Citric acid cycle

The citric acid cycle (CAC) – also known as the TCA cycle (tricarboxylic acid cycle) or the Krebs cycle.

The CAC refers to a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. 

The citric acid cycle provides precursors of certain amino acids, as well as the reducing agent NADH, that are used in numerous other reactions. 

The citric acid cycle is the body’s primary catabolic pathway and is essential in breaking down the building blocks of the cell such as carbohydrates, amino acids, and lipids.

The cycle consumes acetate, in the form of acetyl-CoA, and water, reduces NAD+ to NADH, releasing carbon dioxide. 

The NADH generated by the citric acid cycle is fed into the oxidative phosphorylation electron transport pathway. 

The citric acid cycle creates a pathway to oxidation of nutrients to produce usable chemical energy in the form of ATP.

In eukaryotic cells, the citric acid cycle occurs in the matrix of the mitochondrion. 

The citric acid cycle is a key metabolic pathway that connects carbohydrate, fat, and protein metabolism. 

The reactions of the Kreb’s cycle are carried out by eight enzymes that completely oxidize acetate in the form of acetyl-CoA, into two molecules each of carbon dioxide and water. 

Acetate is a two carbon molecule.

By catabolism of sugars, fats, and proteins, the two-carbon organic product acetyl-CoA, a form of acetate,  is produced which enters the citric acid cycle. 

The citric acid cycle reactions also converts three equivalents of nicotinamide adenine dinucleotide (NAD+) into three equivalents of reduced NAD+ (NADH), one equivalent of flavin adenine dinucleotide (FAD) into one equivalent of FADH2, and one equivalent each of guanosine diphosphate (GDP) and inorganic phosphate (Pi) into one equivalent of guanosine triphosphate (GTP). 

The NADH and FADH2 generated by the citric acid cycle are, in turn, used by the oxidative phosphorylation pathway to generate energy-rich ATP.

One of the primary sources of acetyl-CoA is from the breakdown of sugars by glycolysis which yield pyruvate that in turn is decarboxylated by the pyruvate dehydrogenase complex generating acetyl-CoA.

Acetyl-CoA, is the starting point for the citric acid cycle. 

Acetyl-CoA may also be obtained from the oxidation of fatty acids.

The citric acid cycle stars with the transfer of a two-carbon acetyl group from acetyl-CoA to the four-carbon acceptor compound to form a six-carbon compound, citrate.

Most of the electrons made available by the oxidative steps of the cycle are transferred to NAD+, forming NADH. 

For each acetyl group that enters the citric acid cycle, three molecules of NADH are produced. 

For every NADH and FADH2 that are produced in the citric acid cycle, 2.5 and 1.5 ATP molecules are generated in oxidative phosphorylation, respectively.

There are ten basic steps in the citric acid cycle

The cycle is continuously supplied with new carbon in the form of acetyl-CoA.

Mitochondria possess two succinyl-CoA synthetases: one that produces GTP from GDP, and another that produces ATP from ADP.

Products of the first turn of the cycle are one GTP (or ATP), three NADH, one QH2 and two CO2.

The total number of ATP molecules obtained after complete oxidation of one glucose in glycolysis, citric acid cycle, and oxidative phosphorylation is estimated to be between 30 and 38.

The regulation of the citric acid cycle is largely determined by product inhibition and substrate availability. 

Its major eventual substrate of the cycle is ADP which gets converted to ATP. 

A reduced amount of ADP causes accumulation of precursor NADH which in turn can inhibit a number of enzymes. 

NADH, a product of all dehydrogenases in the citric acid cycle inhibits pyruvate dehydrogenase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and also citrate synthase. 

Citrate is used for feedback inhibition, as it inhibits phosphofructokinase, an enzyme involved in glycolysis. 

Calcium is also used as a regulator in the citric acid cycle, as its levels in the mitochondrial matrix can reach up to the tens of micromolar levels during cellular activation.

Calcium activates pyruvate dehydrogenase phosphatase, activating  the pyruvate dehydrogenase complex. 

Calcium also activates isocitrate dehydrogenase and α-ketoglutarate dehydrogenase, increasing  the reaction rate of many of the steps in the cycle, and therefore increases flux throughout the pathway.

Transcriptional regulation by a link between intermediates of the citric acid cycle and the regulation of hypoxia-inducible factors (HIF). 

HIF plays a role in the regulation of oxygen homeostasis,

HIF is a transcription factor that targets angiogenesis, vascular remodeling, glucose utilization, iron transport and apoptosis. 

HIF is synthesized and its hydroxylation of at least one of two critical proline residues mediates their interaction with the von Hippel Lindau ubiquitin ligase complex.

Several catabolic pathways converge on the citric acid cycle. 

Pyruvate molecules produced by glycolysis are actively transported across the inner mitochondrial membrane, and into the matrix where they are oxidized and combined with coenzyme A to form CO2, acetyl-CoA, and NADH, as in the normal cycle.

Pyruvate to be carboxylated by pyruvate carboxylase to form oxaloacetate, which increases  the cycle’s capacity to metabolize acetyl-CoA when the tissue’s energy needs are suddenly increased by activity.

In the citric acid cycle the intermediates of citrate, iso-citrate, alpha-ketoglutarate, succinate, fumarate, malate, and oxaloacetate are regenerated during each turn of the cycle. 

During the course of the cycle, increases or decreases in the amount of oxaloacetate available to combine with acetyl-CoA to form citric acid,  in turn increases or decreases the rate of ATP production by the mitochondrion, and thus the availability of ATP to the cell.

Acetyl-CoA is the only fuel to enter the citric acid cycle. 

With each turn of the cycle one molecule of acetyl-CoA is consumed for every molecule of oxaloacetate present in the mitochondrial matrix, and is never regenerated. 

It is the oxidation of the acetate portion of acetyl-CoA that produces CO2 and water, with the energy of O2 thus released captured in the form of ATP.

In protein catabolism, proteins are broken down by proteases into their constituent amino acids.

De-aminated amino acids may either enter the citric acid cycle as intermediates, having an anaplerotic effect,  on the cycle, or, in the case of leucine, isoleucine, lysine, phenylalanine, tryptophan, and tyrosine, converted into acetyl-CoA which can be burned to CO2 and water, or used to form ketone bodies, which too can only be burned in tissues other than the liver where they are formed, or excreted via the urine or breath.

These latter amino acids are ketogenic amino acids.

Glucogenic amino acids that enter the citric acid cycle as intermediates can only be removed by entering the gluconeogenic pathway via malate which is transported out of the mitochondrion to be converted into cytosolic oxaloacetate and ultimately into glucose. 

Glucogenic amino acids: De-aminated alanine, cysteine, glycine, serine, and threonine are converted to pyruvate and can consequently either enter the citric acid cycle as oxaloacetate or as acetyl-CoA to be disposed of as CO2 and water.

In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. 

In the liver the glycerol can be converted into glucose by way of gluconeogenesis. 

In many tissues, especially heart and skeletal muscle tissue, fatty acids are broken down through beta oxidation, which results in the production of mitochondrial acetyl-CoA, which can be used in the citric acid cycle. 

The total energy gained from the complete breakdown of one molecule of glucose by glycolysis, the formation of 2 acetyl-CoA molecules, their catabolism in the citric acid cycle, and oxidative phosphorylation equals about 30 ATP molecules, in cells. 

The number of ATP molecules derived from the beta oxidation of a 6 carbon segment of a fatty acid chain, and the subsequent oxidation of the resulting 3 molecules of acetyl-CoA is 40.

Acetyl-CoA cannot be transported out of the mitochondria.

To obtain cytosolic acetyl-CoA, citrate is removed from the citric acid cycle and carried across the inner mitochondrial membrane into the cytosol. 

The cytosolic acetyl-CoA is used for fatty acid synthesis and the production of cholesterol. 

Cholesterol can, in turn, be used to synthesize the steroid hormones, bile salts, and vitamin D.

The amino acids, aspartate and glutamine are used, together with carbon and nitrogen atoms from other sources, to form the purines that are used as the bases in DNA and RNA, as well as in ATP, AMP, GTP, NAD, FAD and CoA.

The majority of the carbon atoms in porphyrins come from the citric acid cycle intermediate, succinyl-CoA, and are an important component of the hemoproteins, such as hemoglobin, myoglobin and various cytochromes.

During gluconeogenesis mitochondrial oxaloacetate is reduced to malate which is then transported out of the mitochondrion, to be oxidized back to oxaloacetate in the cytosol. 

Cytosolic oxaloacetate is then decarboxylated to phosphoenolpyruvate by phosphoenolpyruvate carboxykinase: that is the rate limiting step in the conversion of nearly all the gluconeogenic precursors into glucose by the liver and kidney.

The  citric acid cycle is involved in both catabolic and anabolic processes, it is known as an amphibolic pathway.

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