No abstract

Tliermoplasma acidophilum and Sulfolohus acidocaldarius contain coenzyme A-acylating 2-oxoacid : ferredoxin oxidoreductases similar to those found in halophilic archaebacteria. A common feature of these enzymes is the formation of a free radical intermediate in the course of the catalytic cycle. The electron-accepting ferredoxins and a similar protein from Desulfirococcus mohilis have been purified and characterized. In contrast to the [2Fe-2S] ferredoxin of Halobacterium halnhium, the ferredoxins of thermoacidophilic archaebacteria most likely contain two [4Fe-4SI2+ ( 2 + * ' + ) clusters per molecule. Properties of these proteins are compared with respect to the evolution of archaebacteria.Oxidative decarboxylation of 2-oxoacids, such as pyruvate and 2-oxoglutarate, is a key reaction of intermediary metabolism. Most organisms exploit the high reducing power of these substrates [l] for the reduction of a low-potential electron carrier and concomitant formation of an energy-rich thioester linkage between coenzyme A and the resulting carboxylic acid. In mitochondria and many respiratory eubacteria this sequence of reactions is catalyzed by the well-studied 2-0x0-acid dehydrogenase multienzyme complexes which use NAD as the electron acceptor. They are large aggregates (in most cases 3-7 MDa) of three different enzymes which catalyze the three partial reactions [2].