Cori cycle

The Cori cycle (named after its discoverers, Carl Cori Gerty Cori and ) describes the cycle of glucose and its degradation products between skeletal muscle and liver. The extended description refers to the metabolic pathways of gluconeogenesis, the glutamic acid ( Glu), parts of the citric acid cycle and the urea cycle with.

Of the skeletal muscle is under aerobic conditions is not capable to convert back into glucose lactate: it lacks the enzymes of gluconeogenesis. For this reason, there is a circulation of metabolites between the muscle and the liver - the latter has the corresponding enzyme repertoire. In its original form this organ circulation was called " Cori cycle ". An extended form of the same, the " glucose -alanine cycle " probably of greater importance, since it simultaneously prevents an ammonia poisoning of the muscle by feeding it to the detoxification apparatus of the liver ( urea cycle).

Cori cycle

For muscle activation fast, a certain lack of oxygen in the muscle. Under these conditions, the more anaerobic respiratory chain slows down in the mitochondrion and energy is generated mainly by glycolysis. On the other hand, however, pyruvate is degraded less about the citric acid cycle. Instead, pyruvate reacts anaerobically to lactic acid. This NAD is regenerated for glycolysis. Lactic acid is released as lactate in the bloodstream - this way is shown in abbreviated form in the figure (see pyruvate ). The liver takes up lactate from the blood and converts it back to the way of gluconeogenesis via oxaloacetate into glucose. This glucose can - depending on the current status of the power supply - the energy storage are the liver as glycogen supplied or delivered to the bloodstream to provide the same muscle.

However, it should be noted that it is not dealing with a closed energy cycle. In the gluconeogenesis in the liver, more energy must be applied, as is produced in the glycolysis in muscle. This is because that in the gluconeogenesis the strongly endothermic reaction of pyruvate to phosphoenolpyruvate (PEP) is bypassed energy-consuming, while the energy released during the exothermic reaction of fructose -1 ,6 -BP to fructose -6 -P or glucose-6 -P is not used for glucose. So glucose 4 ATP, 2 GTP, and 2 NADH must be spent in gluconeogenesis from pyruvate for each molecule. In glycolysis, however, arise from one molecule of glucose only 2 ATP and 2 NADH.

Glucose- alanine cycle

Proteins are degraded in the cytosol to amino acids. These in turn are deaminated by transamination and introduced the remaining carbon skeleton in the citric acid cycle. The amino group of the amino acid is transferred to the transamination temporarily the cofactor pyridoxal phosphate (PLP ); PLP is to pyridoxamine phosphate ( PAMP ). The alanine aminotransferase ( ALT, ALT) ( glutamate - pyruvate transaminase, GPT called ) transmits in the muscle, the amino moiety of PAMP on pyruvate. This results in alanine and regenerated PLP, which may take up again new amino groups. Alanine is transported via the blood to the liver where ALAT makes PLP and PAMP alanine and pyruvate, which can be used for gluconeogenesis and is sent as glucose back to the extrahepatic cells.

By ALAT the amino group of PAMP is transferred to α -ketoglutarate. Glutamate is the resultant in the mitochondria of the liver cells by glutamate dehydrogenase ( GLDH ) to α -ketoglutarate, and NH3, and the latter is converted by the carbamoyl phosphate synthetase I with CO2 to carbamoyl flowing in the urea cycle. The second NH2 group of the urea is supplied by a Transaminierungsprodukt of aspartate ( Asp), which is in turn cleaved to arginine and fumarate. From arginine finally urea is cleaved. From fumarate can be regenerated via malate and oxaloacetate to aspartate ( Aspartatzyklus ). Urea is excreted by the kidney.

In contrast to the Cori cycle is not only regenerates when carbohydrate - alanine cycle, but also NH3 transported out of the muscle. This is also energy must be expended in the urea synthesis of the liver, however, to dispose of NH3.

• Biochemical reaction
• Pathway