During the Krebs cycle, Acetyl-CoA combines with oxaloacetate to form citrated and NAD+ is reduced to form NADH. In the electron transport chain, NADH is oxidized to NAD+ by passing it along to a series of protein complexes in the inner mitochondrial membrane. This journey produces 3 ATP per turn, and this is chemiosmosis production. The first protein complex in the chain is the Cytochrome b-c1 complex which transfers electrons from CoQ (Q) in the outer mitochondrial membrane to cytochrome c1. Cytochrome c1 then transfers electrons onto a pool of protons flowing into the mitochondria creating an electrochemical gradient across the membrane, which can synthesize ATP by adding ADP and Pi back together (Ryan, Murphy et al., 2019).

The location of the Krebs cycle and electron transport chain in mitochondria is within the inner mitochondrial membrane. The cyclic nature of the reactions in the Krebs cycle is because of the use of a compound called NAD+, which acts as an electron carrier during the responses, which passes its electrons to cytochrome c1 via NADH and via cytochrome c1 to the ATP synthase enzyme (Gasmi, Peana et al., 2021).

The production of ATP and reduced coenzymes during the cycle is by chemiosmotic production, which occurs in all steps in the electron transport chain ATP is formed by substrate-level phosphorylation in each cycle. NADH (3) and/or ±ADH2 (1) per cycle of the Krebs cycle, 6 molecules ATP are generated per cycle of the Krebs cycle. ADP is added back to ATP and Protein produced during the cycle. This process is known as chemiosmotic production, which occurs through the movement of extra protons across the inner mitochondrial membrane into mitochondria and allows Asp to be converted into NAD+ and FAD to be converted into Ubiquinone. At this point, ADP + Pi and Q are added back together, producing ATP (Ryan, Murphy et al., 2019).

The production of ATP and reduced coenzymes during the Krebs cycle is one way to describe how ATP is formed from nutrients. During each cycle, ATP is produced from the substrate level phosphorylation of TAG, which produces ADP and phosphate. NADH (3) and/or ±ADH2 (1) emerges as a by-product during this process. The production of ATP and reduced coenzymes during the Krebs cycle in many ways describes how the body converts nutrients into energy for cellular use in order to utilize available fuels more efficiently throughout respiration, fermentation, and cell division.

The Chemiosmosis Production of ATP during the electron transport chain is also via the movement of protons moving from outside of mitochondria, out of the mitochondrial matrix down across the inner membrane. In the electron transport chain, protons pumped into mitochondria by ATP synthase go back into internal membrane space, creating a free energy difference that can be harnessed to synthesize ATP with ADP + Pi.

The Chemiosmotic production of ATP during electron transport is achieved by chemiosmotic coupling. During the first half-reaction, protons are pumped into the intermembrane space across the inner membrane. During this process, ATP synthase uses some of the free energy to synthesize ATP. In the second half reaction, the ATP synthase enzyme uses the remaining free energy to drive protons back out of the mitochondrial matrix into intermembrane space in a coupled process that translocates protons and synthesizes ATP from ADP + Pi. The protons in the intermembrane space are free to move in either direction during the chemiosmotic process. However, under normal conditions, protons will flow from the matrix through ATP synthase into the intermembrane space. Protons moving in this direction cause rotation of a portion of the ATP synthase, causing phosphorylation of ADP and Pi, creating ATP. Following this phosphorylation, more protons are needed to produce more ATP so they continue to move through the enzyme, using some of the free energy to drive them back into the matrix (Kasumov, Kasumov et al., 2019).

References

Ryan, D. G., Murphy, M. P., Frezza, C., Prag, H. A., Chouchani, E. T., O’Neill, L. A., & Mills, E. L. (2019). Coupling Krebs cycle metabolites to signalling in immunity and cancer. Nature metabolism, 1(1), 16-33.

Gasmi, A., Peana, M., Arshad, M., Butnariu, M., Menzel, A., & Bjørklund, G. (2021). Krebs cycle: activators, inhibitors and their roles in the modulation of carcinogenesis. Archives of Toxicology, 95(4), 1161-1178.

Kasumov, E., Kasumov, R., & Kasumova, I. (2019). The role of alternative oxidase according mechano-chemiosmotic model of coupling electron transport to ATP synthesis. Photosynthesis and Hydrogen Energy Research for Sustainability, 128.