Which coenzyme is produced in cellular respiration

In a fire, the electrons go up in a smoke of CO 2 and water vapor Fig. In living things, the energy is captured in processes that are largely optimized to minimize heat production. With each transfer to an acceptor of higher electron affinity, the energy of the electrons steps down Fig. Mitochondrial inner membranes and plasma membranes of aerobic bacteria are the epicenters of cellular respiration.

For coenzyme Q to be ready to accept more electrons from reduced cofactors, electrons must flow to protein complex III. Similarly, complex III is ready to accept electrons on the basis of electron flow to complex IV, where four electrons and two protons are transferred from cytochrome C a heme protein to oxygen O 2 , generating water H 2 O.

Thus, without O 2 as the electron acceptor, mitochondrial fuel oxidation would have to be linked to an electron efflux system. Each of the membrane protein complexes consists of many polypeptides with specifically located metal-containing redox-active centers that are intermediates in the chain Fig. See also: Bacteria ; Cell membrane ; Cytochrome ; Protein. The overall process of cellular respiration can be likened to water flowing down a river that drives a turbine. Whereas building and maintaining the turbine are energy-dependent processes, the flow of water works with gravity so long as there is water upstream.

Similarly, although producing and maintaining the mitochondrial enzymes, cell membranes, and cofactors are energy-dependent processes, fuel oxidation and respiratory electron flow are exothermic that is, they liberate heat. Electrons flow in cellular respiration precisely as they flow in other electrical circuits, toward acceptors of higher electron affinity. Just as the cost of turning a water turbine is paid for by water flowing downriver, the cost of pumping protons is paid for by electrons flowing from higher-energy states to lower-energy states.

See also: Proton. Small stepwise increases in electron affinity are manifested by small drops in electron free energy along the respiratory electron chain. Damage produced by reactive oxygen species ROS is an obvious cost of aerobic metabolism, and ROS in the form of hydrogen peroxide H 2 O 2 and phospholipid hydroperoxides are controlled by glutathione reductases and glutathione peroxidases, which depend on NADPH as the reducing agent to reactivate oxidized glutathione.

Protons return through NNT in order to drive this catalytic process in a manner that is directly competitive with production of ATP and heat Fig. See also: Free energy ; Free radical ; Hydrogen peroxide ; Superoxide chemistry.

Respiratory demands vary by type of fuel, by the balance between catabolism and anabolism in which a cell is engaged, and by the degree to which the cell produces cytosolic NADPH anaerobically through processes such as the pentose phosphate pathway in which glucose is metabolized or transformed into NADPH.

See also: Citric acid cycle. In contrast to glucose oxidation, the complete oxidation of triglycerides neutral lipids consisting of three fatty acyl chains esterified to a glycerol backbone is almost entirely aerobic Fig. The ratio of fatty-acid carbons to glycerol carbons in a triglyceride provides an indication of how aerobically demanding triglyceride oxidation is.

Considering that the cytosolic NADH can be effectively reoxidized aerobically via the malate-aspartate shuttle or the glycerolphosphate shuttle and that the glycerol-derived pyruvate can also be oxidized in mitochondria, complete oxidation of a typical triglyceride can demand sufficient oxygen to reoxidize approximately mitochondrial NADH and FADH 2 equivalents.

See also: Lipid ; Lipid metabolism ; Triglyceride triacylglycerol. It should also be pointed out that amino acid oxidation is intermediate in its O 2 requirement between glycolysis and mitochondrial fatty-acid oxidation because some reduced cofactors are produced in the cytosol and others are produced in the mitochondria.

There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found.

One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle. This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver.

This form produces GTP. In particular, protein synthesis primarily uses GTP. Step 6. Step six is a dehydration process that converts succinate into fumarate. Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly. This process is made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion.

Step 7. Water is added to fumarate during step seven, and malate is produced. The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate. Another molecule of NADH is produced. Two carbon atoms come into the citric acid cycle from each acetyl group, representing four out of the six carbons of one glucose molecule. Two carbon dioxide molecules are released on each turn of the cycle; however, these do not necessarily contain the most recently-added carbon atoms.

The two acetyl carbon atoms will eventually be released on later turns of the cycle; thus, all six carbon atoms from the original glucose molecule are eventually incorporated into carbon dioxide. These carriers will connect with the last portion of aerobic respiration to produce ATP molecules.

Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is amphibolic both catabolic and anabolic. In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes to become acetyl Coenzyme A acetyl CoA. Acetyl CoA is a molecule that is further converted to oxaloacetate, which enters the citric acid cycle Krebs cycle.

The conversion of pyruvate to acetyl CoA is a three-step process. Breakdown of Pyruvate : Each pyruvate molecule loses a carboxylic group in the form of carbon dioxide. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. Note: carbon dioxide is one carbon attached to two oxygen atoms and is one of the major end products of cellular respiration. The result of this step is a two-carbon hydroxyethyl group bound to the enzyme pyruvate dehydrogenase; the lost carbon dioxide is the first of the six carbons from the original glucose molecule to be removed.

This step proceeds twice for every molecule of glucose metabolized remember: there are two pyruvate molecules produced at the end of glycolysis ; thus, two of the six carbons will have been removed at the end of both of these steps. Step 3. Note that the citric acid cycle produces very little ATP directly and does not directly consume oxygen.

The citric acid cycle : In the citric acid cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle.

Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants. The first step is a condensation step, combining the two-carbon acetyl group from acetyl CoA with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group -SH and diffuses away to eventually combine with another acetyl group. This step is irreversible because it is highly exergonic.

The rate of this reaction is controlled by negative feedback and the amount of ATP available. If ATP levels increase, the rate of this reaction decreases.

If ATP is in short supply, the rate increases. Citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate. Steps 3 and 4. CoA binds the succinyl group to form succinyl CoA. Step 5. A phosphate group is substituted for coenzyme A, and a high- energy bond is formed.

This energy is used in substrate-level phosphorylation during the conversion of the succinyl group to succinate to form either guanine triphosphate GTP or ATP. There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found. One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle. This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver.

This form produces GTP. In particular, protein synthesis primarily uses GTP. Step 6. Step six is a dehydration process that converts succinate into fumarate. Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly.

This process is made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion.