Krebs’s Cycle, also known as the Tricarboxylic Acid Cycle (TCA Cycle), accounts for more than two thirds of the energy (Adenosine Triphosphate or ATP) generated from fuel oxidation. Oxidation (removal of electrons) of fatty acids, glucose, amino acids, acetate, and ketone bodies all generate Acetyl Coenzyme A (acetyl CoA). Acetyl CoA enters the TCA Cycle inside mitochondrion and in coordination with the demand for ATP (energy). High levels of ATP will cause a negative feedback on the first three enzymes in the pathway, slowing or turning off the cycle. Also, a high amounts of NADH can also cause a negative feedback, while high levels of NAD+ will activate the cycle. This cycle will only be active in the presence of oxygen, when no oxygen is present pyruvate will instead be converted to lactate which only provides two ATP per pyruvate.
Pyruvate Dehydrogenase Complex
Before the TCA cycle can begin, pyruvate (a 3-carbon molecule derived from glycolysis) must be converted to acetyl CoA. During the conversion CO2 and NADH is released to form Acetyl CoA and is now able to be used in the TCA cycle. The conversion of pyruvate to acetyl CoA is achieved through 3 enzymes, vitamins and coenzymes known as the pyruvate dehydrogenase complex. Step 1 involves the enzyme Pyruvate Decarboxylase with thiamine pyrophosphate joining pyruvate and releasing one of its 3 carbons in the form of CO2 in order to produce the two carbon Acetyl. In the second step, Acetyl is joined by enzyme Dihydrolipoyl Transacetylase with lipoic acid and Coenzyme A forming Acetyl CoA. In the third and final step, Acetyl CoA is joined with enzyme Dihydrolipoyl Dehydrogenase Riboflavin and Niacin and removing hydrogen in the form of NADH resulting in the Acetyl CoA that will enter the TCA cycle. Even though the steps in the pyruvate dehydrogenase complex are not part of the TCA cycle its important to mention because the complex repeats itself to form Succinyl CoA in the cycle.
The cycle plays a role in the oxidation of acetyl CoA, gluconeogenesis, amino acid anabolism, lipogenesis. In the TCA cycle, the acetyl CoA is broken down into: two CO2 molecules; energy is conserved as three NADH, one FAD(2H) and one GTP. An easy way to remember the steps of the Cycle is “Can I Keep Selling Sex For Money Officer?” (Citrate, Isocitrate, α-Ketoglutarate, Succinyl CoA, Succinate, Fumarate, Malate, Oxaloacetate). As mentioned earlier, this takes place inside the mitochondrion and starts with Acetyl CoA (a substrate of glucose, fatty acids, ketone bodies, amino acids, a acetate).
In the first step of the cycle, the Acetyl portion of Acetyl CoA combines with the four carbon intermediate Oxaloacetate and the enzyme Citrate Synthase. The CoA portion of the Acetyl CoA is removed to form the six carbon molecule Citrate. Citrate is then rearranged by enzyme Aconitase to form Isocitrate (also 6 carbon). Enzyme isocitrate dehydrogenase catalyzes the oxidation of the alcohol group and the subsequent cleavage of the carboxyl group releasing CO2 and forming α-Ketoglutarate (now only 5 Carbons after losing one in the form of CO2). Next, α-Ketoglutarate must be converted into Succinyl CoA in a method similar to the pyruvate dehydrogenase complex.
α-Ketoglutarate Dehydrogenase Complex
The oxidative decarboxylation of α-Ketoglutarate to Succinyl CoA is catalyzed by the α-Ketoglutarate Dehydrogenase Complex. This complex contains the same coenzymes: thiamine (vitamin B1), pyrophosphate, lipoic acid, NAD+,FAD; found in the Pyruvate Dehydrogenase Complex. The process is also the same, thiamine pyrophosphate joins α-Ketoglutarate releasing one of its five carbons in the form of CO2 in order to produce the four carbon Succinyl which is then joineded with lipoic acid and Coenzyme A forming Succinyl CoA. Finally, Succinyl CoA is joined with Riboflavin and Niacin and removing hydrogen in the form of NADH resulting in the four carbon Succinyl CoA that will continue the cycle. The coenzymes can be remembered with the mnemonic “Tender Love and Care For Nancy” (Thiaminepyrophosphate, Lipic Acid, Coenzyme A, FAD, NAD+).
The cycle continues with succinate thiokinase joining Succinyl CoA removing the CoA and releasing energy in the form of GTP by breaking its thioester bond. The high energy phosphate bond of GTP is energetically equivalent to that of ATP and can be used directly for energy requiring reactions such as protein synthesis. We are now left with the four carbon Succinate up to this point the two carbons that were donated by the Acetyl CoA that entered the cycle have now been removed but more energy can still be released in the form of Hydrogen. Along with the two CO2 that has already been released, two NADH, one GTP and one Coenzyme A have also been released up to this point.
The final steps of the sequence that convert Succinate to Oxaloacetate begin with the oxidation of Succinate to Fumarate. If you recall, oxidation is the losing of electrons (“OIL RIG”) and electrons are usually released in the form of Hydrogen. The enzyme that catalyzes this reaction is Succinate Dehydrogenase, dehydrogenase is synonymous with the removal of hydrogen, which forms Malate and releases FADH in the process. In the last reaction, the alcohol group of Malate is oxidized to a keto group through the donation of electrons to NAD+, releasing NADH and forming Oxaloacetate and the cycle is ready to repeat itself.
The result of the TCA cycle is the production of three NADH, one FADH, one GTP and two CO2 from one Acetyl CoA. Each NAD+ accepts a pair of electrons while FAD only accepts 1 electron. The NADH and FADH pass through the electron transport chain gaining approximately 2.5 ATP and 1.5 ATP respectively by donating these electrons to oxygen (O2) which is later reduced to water (H2O).