Localization of the Enzymes That Catalyze Hydrogen and Electron Transport in Hemophilus parainfluenzae and the Nature of the Respiratory Chain System

Abstract

Hemophilus parainjuenzae possess a respiratory chain system with many characteristics similar to those of the mammalian system (1) and of a number of other bacteria (2): it is composed of typical flavoprotein dehydrogenases and six cytochrome pigments, three of which appear to be oxidases (3-5). These pigments are bound to particles which presumably are derived from the cytoplasmic membrane. Unusual aspects of the respiratory chain of H. parainjluenxae are the accumulation, under some conditions of growth, of large amounts of one cytochrome that is not reducible in the presence of substrates (4,6,7), and the variability of the proportions of the different pigments observed under different growth conditions and in different phases of growth (57). Further studies, reported here, show that bacteria having widely varying proportions of the different cytochromes can have respiration rates that are similar with a number of substrates. These bacteria are readily permeable to substrates and pyridine nucleotides (8). Thus it is possible to compare the reactions of the membrane-bound system in intact bacteria and in the small membrane fragments derived from it upon rupture of the cells. Such studies show little modification of the reactivity immediately after preparation of the membrane fragments, but upon standing some pigments become dissociated from the membrane. As long as the pigments are associated with the membrane in proper orientation, electron transport can proceed rapidly, and the cytochromes and flavoproteins can be seen to undergo oxidation and reduction during electron transport. In either intact cells or particles, the over-all rate of electron transport is always limited by the reaction of the membrane-bound flavoprotein dehydrogenases with the appropriate substrate. Pyridine nucleotide is reduced by dehydrogenases which are not membrane-bound, and the rates of reduction of diphosphopyridine nucleotide are low. Taken together, all of the studies on the respiratory chain system of H. parainj~nzae show that its properties resemble those of mammalian mitochondria in most aspects. However, there is compelling evidence that in these bacteria the system is not composed of fixed units of pigments with a definite “stoichiometry” of the components.