Biochemistry/Tricarboxylic Acid Cycle

Citric acid cycle, also known as Tricarboxylic Acid Cycle(TCA), is a metabolic pathway where oxidation of sugars, fats and proteins proceeds. Components of the cycle locate in mitochondrial matrix, in eukaryote organisms. But it is important to know that eukaryotes includes cytoplasmic isozymes of citric acid cycle enzymes, in cytoplasm. For prokaryotes, citric acid elements have to present at the cytoplasm.



Pyruvate, obtained by glycolysis and other metabolic functions, cannot enter directly to citric acid cycle. Pyruvate is converted to acetyl- CoA via the help of pyruvate dehydrogenase complex in an irreversible way. At the end of this reaction pyruvate at the cytosol degraded and newly formed acetyl CoA is released into mitochondria. Then acetyl CoA is able to encounter with citric acid enzymes, because there is no barrier to prevent this event.

The first reaction of citric acid cycle is citrate synthesis with the condensation of acetyl CoA and oxaloacetate. First step has highly negative delta G value which is the one of the sign shows us this step is irreversible.

The second reaction uses citrate as a substrate and produce iso- citrate. However, the enzyme catalysing the reaction carries out its job in two steps. In the beginning citrate is used and cis- aconitate is formed. As the further step cis- acotinase used in order to produce isocitrate. This explains where the name of enzyme originates. Here, delta G is positive however reaction continues to go in the direction of products, how? The answer is based on Le Chatelier' s principle. According to Le Chatelier' s principle, the reaction tends to go to the side which is lowering in amount. In this step, the depletion of iso- citrate via the action of isocitrate dehydrogenase, which is the is processor of the next step, leads reaction to go to the side of products.

At the 3rd step, isocitrate is converted to alpha-ketoglutarate by means of isocitrate dehydrogenase. The reaction has negative delta G thus it is irreversible.

The fourth step contains the oxidation of alpha-ketoglutarate to succinyl-CoA via the help of the enzyme, named as alpha-ketoglutarate dehydrogenase. The 4th is known as last irreversible step of Krebs cycle.

In the 5th reaction, passage from succinyl-CoA to succinate is seen. This step is reversible however proceeds to product's side because of Le Chatelier' s principle.

The 6th step is catalyzed by an enzyme which is both the member of TCA cycle and Electron Transport Chain (ETC). The enzyme has two name as Complex II and succinate dehydrogenase. It uses succinate and releases fumarase.

In seventh step, fumarate to malate conversion is observed. The reaction is reversible. However, the enzyme found in this step is highly stereospecific and only uses L-malate and  trans- maleate(fumarate) as substrate.

The 8th step is the last step where oxidation of malate to oxaloacetate occurs.

Regulation
Regulation of Krebs cycle achieved by regulating enzymes of irreversible steps of the cycle. But also via regulation pyruvate dehydrogenase can be controlled because this enzyme provides main substrate, acetyl CoA, for the cycle. For regulation of pyruvate dehydrogenase, please investigate Biochemistry/Conversion of pyruvate to acetyl CoA page.



Dual nature of Krebs cycle
Krebs cycle functions as both anabolic and catabolic pathway. This event makes it an amphibolic pathway. If they are used as precursors for other compounds, how does the cycle proceed? The answer is anaplerotic reactions, which are responsible for the replenishment of pathway intermediates and utilizes NADP+ for this mission.