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The citric acid cycle is the second step in cellular metabolism, following glycolysis. The cycle occurs in the cytosol. The goal is to produce ATP which can be used in biosynthetic reactions. The cycle produces two molecules of carbon dioxide (waste products), three molecules of NADH, one molecule of GTP, and one molecule of FADH2.
The first step of the cycle entails the enzyme citrate synthase removing a proton from the CH3 on acetyl CoA. This creates a CH2 with a negative charge. The CH2 bonds to the carbonyl carbon of the oxaloacetate. The reaction is driven forward by the hydrolysis of coenzyme A (CoA).
The enzyme aconitase facilitates an isomerization reaction which involves the hydrolysis and then condensation of a water molecule. This process changes the location of the hydroxyl group from 3’ to 4’.
Step three is the first of four oxidation steps. The carbon 4’, which is hosting the hydroxyl group, is converted into a carbonyl group by the enzyme isocitrate dehydrogenase. One molecule of NADH is created in this step.
Step four creates another NADH plus one waster carbon dioxide. A-ketoglutarate complex allows for an oxidation reaction which produced the above mentioned products. It also facilitates a high-energy thioester bond between the terminal sulfate and the CoA, creating succinyl-CoA.
In step five, one molecule of GTP is created. Succinyl-CoA synthetase allows for an extraneous phosophate molecule to be added to GDP. Succinate is also created.
Succinate dehydrogenase allows for the removal of two hydrogen atoms from the previously mentioned product succinate. In such a step, FADH2 is created, and the new product is fumarate.
Step seven is not an oxidation step. Water is added through the help of fumarase, and fumarate is changed into malate.
The last step, which is once again an oxidation step, malate dehydrogenase carbon 2’ is converted into a carbonyl group. This creates another molecule of NADH and produces oxaloacetate, which is needed to restart the citric acid cycle.
In anaerobic organisms, the citric acid cycle does not occur, instead fermentation occurs. The citric acid cycle is also known as tricarboxylic acid cycle or the Krebs cycle. The citric acid cycle does not directly utilize oxygen, however it is still necessary in order for it to proceed. The reason for this is that in order for NADH to be reduced back into NAD+, oxygen must be present. If NAD+ is not regenerated, the cycle can’t proceed, thus fermentation evolved.
The oxygen in CO2, which is released as a waste product, is not from molecular oxygen. Instead, water which is present in the cell is used when it is added in steps one, two, and seven. On top of the above listed compounds, oxaloacetate and a-ketoglutarate are very important byproducts of the citric acid cycle. Both of these products are necessary for anabolic reactions in the cell.
The citric acid cycle could not act alone, without glycolysis it would not have it’s substrates. Without electron transport it would not have a beneficial purpose for the cell. For this reason it is the second process in a triage of processes composing cellular metabolism.
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