Introduction to Cell Respiration

By Yujean Kim

When we run a marathon, our leg muscles contract to keep our legs moving. When we eat dinner, our intestines contract to undergo peristalsis. All of these fundamental processes that occur in our body are able to function due to this one metabolic reaction: cellular respiration.

Cell respiration is the controlled release of energy from organic compounds to produce ATP (adenosine triphosphate). It is extremely important to produce ATP in our body, as ATP is a high energy molecule that functions as an immediate source of energy or power for cellular processes. ATP is a molecule with adenosine covalently bonded to three phosphate groups. The bond that links the phosphate groups together stores a great amount of energy. Therefore when ATP is hydrolyzed into ADP (adenosine diphosphate) and inorganic phosphate (Pi), energy stored is released and can be used for many cellular processes (Ib and Guides, 2014). Cell respiration can release energy from organic compounds to produce ATP in two ways: anaerobic respiration and aerobic respiration.

The very first step that takes place during cellular respiration is glycolysis. Simply put, glycolysis is the splitting of glucose into two to produce pyruvate:

glucose + 2ADP + 2Pi + 2NAD 2 pyruvate + 2ATP + 2NADH

However, this simple definition is just the tip of the iceberg. Glycolysis actually consists of many complex intermediate steps which are regulated by many enzymes. These enzymes are in turn regulated through the help of kinase and phosphorylase. Kinases are enzymes that phosphorylate molecules using organic molecules such as ATP. They often add a phosphate group to serine, threonine or tyrosine to activate the enzyme,  increasing enzyme activity. Phosphorylases also phosphorylate molecules, but do so using inorganic molecules.

The glycolysis pathway is split into three processes: preparatory, cleavage and the pay-off step (see Figure 1) (Voet and Voet, 1990). The preparatory step starts off by converting carbohydrate into glucose 6-phosphate (G6P). Carbohydrates are obtained from many sources, such as glucose (food), glycogen (liver) or starch (amyloplasts). Hexokinase phosphorylates glucose into G6P whereas glycogen phosphorylase phosphorylates glycogen into G6P. Hexose-P-isomerase then catalyzes conversion of G6P to fructose 6-phosphate (F6P). Finally, F6P is phosphorylated using ATP to create fructose-1,6-biphosphate by the enzyme phosphor-fructokinase. This ends the preparatory step of the glycolysis pathway.

Figure 1: Glycolysis pathway: preparatory, cleavage and pay-off step

Fructose-1,6-biphosphate then enters the cleavage pathway. During the cleavage pathway, the 6 carbon molecule, fructose-1,6-biphosphate, is split into two carbon 3 groups: GAP (glyceraldehyde phosphate) and DHAP (dihydroxyacetone phosphate). DHAP is then converted into GAP in a reaction catalyzed by triose phosphate isomerase. This ultimately leads GAP into the pay-off stage, ultimately producing pyruvate. Overall, glycolysis produces a net yield of 2 ATP and 2 NADH. Once pyruvate is produced, it is destined to participate in two further processes: aerobic oxidation and anaerobic respiration.

Once glycolysis finishes, pyruvate will undergo anaerobic respiration (“anaerobic” refers to the lack of oxygen). Anaerobic respiration is a type of cell respiration that partially breaks down organic molecules in the cytoplasm without the use of oxygen. Because organic molecules are not completely broken down during anaerobic respiration, a small yield of ATP is produced instead. The products of anaerobic respiration vary  between species. 

Under aerobic conditions, pyruvate is oxidized to water and carbon dioxide via the Krebs cycle and oxidative phosphorylation.  Aerobic respiration is a type of cell respiration that uses oxygen to oxidize an organic molecule such as glucose. Oxygen fully breaks down a glucose molecule therefore it produces a high yield of ATP. Therefore, aerobic respiration can be simplified with the following equation:

glucose + oxygen -> carbon dioxide + water + ATP

Once pyruvate is produced from glycolysis, it is transferred to the mitochondrial matrix, where it undergoes a series of reactions collectively called the link reaction. During the link reaction, the pyruvate will form acetyl coenzyme A (acetyl-CoA) with the help of pyruvate dehydrogenase complex. Pyruvate dehydrogenase is an extremely big enzyme – in fact it is as big as a ribosome. It plays a role in linking glycolysis to the Krebs cycle. It does this by decarboxylating pyruvate into a 2 carbon group fragment. The 2 carbon group fragment then undergoes oxidation which reduces the NAD+ molecule into NADH. This ultimately forms an acetyl group. The acetyl group finally combines with coenzyme A which produces acetyl coenzyme A.

The acetyl coenzyme A will then enter the Krebs cycle in the mitochondrial matrix. Just like glycolysis, the Krebs cycle consists of a series of reactions supported by many different enzymes, as illustrated in Figure 2. It all starts off with acetyl coenzyme A combining with oxaloacetate to form a citrate molecule. The citrate molecule then undergoes a series of decarboxylation and oxidation reaction to form a C4 molecule. The C4 molecule further undergoes further oxidation reaction to form oxaloacetate. The cycle then repeats by combining oxaloacetate with acetyl coenzyme A again. 

Figure 2: The link reaction and the Krebs cycle

Once the Krebs cycle finishes, oxidative phosphorylation occurs. Oxidative phosphorylation takes place in the inner mitochondrial membrane and between the mitochondrial matrix and transmembrane space. Within the inner mitochondrial membrane, many proteins are embedded within the membrane. This includes the NADH dehydrogenase protein transmembrane complex, Succinate dehydrogenase, cyt-b/c complex, cytochrome-c oxidase and finally the F-type ATP synthase. Together these embedded proteins ultimately form an electron transport chain as illustrated in Figure 4.

Figure 4: Electron transport chain

The electron transport chain all starts off in the NADH dehydrogenase complex, which is one of the largest transmembrane complexes. The structure was only recently studied, as purifying the transmembrane complex is extremely hard. NADH dehydrogenase initially converts NADH to NAD+, which releases two electrons. These electrons flow through the transmembrane complex and cause conformational changes to each of the subunits in the transmembrane. These conformational changes cause H+ ions to flow through the inner membrane, to the transmembrane space. The electrons are then transferred to ubiquinone (UQ), which is also embedded within the inner mitochondrial membrane. By accepting two electrons, the ubiquinone is converted to ubiquinol (UQH2).

Next to the NADH dehydrogenase complex is the succinate dehydrogenase protein, responsible for converting the succinate formed during the Krebs cycle into fumarate. This releases two electrons which travel through the transmembrane complex. Therefore, just like the NADH dehydrogenase, succinate dehydrogenase will convert the ubiquinone to ubiquinol by accepting two electrons. However no H+ ions are passed given a less favorable ΔG.

Next to succinate dehydrogenase are the cytochrome-b/c complex and cytochrome-c oxidase. They also cause H+ ions to flow across the inner membrane to the transmembrane space. Accordingly, the electron transport chain will move electrons from carrier to carrier where energy is used to transfer protons across mitochondrial matrix to intermembrane space. This will accumulate protons in the intermembrane space which will create an electrochemical gradient. While this occurs, de-energized electrons in the cytochrome-c oxidase will be accepted by oxygen molecules to form H2O. This maintains electrochemical gradient and keeps the electron transport chain flow going, allowing more electrons to flow along the electron transport chain and NAD+/FADH to be regenerated. The oxygen also maintains the hydrogen gradient by absorbing H+ ions in the mitochondrial matrix.

Finally, hydrogen ions that accumulate in the intermembrane space will diffuse across the ATP synthase, moving down the electrochemical gradient. The energy released will phosphorylate ADP. Consequently, a high yield of ATP is formed and concludes the process of aerobic respiration.

In conclusion, cell respiration is a metabolic reaction that is much more complex than it may seem. However due to this one magnificent cell reaction, we are able to carry out all fundamental processes that occur in our body!

References:

Ib, O. and Guides, S. (2014) F O R T H E I B D I P LO M A 2014 edition 2.

Voet, D. and Voet, J. G. (1990) Biochemistry 4th edition.

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