Oxidative Phosphorylation And Electron Transport Chain PdfBy Casiano Q. In and pdf 04.05.2021 at 16:03 9 min read
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ATP synthesis is not an energetically favorable reaction: energy is needed in order for it to occur.
- Electron carriers and energy conservation in mitochondrial respiration
- Oxidative Phosphorylation and Electron Transport
- Oxidative phosphorylation
The electron transport chain is the main source of ATP production in the body and as such is vital for life. The previous stages of respiration generate electron carrier molecules, such as NADH, to be used in the electron transport chain. Clinically, some molecules can interfere with the electron transport chain, which can be life threatening due to its importance and these are discussed in detail later. The electron transport chain is located in the mitochondria.
Electron carriers and energy conservation in mitochondrial respiration
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Fresh and frozen human heart tissue was used. Cell lysate or mitochondria were isolated using standard techniques. Proteins critical to the regulation of mitochondrial metabolism and function were also evaluated in tissue lysates.
PCR analysis revealed an increase in mt DNA lesions and the frequency of mitochondrial common deletion, both established markers for impaired mitochondrial integrity in CAD compared to non-CAD patient samples. CAD 0. Expression of ETC complex subunits and respirasome formation were increased; however, elevations in the de-active form of complex I were observed in CAD. We observed a corresponding increase in glycolytic flux, indicated by a rise in pyruvate kinase and lactate dehydrogenase activity, indicating a compensatory increase in glycolysis for cellular energetics.
Together, these results indicate a shift in mitochondrial metabolism from oxidative phosphorylation to glycolysis in human hearts subjects with CAD. Mitochondria are the major source of cellular energy in the heart, producing ATP via the electron transport chain ETC. Mitochondrial damage can impair cellular function and has been linked to several cardiovascular diseases 1 , 2 , 3 , 4.
For example, in coronary artery disease CAD 5 , studies have identified an increased frequency of the mitochondrial DNA mt DNA common deletion typically, a well-characterized 4, base pair deletion and elevated mt DNA damage when compared to healthy controls 6 , 7. Similarly, a relative rise in mt DNA deletion frequency was found in human heart tissue with chronic ischemia 1. Although CAD is associated with mt DNA damage and deletions, the specific effect on mitochondrial function has not been examined.
During mitochondrial respiration, electrons from complexes I and III can react with molecular oxygen to form superoxide, a damaging reactive oxygen species ROS 9.
Several animal models and human studies have demonstrated that energy demands in the failing heart are increasingly dependent on glycolysis However, a detailed characterization of the cellular changes occurring in the cardiac tissue of human CAD patients is lacking.
Previous studies from our group have shown an increased level of mt ROS in CAD patients as determined by the increase release of mitochondrial H 2 O 2 , a very important pro-inflammatory signaling factor 15 , The goal of this study was to determine the bioenergetic profile of cardiac mitochondria in tissue from subjects suffering from CAD compared to non-CAD controls.
The study was performed in accordance with the ethical principles for medical research involving human subjects. The Institutional Review Board of the Medical College of Wisconsin and Froedtert Hospital deemed collected or otherwise discarded tissue to be exempt and as a result no patient consent was required.
The samples were collected from surgical discarded tissues and rejected donor hearts in areas without any obvious sign of infarct or other damage. Atrial and ventricular tissues from subjects without known cardiovascular risk factors or clinical diagnosis of CAD were used for Non-CAD groups.
Sections of fresh or frozen CAD and non-CAD hearts were used to isolate intact mitochondria or mitochondrial proteins, respectively. Protein concentration was determined by the Bradford method with BSA as a standard.
Mitochondria isolated from fresh cardiac tissue as described above were used to measure mitochondrial respiration using Clark-type O 2 electrode model ; Strathkelvin Instruments, Glasgow, Scotland in a chamber model MTA, Strathkelvin Instruments as previously described The intactness of OXPHOS or coupling was appraised by determining the respiratory control index RCI , the ratio of the maximum oxygen consumption rate, state 3 respiration, to the oxygen consumption rate after most of the added ADP is converted to ATP, state 4 respiration.
The purity of mitochondria was not assessed in this experiment as only intact mitochondria are able to respire. The activities of the mitochondrial electron transport chain complexes were measured using a native gel-based assay as previously described At the end of the run, one gel was used as a loading control for total protein and stained with Coomassie blue and the second gel was stained for activity.
Image J software was used to quantify the density of the activity-bands and normalize them to the corresponding total protein loaded on the Coomassie gel. The tissue was minced and homogenized with an Elvehjem potter.
Denaturing and Clear Native CN electrophoresis was done according to published protocol 19 , The activity of Complex I was measured with a Spectramax plus microplate reader Molecular Devices, Sunnyvale CA at room temperature using published protocols 21 , Generally, the test-volume was 0.
Samples were frozen and thawed twice prior to testing. The active and de-active form of Complex I was accessed by incubating a mitochondrial aliquot with and without 1 mM N-ethyl-maleimide NEM in 0. Protein expression was analyzed as previously described Specific primers were used to amplify a large fragment of mt DNA 8. These values were then used to estimate mathematically, assuming a Poisson distribution as previously described 26 , the number of lesions present in mt DNA.
Relative quantitative standard curves were generated by dilution series of standard pool human DNA. Low deletion region signal was used to control for mt DNA content in samples, and common deletion load per genome was expressed as the ratio of high deletion to low deletion content.
All samples were assayed in triplicate, and replicate means were used for analysis. Individual libraries were prepared for each sample, indexed for multiplexing, and then sequenced on an Illumina HiSeq. Default parameters were used with the exception of a Bowtie2 offset of 1, trading index size for increased alignment speed. Expression abundances were quantified at the whole transcript-level as effect counts using eXpress version 1.
The transcript-level count data were aggregated per gene and rounded to an integer to produce gene-level count matrix. Statistical significance was determined at an FDR threshold of 0. Statistical analyses were performed using Graphpad Prism version 7 software.
Other experiments were performed in previously frozen cardiac tissue. Mitochondria are dynamic organelles that change their morphology by fusion and fission, especially following stress 30 , Altered fusion and fission of mitochondria are associated with elevated levels of ROS production and have been linked to the development of various cardiovascular phenotypes in animal models To evaluate the mitochondrial dynamics associated with CAD, we measured expression of markers regulating mitochondrial dynamics: Drp1, Mfn1, Mfn2 and Opa1 Fig.
Both fission phosphorylated and total DRP1 Fig. These data suggest consistent changes in mitochondrial dynamics in the CAD hearts. Mitochondrial dynamic markers expression in left ventricles of human hearts. Non-CAD human hearts. Mitochondria are the main source of ATP in cardiac cells, and oxidative stress is known to impair ATP generation 33 , Similar patterns were observed in the right ventricle and in the left and right atria and of the heart from these same subjects Supp Fig.
Mitochondrial bioenergetics in human hearts. Similar changes were observed in left and right atria as well as in the right ventricle of the heart Supp Fig. These data suggest that the deficit in ATP observed in the hearts from CAD subjects is caused, at least in part, by a significant defect in the mitochondrial OXPHOS machinery, suggesting that the observed reduction in ATP levels arises from impaired mitochondrial respiration. To further study the effect of CAD on mitochondrial respiration, we evaluated the activity of each complex I-V of the ETC in in-gel assays of native gel.
There were no significant differences in complex IV and V activity between the two groups Fig. A decline in respirasomes has been observed due to aging or IR injury 38 , while synthasomes assemble due to bioenergetic demands We separated mitochondrial protein complexes by CN electrophoresis and analyzed supercomplex levels by immunoblotting Supercomplexes activity and formation in human hearts. Since we had found decreased complex I activity but increased complex I expression and assembly, we then tested complex I activity and the connectivity of complex I to ubiquinone and cytochrome c using spectrophotometry.
First, we measured the activity of complex I to oxidize electrons and to transfer them to ubiquinone Fig. We found that even though more complex I was expressed and assembled, this ETC complex was more likely to be in its de-active state in CAD samples Fig.
As shown in Fig. Although the antibody used consists of a cocktail of antibodies for the 5 different subunits, it was difficult to represent the blot as a whole with one exposure time see Supp Fig. Thus, we chose to show every row corresponding to each subunit individually and, as results for ATP5A were inconsistent using the cocktail of antibodies see Supp Fig. Oxidative phosphorylation complexes expression in human hearts. To test this hypothesis, we assessed the pyruvate kinase activity, the final step of glycolysis that yields an additional ATP.
Glycolytic pathway in human hearts. Like the pyruvate kinase activity Fig. The results further point to metabolic switch towards more reliance on glycolysis as the major source of ATP with concomitant increase in lactate production in the CAD hearts.
These data support our functional data of a metabolic shift and impaired energy production that we observed in the LV tissue of CAD patients, and provides multiple molecular mediator candidates for future analysis. To the best of our knowledge, this is the first study to fully characterize changes in mitochondrial respiration and mt DNA integrity and its detailed effect on mitochondrial metabolism within the human heart from subjects with and without CAD.
There are five major novel findings of this study. Second, regulators of mitochondrial networks fission and fusion are both upregulated in CAD. Fourth, reduced mitochondrial respiratory activity is compensated by an increase in glycolytic flux in the hearts from CAD subjects.
Lastly, the mitochondrial genome and the RNAseq analyses confirm a general metabolic defect in the hearts from CAD subjects. The human heart must continuously pump blood to supply oxygen and nutrients to tissue; as such, it has substantial energy demands, even at rest Thus, it is no surprise that mitochondrial dysfunction is associated with many cardiovascular diseases reviewed in Compared to previous reports, our observed ATP levels were surprisingly low.
A possible explanation is we have evaluated ATP levels directly ex vivo using frozen heart tissues vs. Hence, we can only speculate about the absolute levels of ATP in the CAD hearts as our analysis was purely comparative.
Because of their instability and very short half-life, the means used to measure ROS are limited, especially in clinical conditions.
Previous studies showed that plasma levels of oxidized LDL 51 , 52 , malondialdehyde 51 , and advanced oxidation protein products 57 were significantly higher in patients with CAD than in those without CAD. Of relevance to the data of the present study, impairment of complexes I and III due to ROS have been previously reported 58 , 59 and suggested to cause electron leak from the ETC, leading to a feed-forward cycle of oxygen radical-induced damage to mitochondrial membrane components.
Moreover, oxidative post-translational modification of complex I and complex II has been shown to affect enzymatic catalysis and enzyme-mediated ROS production 60 , 61 , 62 , 63 , 64 , 65 , which in turn contributes to compromised mitochondrial respiration and ATP synthesis, and in the pathogenesis of many diseases, including diabetes and heart failure 2 , 6 , 66 , 67 , 68 ,
Oxidative Phosphorylation and Electron Transport
The chemical system for the transformation of energy in eukaryotic mitochondria has engaged researchers for almost a century. This summary of four lectures on the electron transport system in mitochondria is an introduction to the mammalian electron transport chain for those unfamiliar with mitochondrial oxidative phosphorylation. It gives references chosen to reflect the history of the field and to highlight some of the recent advances in bioenergetics. The electron transport chain converts the energy that is released as electrons are passed to carriers of progressively higher redox potential into a proton gradient across the membrane that drives adenosine triphosphate ATP synthesis. The three processes of proton pumping are now known after the successful determination of the structures of the large membrane protein complexes involved. Mitochondria and their proteins play roles not only in the production of ATP but also in cell survival, for which energy supply is the key. The chemiosmotic mechanism for ATP synthesis is key to aerobic energy conversion in all cells, supplying the majority of the energy required for survival, repair, growth, and reproduction of the organism.
Due to the existence of electron leak and proton leak, not all electrons in the ETC can be transferred to the final electron acceptor O 2 and the energy released by the transferred electrons cannot be completely coupled with ATP generation. However, both the ROS generated by electron leak and the UCPs implicated in proton leak play an important role in the physiology and pathology of cells. Therefore, it is extremely important to understand the process of electron transfer in the ETC and the mechanism of electron leak and proton leak. In this review, the basic components of the ETC are discussed and the process of electron transfer in each complex, including the structure, composition and function of each complex is reviewed.
In eukaryotes , this takes place inside mitochondria. Almost all aerobic organisms carry out oxidative phosphorylation. This pathway is so pervasive because it releases more energy than alternative fermentation processes such as anaerobic glycolysis.
The electron transport chain uses the electrons from electron carriers to create a chemical gradient that can be used to power oxidative phosphorylation. Oxidative phosphorylation is a highly efficient method of producing large amounts of ATP, the basic unit of energy for metabolic processes. During this process electrons are exchanged between molecules, which creates a chemical gradient that allows for the production of ATP.
Molecular Cell Biology. 4th edition.
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