Energetics of respiration and oxidative phosphorylation in mycobacteria

Mycobacteria inhabit a wide range of intracellular and extracellular environments. Many of these environments are highly dynamic, and therefore mycobacteria are faced with the constant challenge of redirecting their metabolic activity to be commensurate with either replicative growth or a nonreplicative quiescence. A fundamental feature in this adaptation is the ability of mycobacteria to respire, regenerate reducing equivalents, and generate ATP via oxidative phosphorylation. Mycobacteria harbor multiple primary dehydrogenases to fuel the electron transport chain, and two terminal respiratory oxidases, an aa₃-type cytochrome c oxidase and a cytochrome bd-type menaquinol oxidase, are present for dioxygen reduction coupled to the generation of a proton motive force (PMF). Hypoxia leads to the downregulation of key respiratory complexes, but the molecular mechanisms regulating this expression are unknown. Despite being obligate aerobes, mycobacteria have the ability to metabolize in the absence of oxygen, and a number of reductases are present to facilitate the turnover of reducing equivalents under these conditions (e.g., nitrate reductase, succinate dehydrogenase/fumarate reductase). Hydrogenases and ferredoxins are also present in the genomes of mycobacteria, suggesting the ability of these bacteria to adapt to an anaerobic type of metabolism in the absence of oxygen. ATP synthesis by the membrane-bound F₁F₀-ATP synthase is essential for growing and nongrowing mycobacteria, and the enzyme is able to function over a wide range of PMF values (aerobic to hypoxic). The discovery of lead compounds that target respiration and oxidative phosphorylation in Mycobacterium tuberculosis highlights the importance of this area for the generation of new frontline drugs to combat tuberculosis.