STRUCTURE AND SYMMETRY OF B-STEAROTHERMOPHILUS PYRUVATE-DEHYDROGENASE MULTI-ENZYME COMPLEX AND IMPLICATIONS FOR EUCARYOTE EVOLUTION

The pyruvate dehydrogenase multienzyme complex was purified from B. stearothermophilus. The enzyme was found to be of high molecular weight (s20,w 0 = 75S) and to contain four different types of polypeptide chain, with subunit molecular weights estimated as 57,000, 54,000, 42,000 and 36,000, respectively. The subunit of molecular weight 57,000 was shown to derive from the lipoate acetyltransferase component (EC 2.3.1.12), whereas the subunit of molecular weight 54,000 was identified as lipoamide dehydrogenase (EC 1.6.4.3). The other two polypeptide chains are likely to be the subunits of pyruvate decarboxylase (EC 1.2.4.1). The purified lipoate acetyltransferase component was also of high molecular weight (s20,w 0 = 35S), and both it and the intact enzyme complex were readily visualized in negatively-stained preparations in the electron microscope. The lipoate acetyltransferase component, in particular, clearly showed the 5 fold, 3 fold and 2 fold rotation axes of a regular pentagonal dodecahedron with a diameter of 23 nm. The symmetry of the enzyme complex is apparently icosahedral. In all these properties the enzyme from B. stearothermophilus (Gram-positive) strikingly resembles the pyruvate dehydrogenase complex from the mitochondria of eucaryotic cells, and stands in marked contrast to the enzyme from E. coli (Gram-negative). A growing body of evidence indicates that the quaternary structures of enzymes from Gram-positive bacteria and the mitochondria of eucaryotes share distinctive common features that set them apart from the corresponding enzymes from Gram-negative bacteria. Adopting the serial endosymbiosis theory for the evolution of the mitochondrion, it follows that the forerunner of mitochondria may have been a Gram-positive rather than a Gram-negative bacterium. © 1979.