Aerobic respiration is a cellular process for harvesting energy. Electrons are extracted from an electron donor and transferred to O2 as the terminal electron acceptor. This process generates a membrane potential across the cytoplasmic membrane termed proton motive force (pmf). The pmf is then used to drive ATP synthesis via the membrane-bound ATP synthase (electron transport phosphorylation).

Key Concepts

  • Aerobic respiratory chains are located in the cytoplasmic membrane and are used to generate a proton motive force (pmf).
  • Aerobic electron transport chains always contain a dehydrogenase enzyme that donates electrons to cytochrome oxidase that reduces O2to H2O.
  • Ubiquinone (Q) mediates electron mediator between the two enzymes.
  • The proton motive force drives ATP synthesis via the membrane-bound ATP synthase.
  • Cytochrome oxidases with different affinities for O2 allow E. coli to grow under aerobic and microaerophilic conditions.

Principles of Aerobic Respiration

Aerobic respiration is a process that provides energy for cell growth under aerobic and microaerophilic conditions. Oxygen is always used as the terminal electron acceptor.

The E. coli aerobic electron transport chains consist of two distinct enzyme types, a dehydrogenase that oxidizes an electron donor (D) such as NADH, and a cytochrome oxidase, which reduces the electron acceptor O2 to H2O. Electrons are transferred from one enzyme to the other via the lipophilic cofactor ubiquinone (Q). The combined reactions may be represented by:

DH + O2 → D + H2O
Electron transport chains are modular. E. coli contains three distinct cytochrome oxidases that differ in their affinity for O2, their ability to pump protons, their cytochrome content, and their gene expression profiles. Each cytochrome oxidase can receive electrons from a variety of alternative electron donating dehydrogenases. The type of electron donor used depends on the availability of its substrate – it can be organic or inorganic in nature – for example, succinate or hydrogen.

The aerobic electron transport chains generate the cellular proton motive force (pmf), which in turn drives ATP synthesis via ATP synthase. The pmf is also used to power flagella rotation needed for cell motility as well as to energize some types of nutrient uptake systems.

How E. Coli Respires with Oxygen

When E. coli grows by oxidizing carbohydrates such as glucose, NAD is reduced to NADH. This cellular intermediate then serves as the electron donor for aerobic respiration.

2 NADH + 2 H+ + O2  →  2 NAD+ + 2 H2O

Simple

For example, when glucose is metabolized via the glycolysis pathwayto pyruvate, two molecules of NADH are made. The pyruvate is subsequently decarboxylated to acetyl-CoA, and then further oxidized to CO2 by the TCA cycle. These latter reactions provide additional molecules of NADH that support aerobic respiration. The TCA cycle intermediate, succinate – can also serve as an electron donor for aerobic respiration.

Enzymes Employed in Aerobic Respiration

The cytochrome oxidases:

The Cytochrome bo oxidase couples the two-electron oxidation of ubiquinol (QH2) with the four-electron reduction of molecular oxygen to water. It has a relatively low affinity for O2 and is most abundant when cells are grown when in high aeration conditions (i.e., oxygen rich). The membrane bound enzyme generates a proton-motive force by consuming protons in the cytoplasm. It also functions as a proton pump, effecting a net movement of 2H+/e– across the cytoplasmic membrane. Expression of the cyooperon is repressed by Fnr and the ArcA/ArcB two component system under anaerobic conditions. Expression also varies with the carbon source used for growth – highest on non-fermentable carbon sources and lowest on glucose. Expression is induced by iron limitation.

Cytochrome bd-I oxidase also catalyses the two electron oxidation of ubiquinol and the four electron reduction of oxygen to water. It generates a proton-motive force by consuming protons in the cytoplasm but unlike CyoABCD, it cannot pump protons.  Expression of the cydAB operon is repressed by Fnr anaerobically, induced under low oxygen conditions via the ArcA/ArcB two component system, and repressed by H-NS under aerobic conditions. This control provides for maximal gene expression under microaerobic conditions.

The physiological role of cytocrome bd-II oxidase is unclear. While it does function as a quinol:oxygen oxidoreductase, it does not appear to contribute to the generation of a proton motive force. It may play a role in uncoupling catabolism from ATP synthesis. The two subunits of cytochrome bd-II encoded by the appC and appBgenes are structurally similar to the cytochrome bd-I terminal oxidase CydA and CydB subunits, respectively. The appCB-appAoperon is regulated by the transcriptional activator AppY whereby gene expression is induced upon entry into the stationary phase, and by starvation for carbon and/or phosphate.

The dehydrogenases:

NADH dehydrogenase I (Ndh-1) catalyzes the transfer of electrons from NADH to the quinone pool in the cytoplasmic membrane, thus regenerating the oxidized form of the cofactor, NAD+. This membrane enzyme forms a proton electrochemical gradient (pmf) by pumping protons from the cytoplasm to the periplasm. It can participate in either aerobic or anaerobic cell respiration. The purified enzyme can be resolved into three components: a soluble fragment composed of the NuoE, F and G subunits, which catalyzes the oxidation of NADH – representing the electron input part of the enzyme; an amphipathic connecting fragment composed of the NuoB, CD and I subunits; and a hydrophobic membrane fragment composed of the NuoA, H, J, K, L, M and N subunits. The soluble subunits contain all iron-sulfur clusters and the FMN cofactor. Expression of the nuo operon is regulated by oxygen, nitrate, fumarate, and other factors including C4 dicarboxylates.

NADH dehydrogenase II catalyzes the transfer of electrons from NADH to the quinone pool in the cytoplasmic membrane. In contrast to Ndh-1, Ndh-2 is a single subunit enzyme and does not pump protons. It functions during aerobic cell respiration, and its primary function may be help maintain the [NADH]/[NAD+] balance of the cell.

Succinate dehydrogenase also participates in aerobic electron transport by donating electrons to the cytochrome oxidases.

succinate + O2 → fumarate + H2O

Also called succinate:quinone oxidoreductase (SQR), succinate dehydrogenase catalyses the oxidation of succinate to fumaratewith the concomitant reduction of ubiquinone (Q) to ubiquinol. Sdh plays an important role in cellular metabolism by directly connecting the TCA cycle with the respiratory electron transport chain. E. coli Sdh is a membrane bound heterotetrameric enzyme. Subunits SdhA and SdhB are hydrophilic and attached to the cytoplasmic surface of the plasma membrane via interactions with the two hydrophobic integral membrane subunits, SdhC and SdhD. SdhA contains the FAD cofactor and the dicarboxylic acid binding site. A single heme b556 cofactor bridges the SdhC and SdhD subunits. The sdhCDAB operon is optimally expressed during aerobic cell growth whereby the ArcA/ArcB two component systemrepresses expression in the absence of oxygen. Gene expression is also subject to catabolite control.

For example, L-lactate dehydrogenase, glycerol-phosphate dehydrogenase, and proline dehydrogenase oxidize their respective substrates and donate electrons to the quinone pool in the cytoplasmic membrane.

Credits:

Authored by Robert Gunsalus and Imke Schröder
©The Escherichia coli Student Portal

This project acknowledges support from:
NIH Grant Award GM077678 to SRI, International
Peter Karp and coworkers at EcoCyc.org
The UCLA Department of MIMG