Research Article

An electron transfer path connects subunits of a mycobacterial respiratory supercomplex

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Science  30 Nov 2018:
Vol. 362, Issue 6418, eaat8923
DOI: 10.1126/science.aat8923

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An electron bridge in place of a ferry

Respiratory complexes are massive, membrane-embedded scaffolds that position redox cofactors so as to permit electron transfer coupled to the movement of protons across a membrane. Gong et al. used cryo–electron microscopy to determine a structure of a stable assembly of mycobacterial complex III–IV, in which a complex III dimer is sandwiched between two complex IV monomers. A potential direct electron transfer path stretches from the quinone oxidizing centers in complex III to the oxygen reduction centers in complex IV. A loosely associated superoxide dismutase may play a role in detoxifying superoxide produced from uncoupled oxygen reduction.

Science, this issue p. eaat8923

Structured Abstract

INTRODUCTION

Cellular respiration is a core feature in the metabolism of many organisms that allows for the generation of a proton gradient across a membrane. During respiration, electrons are transferred from electron donors to oxygen through an electron transport chain. The energy created allows protons to be pumped across a membrane (cellular or mitochondrial). In electron transport chains, quinones and cytochrome c are two of the electron carriers that shuttle electrons to and from large macromolecular structures that are embedded in the membrane. The components that allow respiratory chains to function in the mitochondria are well characterized, but the situation is less clear and more varied in prokaryotic systems. A soluble cytochrome c pathway for electron transfer similar to that in mitochondria is commonly found in Gram-negative bacteria. Gram-positive bacteria such as Mycobacteria are devoid of a soluble cytochrome c but instead possess cytochrome c proteins that are anchored onto the membrane or have a fused cytochrome c domain to mediate electron transfer between two of the major complexes, which are referred to as CIII and CIV.

Structures of eukaryotic respiratory supercomplexes have been reported, but cytochrome c is not visible in any of these structures. Thus, a complete pathway for electron flow has not yet been visualized. CIII–CIV supercomplexes have been isolated from Mycobacterium smegmatis, Corynebacterium glutamicum, and Mycobacterium tuberculosis and shown to couple quinol oxidation to oxygen reduction without an external electron shuttle, suggesting that the flow of electrons is internalized in this type of complex. The determination of the structure of this complex reveals a path for electron transfer between the subunits of these supercomplexes.

RATIONALE

The structural information provided here is required to understand the molecular details of electron transport in Mycobacteria. We have selected the supercomplex CIII–CIV from M. smegmatis because it is highly similar to the CIII–CIV complex from the human pathogen M. tuberculosis. This complex was amenable to expression and purification and analysis by means of cryo–electron microscopy (cryo-EM).

RESULTS

We have determined a cryo-EM structure of a respiratory supercomplex isolated from M. smegmatis. The structure allows the complete visualization of 20 subunits that associate to form the complex. Central to the supercomplex is a CIII dimer that is flanked on either side by individual CIV subunits. Fused c-type cytochrome domains bridge and mediate electron transfer from CIII to CIV. The structure also reveals three previously unidentified associated subunits that contribute to the stability of the supercomplex and the presence of superoxide dismutase (SOD), which may be responsible for the detoxification of superoxide formed by CIII.

CONCLUSION

This study of a respiratory supercomplex in Mycobacteria reveals cofactors positioned at distances that permit electron tunneling, enabling direct intrasupercomplex electron transfer from menaquinol to oxygen without the need for a separate cytochrome c electron shuttle. The presence of a bound SOD to the respiratory supercomplex suggests a mechanism of mycobacterial resistance against exogenous and endogenous oxidative stress in macrophages and host immune responses. The structure of the quinone binding sites provides a framework for rational structure-based M. tuberculosis drug discovery. A binding site can be proposed for the candidate antimycobacterial drug Q203, which acts by inhibiting the activity of this supercomplex.

Structure of mycobacterial respiratory supercomplex CIII2CIV2SOD2.

Overall architecture of the bcc-aa3–type respiratory CIII–CIV supercomplex from M. smegmatis. The cryo-EM map of the supercomplex shows a linear twofold dimerized form of CIV1–CIII2–CIV1.

Abstract

We report a 3.5-angstrom-resolution cryo–electron microscopy structure of a respiratory supercomplex isolated from Mycobacterium smegmatis. It comprises a complex III dimer flanked on either side by individual complex IV subunits. Complex III and IV associate so that electrons can be transferred from quinol in complex III to the oxygen reduction center in complex IV by way of a bridging cytochrome subunit. We observed a superoxide dismutase-like subunit at the periplasmic face, which may be responsible for detoxification of superoxide formed by complex III. The structure reveals features of an established drug target and provides a foundation for the development of treatments for human tuberculosis.

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