Regulation of mitochondrial oxidative phosphorylation through tight control of cytochrome c oxidase in health and disease - Implications for ischemia/reperfusion injury, inflammatory diseases, diabetes, and cancer

Abstract

Mitochondria are essential to cellular function as they generate the majority of cellular ATP, mediated through oxidative phosphorylation, which couples proton pumping of the electron transport chain (ETC) to ATP production. The ETC generates an electrochemical gradient, known as the proton motive force, consisting of the mitochondrial membrane potential (ΔΨ_m, the major component in mammals) and ΔpH across the inner mitochondrial membrane. Both ATP production and reactive oxygen species (ROS) are linked to ΔΨ_m, and it has been shown that an imbalance in ΔΨ_m beyond the physiological optimal intermediate range results in excessive ROS production. The reaction of cytochrome c oxidase (COX) of the ETC with its small electron donor cytochrome c (Cytc) is the proposed rate-limiting step in mammals under physiological conditions. The rate at which this redox reaction occurs controls ΔΨ_m and thus ATP and ROS production. Multiple mechanisms are in place that regulate this reaction to meet the cell's energy demand and respond to acute stress. COX and Cytc have been shown to be regulated by all three main mechanisms, which we discuss in detail: allosteric regulation, tissue-specific isoforms, and post-translational modifications for which we provide a comprehensive catalog and discussion of their functional role with 55 and 50 identified phosphorylation and acetylation sites on COX, respectively. Disruption of these regulatory mechanisms has been found in several common human diseases, including stroke and myocardial infarction, inflammation including sepsis, and diabetes, where changes in COX or Cytc phosphorylation lead to mitochondrial dysfunction contributing to disease pathophysiology. Identification and subsequent targeting of the underlying signaling pathways holds clear promise for future interventions to improve human health. An example intervention is the recently discovered noninvasive COX-inhibitory infrared light therapy that holds promise to transform the current standard of clinical care in disease conditions where COX regulation has gone awry.

Graphical abstract

Fig. 1

ATP/ADP allosteric regulation of COX and Cytc as an energetic sensor to regulate ATP production. Under low ATP/ADP ratios in the cell, ADP acts as an allosteric activator by binding to COX and the reaction between COX and Cytc occurs at an increased rate resulting in more protons pumped into the intermembrane space (IMS). This generates an increased electrochemical gradient, and ATP synthase (Complex V) is able to generate more ATP. Under high ATP/ADP ratios, ATP acts as an allosteric inhibitor to both COX and Cytc and decreases the rate at which Cytc donates its electron to COX, resulting in fewer protons pumped into the IMS, thereby directly decreasing ATP production by complex V.

Fig. 2

COX subunits with isoforms including position on enzyme, tissue specificity and function. Subunit IV: pink, Subunit VIa: yellow, Subunit VIb: grey, Subunit VIIa: blue, Subunit VIII: light green. Crystal structure of bovine heart COX monomer (PDB DOI: https://doi.org/10.2210/pdb6JY3/pdb). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

Molecular changes of COX and Cytc during ischemia and reperfusion and the application of inhibitory infrared light to prevent ΔΨ_m hyperpolarization and increased ROS as a therapeutic intervention. Under physiological conditions, ΔΨ_m is maintained in the optimal intermediate range (80–120 mV), allowing efficient ATP production and minimal ROS (left). ΔΨ_m is primarily regulated by PTMs such as phosphorylations on COX and Cytc to maintain this optimal intermediate range. During ischemia, increased calcium influx into mitochondria activates stress pathways including phosphatases, resulting in dephosphorylation of mitochondrial proteins. This includes Cytc and COX, priming them for hyperactivity. During reperfusion, oxygen, the substrate for COX returns resulting in hyperpolarization of ΔΨ_m and an exponential increase in ROS. COX inhibitory infrared light applied during reperfusion counteracts mitochondrial hyperactivity, limits ROS production, and is neuroprotective.