What is the respiratory chain?


The respiratory chain is a process for generating energy in our body cells. It joins the citric acid cycle and is the last step in the breakdown of sugar, fats and proteins. The respiratory chain is located in the inner membrane of the mitochondria. In the respiratory chain, reduction equivalents (NADH + H + and FADH2) that have formed in the meantime are oxidized again (electrons are given off), whereby a proton gradient can be built up. This is ultimately used to form the universal energy carrier ATP (adenosine triphosphate). Oxygen is also required so that the respiratory chain can run completely.

Sequence of the respiratory chain

The respiratory chain is integrated into the inner mitochondrial membrane and consists of a total of five enzyme complexes. It follows on from the citric acid cycle, in which the reduction equivalents NADH + H + and FADH2 are formed. These reduction equivalents temporarily store energy and are oxidized again in the respiratory chain. This process takes place in the first two enzyme complexes in the respiratory chain.

Complex 1: NADH + H + reaches the first complex (NADH ubiquinone oxidoreductase) and releases two electrons. At the same time, 4 protons are pumped from the matrix space into the intermembrane space.

Complex 2: FADH2 releases its two electrons at the second enzyme complex (succinate-ubiquinone-oxidoreductase), but no protons get into the intermembrane space.

Complex 3: The electrons released are passed on to the third enzyme complex (ubiquinone cytochrome c oxidoreductase), where another 2 protons are pumped from the matrix space into the intermembrane space.

Complex 4: Ultimately, the electrons get to the fourth complex (cytochrome c oxidase). Here the electrons are transferred to oxygen (O2), so that water (H2O) is created with two additional protons. In the process, 2 protons again enter the intermembrane space.

Complex 5: A total of eight protons have now been pumped from the matrix space into the intermembrane space. The basic requirement for the electron transport chain is the increasing electronegativity of the enzyme complex. This means that the ability of the enzyme complexes to attract negative electrons is getting stronger.
In addition to the first end product, water, a proton gradient was built up in the intermembrane space through the respiratory chain. This stores energy that is used to build up ATP (adenosine triphosphate). This is the job of the fifth and final enzyme complex (ATP synthase). The fifth complex spans the mitochondrial membrane like a tunnel. Through this, driven by the difference in concentration, the protons flow back into the matrix space. This creates ATP from ADP (adenosine diphosphate) and inorganic phosphate, which is available to the entire organism.

What does the proton pump do?

The proton pump is the fifth and last enzyme complex in the respiratory chain. Through this, the protons flow back from the intermembrane space into the matrix space. This is only made possible by the previously established difference in concentration between the two reaction spaces. The energy stored in the proton gradient is used to ultimately synthesize ATP (adenosine triphosphate) from phosphate and ADP.
ATP is our body's universal energy carrier and is essential for a variety of reactions. Since it is generated at the proton pump, it is also known as ATP synthase.

Balance of the respiratory chain

The decisive end product of the respiratory chain is ATP (adenine triphosphate), which is a universal energy carrier in the body. ATP is synthesized with the help of a proton gradient that arises during the respiratory chain. NADH + H + and FADH2 are differently efficient. NADH + H + is oxidized back to NAD + in the respiratory chain at the first enzyme complex and pumps a total of 10 protons into the intermembrane space. When FADH2 is oxidized, the yield is lower because only 6 protons are transported into the intermembrane space. This is because FADH2 is introduced into the respiratory chain at the second enzyme complex and thus bypasses the first complex. In order to synthesize an ATP, 4 protons have to flow through the fifth complex.
Consequently, per NADH + H + 2.5 ATP (10/4 = 2.5) and per FADH2 1.5 ATP (6/4 = 1.5) are produced.
When a sugar molecule is broken down via glycolysis, citric acid cycle and respiratory chain, a maximum of 32 ATP can be generated, which is available to the organism.

What role do the mitochondria play?

Mitochondria are cell organelles found in animal and plant organisms. Various energy processes take place in the mitochondria, including the respiratory chain. Since the respiratory chain is the decisive process for generating energy, mitochondria are also called the "power plants of the cell". They have a double membrane, so that a total of two separate reaction spaces are created. Inside is the matrix space and the intermembrane space between the two membranes. These two spaces are fundamental for the respiratory chain to run. Only in this way can a proton gradient be built up, which is important for ATP synthesis.

Read more about the topic in this article: Structure of the mitochondria

What does cyanide do in the respiratory chain?

Cyanides are dangerous toxins, including compounds of hydrogen cyanide. They are able to bring the respiratory chain to a standstill.
Specifically, the cyanide binds to the iron of the fourth complex of the respiratory chain. As a result, the electrons can no longer be transferred to molecular oxygen. As a result, the entire respiratory chain can no longer run.
The result is a lack of the energy carrier ATP (adenosine triphosphate) and so-called "internal suffocation" occurs. Symptoms such as vomiting, unconsciousness and cramps occur very quickly after cyanide poisoning and, if left untreated, lead to rapid death.

What is a respiratory chain defect?

A respiratory chain defect is a rare metabolic disease that often manifests itself in childhood. The causes are changes in the genetic information (DNA). The mitochondria are restricted in their function and the respiratory chain does not function properly. This is particularly noticeable in organs that consume a lot of energy in the form of ATP (adenosine triphosphate).
Typical symptoms are, for example, muscle pain or muscle weakness.
Therapy for this disease is difficult because it is a hereditary disease. It should be ensured that there is a sufficient supply of energy (e.g. through glucose). Otherwise purely symptomatic treatment is appropriate.