochem. 139, 509-518]. An improved structural preservation of the enzyme was observed in the presence of either of the activators acetyl-CoA (250 microM) and palmitoyl-CoA (1-5 microM). At higher than 5 microM palmitoyl-CoA, although activity was further increased, dissociation of enzyme tetramers was evident, presumably because of the detergent effect of the long-chain acyl moiety. Two inhibitors of yeast pyruvate carboxylase, L-aspartate (10 mM) and 2-oxoglutarate (40 mM), added alone or together decreased significantly the proportion of intact tetramers even in the presence of acetyl-CoA or palmitoyl-CoA. When yeast pyruvate carboxylase was incubated with avidin, the formation of unbranched linear concatemers occurred at avidin:enzyme ratios between 2:1 and 1:2. Avidin molecules were sometimes bound asymmetrically to the enzyme, appearing to complex only one biotin group on each side of the enzyme. This appeared to permit kinking and circularization of some concatemers.

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 acetyl-coenzyme 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.