7.6. Cori cycle

Cori cycle (glucose-lactate cycle) has opened a Czech scientist and Nobel Prize winner Theresa Cori.

During intense muscular work and in the absence or insufficient number of mitochondria (e.g., in erythrocytes) glucose undergoes anaerobic glycolysis with lactate formation. When there is the accumulation of lactate in the muscles lactic-acidosis occurs. Sensory nerve endings are irritated, causing pain in the muscles.

Lactate is transferred by blood to the liver and is converted to pyruvate and then into glucose. Glucose synthesis is gluconeogenesis. Then glucose with the blood flow can return to the working muscle. Direction of lactate dehydrogenase reaction in the working muscles and the liver determined by the concentration ratio of reduced and oxidized forms of NAD⁺: ratio of NAD⁺ / NADH in contracting muscle is more than in liver.

7.7. Alcoholic fermentation

Alcoholic fermentation is carried out by yeast organisms, as well as some of the mold:

The reaction mechanism is similar to glycolysis. The divergence starts after the stage of pyruvate formation. During glycolysis pyruvate is reduced to lactate by the enzyme LDH (lactate dehydrogenase) and the coenzyme NADH. During alcoholic fermentation pyruvate undergoes decarboxylation, acetaldehyde is formed as a result, and then during its reduction ethanol is formed:

During lactic acid fermentation pyruvate is not decarboxylated, and, as in glycolysis in animal tissues, it is redused with the participation of LDH due to the hydrogen of NADH.

C6h12o6 → 2 ch3chohcooh

7.8. Pentose phosphate pathway (hexose monophosphate shunt)

Pentose phosphate pathway is an alternative way of glucose oxidation.

Its functions are:

- to supply to cells coenzyme NADPH, used as a hydrogen donor in biosynthetic reactions;

- to provide cells with pentose phosphates for the synthesis of nucleotides and nucleic acids.

Pentose phosphate pathway does not lead to the synthesis of ATP.

Pentose phosphate pathway of pentose formation occurs in the liver, adipose tissue, lactating mammary gland, tests, adrenal cortex, erythrocytes. The enzymes of pentose phosphate pathway are located in the cytosol.

In the pentose phosphate pathway of glucose conversion oxidative and non- oxidative pathways are released.

Oxidative phase includes two reactions of dehydrogenation. Coenzyme of dehydrogenase is NADP⁺. Pentoses are formed by oxidative decarboxylation.

1. Dehydrogenation of glucose-6-phosphate with the participation of glucose-6-phosphate dehydrogenase and coenzyme NADP⁺ gives 6-phosphoglucono--lactone and NADPH:

The 1^st reaction is the most regulatory in the HMP shunt. NADPH is an inhibitor of glucose-6-phosphate dehydrogenase.

2. 6-Phosphogluconolactone is unstable and is hydrolyzed to 6-phosphogluconate under the action of 6-phosphoglucono lactonase:

3. Dehydrogenation and decarboxylation of 6-phosphogluconate to form ribulose-5-phosphate (pentose) and NADPH with the participation of decarboxylating 6-phospho-gluconate dehydrogenase:

4. Xyluloso-5-phosphate (pentose) is formed from ribulose-5-phosphate under the action of epimerase. Under the influence of ketoisomerase ribulose-5-phosphate is converted to ribose-5-phosphate (pentose). Between the forms of pentose phosphates equilibrium is established:

At this stage, the pentose phosphate pathway can be completed. Under anaerobic conditions, there a non-oxidative stage of pentose phosphate cycle comes. Non-oxidative stage includes reactions of transfer of 2 - and 3-carbon fragments from one molecule to another. The process is reversible, and hexoses can be formed from pentoses. Intermediates of glycolysis are formed (fructose-6-phosphate, fructose-1,6-bisphosphate, phosphotrioses), and substances that are specific to the pentose phosphate pathway (sedoheptulose-7-phosphate, pentose-5-phosphate, erythrose-4-phosphate).

Six molecules of glucose-6-phosphate in the pentose phosphate cycle form 6 molecules of ribulose-5-phosphate and 6 molecules of CO₂. CO₂ is formed from C-1 carbon atoms of six molecules of glucose-6-phosphate. Then 5 molecules of glucose-6-phosphate are regenerated by 6 molecules of ribulose-5-phosphate.

The overall equation:

6 Glucose 6-phosphate + 12 NADP⁺ + 7 H₂O 

 5 Glucose 6-phosphate + 12 NADPH + 12H⁺ + 6 CO₂

The intermediates of the cycle (fructose-6-phosphate and glyceraldehyde 3-phosphate) are included in glycolysis.