Targeting Energy Metabolism in Mycobacterium tuberculosis, a New Paradigm in Antimycobacterial Drug Discovery

Abstract

Drug-resistant mycobacterial infections are a serious global health challenge, leading to high mortality and socioeconomic burdens in developing countries worldwide. New innovative approaches, from identification of new targets to discovery of novel chemical scaffolds, are urgently needed. Recently, energy metabolism in mycobacteria, in particular the oxidative phosphorylation pathway, has emerged as an object of intense microbiological investigation and as a novel target pathway in drug discovery. New classes of antibacterials interfering with elements of the oxidative phosphorylation pathway are highly active in combating dormant or latent mycobacterial infections, with a promise of shortening tuberculosis chemotherapy. The regulatory approval of the ATP synthase inhibitor bedaquiline and the discovery of Q203, a candidate drug targeting the cytochrome bc₁ complex, have highlighted the central importance of this new target pathway. In this review, we discuss key features and potential applications of inhibiting energy metabolism in our quest for discovering potent novel and sterilizing drug combinations for combating tuberculosis. We believe that the combination of drugs targeting elements of the oxidative phosphorylation pathway can lead to a completely new regimen for drug-susceptible and multidrug-resistant tuberculosis.

Keywords: Mycobacterium tuberculosis; antibiotics; energy metabolism; oxidative phosphorylation.

FIG 1

Oxidative phosphorylation in M. tuberculosis. Electrons derived from NADH are fed into the electron transport chain by NADH dehydrogenase, leading to the reduction of the menaquinone pool (MK/MKH₂). In M. tuberculosis, the type I NADH dehydrogenase, the homologue of complex I in mitochondria, is dispensable for growth. Instead, mycobacteria employ the type II NADH dehydrogenase (NDH-2), which is present in two copies in M. tuberculosis. The menaquinone pool can also be reduced by alternative electron donors, e.g., via the succinate dehydrogenase (SDH). M. tuberculosis has two succinate dehydrogenase enzymes (Sdh-1 and Sdh-2) and one fumarate reductase, which catalyzes the reverse reaction. From the menaquinone pool, electrons can be transferred to the cytochrome bc₁ complex. In mycobacteria, the cytochrome bc₁ complex forms a supercomplex with the cytochrome aa₃-type terminal oxidase, which transfers the electrons onto oxygen. Alternatively, oxygen can be reduced by a cytochrome bd-type terminal oxidase, which directly accepts electrons from the menaquinone pool. During electron transport along the respiratory chain, protons are pumped across the membrane, leading to a proton motive force (PMF). The energy of the PMF can be used by ATP synthase for synthesis of ATP.

FIG 2

Effect of inhibitors of oxidative phosphorylation on function and expression of efflux pumps. (A) Inhibitors of oxidative phosphorylation can act at 2 different levels: limiting the electron flow through the respiratory chain or inhibiting directly the production of ATP. Inhibitors of the ATP synthase (red lines, direct inhibition) cause the ATP pool to drop, thus decreasing the function of primary transporters (blue dashed lines, indirect inhibition), which use ATP directly to fuel transport through the membrane. The inhibitors of the electron flow in the respiratory chain lower the generation of proton motive force (PMF), which results in less function of secondary transporters (PMF driven) and in less ATP production, which affects the function of ATP-driven efflux pumps. As a result, efflux pumps cannot extrude their substrates as efficiently as in the absence of inhibitors of oxidative phosphorylation, and if used in combination with other drugs, the intracellular concentration of these drugs can be increased and their activity can be enhanced. Note that in mycobacteria ATP synthase synthesizes ATP from ADP and P_i and apparently does not invert its function (79). (B) Effect of bedaquiline on the transcription levels of efflux pumps in M. tuberculosis. RNA levels of ATP- and PMF-driven efflux pumps after 0, 30, 180, and 360 min of treatment with bedaquiline are shown. The RNA profile shows temporary downregulation for the majority of efflux pump genes after 30 min of exposure to bedaquiline. The color scales represent log₂ fold changes in gene expression.