Oxygen metabolism in cells and adaptations to hypoxia

Figure 5 Binding curve of oxygen to haemoglobin

Due to the cooperative character of haemoglobin, the binding curve shows a sigmoidal course. Various factors affect the affinity and can shift the curve to the left or right.

3.1.5. Oxygen metabolism in cells and adaptations to hypoxia

Every cell in the body requires oxygen to produce Adenosine triphosphate (ATP), the cell’s

“currency of energy”. Adult humans metabolize about 200 grams of oxygen per day (X. D.

Wang and Wolfbeis 2014). Therefore, oxygen molecules diffuse passively into the cells. The driving force is the difference in concentration of oxygen between the intracellular and extracellular milieu. For fat-soluble substances such as oxygen molecules, the entire cell membrane of the consumer cell is available for this mechanism (Pittman 2011). The oxygen molecules are either dissolved directly in the blood stream or are bound to haemoglobin, from where it dissolves at the place of consumption due to the Bohr effect.

A simplified schematic of the following procedures according to Alberts et al., 2017 is shown in figure 6 .

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Through the blood plasma and the intercellular fluid, the oxygen molecules diffuse into the cytosol of the cells. From there, they enter the mitochondria, where they serve as maintainers of the so-called respiratory or electron transport chain and are "consumed".

Pyruvate and fatty acids from the cytosol are degraded in the mitochondria to Acetyl-CoA. The Acetyl-CoA is then metabolized in the so-called Citric acid cycle, which reduces Nicotinamide adenine dinucleotide (NAD⁺) to NADH (or Flavin adenine dinucleotide (FAD) to FADH2; not shown). By means of the so-called oxidative phosphorylation energy-rich electrons from NADH (or FADH2) are then transported to the direction of oxygen along the electron transport chain in the inner membrane of the mitochondria. This electron relocation produces a proton gradient that is used to power the formation of ATP by the ATP synthase, using Adenosine diphosphate (ADP) and phosphate. Along the electron transport chain, the electrons pass through different protein complexes, with Complex I absorbing the electrons of NADH and Complex II absorbing electrons coming from FADH2. Complex II, unlike Complex I, does not pump protons into the intermembrane space. The lipophilic molecule Q10 (Ubiquinone) transfers the electrons to Complex III, which in turn serves as a proton pump. The protein Cytochrome-C finally transports the electrons from Complex III to Complex IV, in which the so-called Cytochrome-C oxidase catalyses the transfer of electrons to the molecular oxygen and thus reduces it to water (H2O). Molecular oxygen is thus an essential reagent in the respiratory chain.

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Figure 6 Oxygen molecules in the respiratory chain within the mitochondria

From different sources Acetyl-CoA is synthesized inside the mitochondrion. Acetyl-CoA is converted to NADH in the citric acid cycle by which electrons are released. These electrons transpass four complexes (I-IV) located on the inner membrane where H⁺- ions are released simultaneously. The free H⁺- ions pass through the intermembrane space to the ATP synthase, which transfers externally derived phosphate to ADP, thereby producing ATP, the cell's energy currency. To maintain the flow of electrons in the complexes, oxygen ultimately serves as an electron acceptor and is converted to water.

If there is an undersupply of oxygen, the ATP synthesis can be ensured shortly via the anaerobic degradation of glucose to lactate. However, the amount of ATP gained thereby is only about 5% of the amount that is otherwise obtained via the oxidative ATP synthesis (G. L.

Semenza 2009; Mollenhauer and Kiss 2010). In addition to this lack of energy, there is also the formation of large amounts of cell toxic reactive oxygen species (ROS), because electrons are then transferred to elemental oxygen (J. Kim et al. 2006). The effect of the ROS can lead to irreversible cell damage, that can be seen sometimes in organ transplantations during inadequate oxygen supply ex vivo (Schmidt et al. 2008). Mammals, however, have a high adaptability to extreme oxygen conditions (e.g. high flying birds or deep diving whales) (Ramirez, Folkow, and Blix 2007).

An important role in the adaptation of the cells to decreasing oxygen concentrations is played by a very prominent transcription factor, the so-called Hypoxia-inducible factors (HIF) (Michael Swindle and Smith 2008). These factors consist of an α- and a β- subunit. The latter

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is constitutively expressed by the cells, while the α-subunit is subject to oxygen regulation (G.

L. Wang et al. 1995). If enough oxygen is present, the HIF prolyl hydroxylases (PHD 1-3) will hydroxylate this subunit (Domann and Place 2013) and then bind it to von Hippel-Lindau tumour suppressor protein (pVHL) (Maxwell et al. 1999), ultimately proteasomally degrading the HIF α-subunit (Forsythe et al. 1996; Bruick 2001). If the α-subunits are not degraded by the PHD in the absence of oxygen, they bind to the β-subunit and are transported into the cell nucleus. There, HIF binds to the so-called hypoxia-responsive elements (HRE) in promoters of target genes whose transcription causes various adaptations to oxygen deficiency like the Vascular endothelial growth factor VEGF, which leads to an angiogenesis around the hypoxic cells and thus promotes an increased blood supply (Mohamed et al. 2004; Shweiki et al. 1992;

Gregg L. Semenza 2014; Brahimi-Horn, Chiche, and Pouysségur 2007; Goto et al. 1993; Prior, Yang, and Terjung 2004). In addition an increased formation of erythropoietin can be seen (G.

L. Semenza 2009). In hibernating animals with reduced respiration the presence of HIF leads to a decrease of high oxygen-consuming metabolic processes (Andrews 2004).

How cells respond to hypoxia is influenced by their oxygen consumption rate (OCR), which has been studied for some cell lines (Wagner, Venkataraman, and Buettner 2011b). Cells with a high metabolism, such as hepatocytes, have a very high OCR between 200-400 attomoles (amol)/cell/s (Metzen et al. 1995). Other cell lines show OCR between 1 - 120 amol/cell/s (Wagner, Venkataraman, and Buettner 2011b). Interestingly, the HIF-PHD system reacts much earlier to decreasing O2 partial pressure than it would be needed to maintain the function of cytochrome-C oxidase. The pO2 or the oxygen concentration when the rate of cytochrome-C oxidase activity is ½ (P50/KM) is about 0.075-0.75 mmHg respectively 0.0097-0.097 mol/L (Scandurra and Gnaiger 2010). KM values for the HIF-PHD system are between 0.100-0.240 mol/L (Ehrismann et al. 2007; Hirsilä et al. 2003). This corresponds to an oxygen partial pressure between 70 and 75 mmHg (Place, Domann, and Case 2017).