Rapoport-Luebering cycle

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Structural formula of 2,3-bisphosphoglycerate, the intermediate of the Rapoport-Luebering cycle

The Luebering-Rapoport pathway , also known as Rapoport Luebering shunt , Rapoport Luebering shuttle , Phosphoglyceratzyklus or 2,3-BPG cycle referred to, is in the biochemistry an especially in red blood cells of (erythrocytes) mammals expiring pathway , a sequence of enzymatically controlled chemical reactions . It is a by-route of glycolysis consisting of three partial reactions , which is of central importance for the production of energy and the carbohydrate metabolism of almost all living things. The Rapoport-Luebering cycle is one of the biochemical processes for the breakdown of glucose in the animal organism.

Its main reaction is the formation of the intermediate product 2,3-bisphosphoglycerate (2,3-BPG) from the 1,3-bisphosphoglycerate formed in glycolysis, controlled by the enzyme bisphosphoglycerate mutase . The 2,3-BPG produced in the Rapoport-Luebering cycle acts as an important biochemical effector in the regulation of the binding capacity (affinity) of the blood pigment hemoglobin for the breathing gas oxygen , in particular for its long-term adaptation to oxygen deficiency conditions, and is therefore responsible for the release of oxygen from the erythrocytes into the tissue of importance. It is also involved in the enzymatic control of glycolysis and functions as an energy and phosphate store in the erythrocytes.

The discovery of the Rapoport-Luebering cycle and the importance of the 2,3-BPG for the energy balance of the red blood cells in the 1940s by the biochemist Samuel Mitja Rapoport and his assistant Janet Luebering was of great medical importance because of the understanding of these processes the shelf life of blood products could be increased considerably.

Biochemical aspects

procedure

Schematic representation of the Rapoport-Luebering cycle

The Rapoport-Luebering cycle is a bypass route to glycolysis in the red blood cells of mammals, including humans . Starting from 1,3-bisphosphoglycerate (1,3-BPG) from glycolysis, it leads to the formation of 2,3-bisphosphoglycerate (2,3-BPG). This gives rise to the phosphoglyceric acid compounds 3-phosphoglycerate (3-PG) , which are part of the glycolysis reaction , and 2-phosphoglycerate (2-PG) through its isomerization .

The enzyme bisphosphoglycerate mutase (BPGM) responsible for these reactions is essentially restricted to erythrocytes and erythropoietic tissue and, as a trifunctional enzyme, has three different activities. Depending on the pH value, it functions either as a synthase (2,3-BPG synthase, synonym bisphosphoglycerate mutase; EC number 5.4.2.4) for the conversion of 1,3-BPG to 2,3-BPG or as a phosphatase (2 , 3-bisphosphoglycerate phosphatase; EC number 3.1.3.13) for the conversion of 2,3-BPG to 3-PG. In addition , as a mutase (monophosphoglycerate mutase ; EC number 5.4.2.1) it catalyzes the equilibrium reaction between 3-PG and 2-PG.

The main activity of BPGM is the synthase reaction from 1,3-BPG to 2,3-BPG, which is irreversible . The last step in the Rapoport-Luebering cycle, the conversion of 3-PG to 2-PG, is a partial glycolysis reaction that also takes place in other cells via the enzyme phosphoglycerate mutase . In addition, low activities as 2,3-BPG synthase and phosphatase were found for phosphoglycerate mutase, which is similar to BPGM in terms of its molecular mass , its subunit structure and its amino acid sequence . It thus probably functions as a trifunctional enzyme similar to BPGM, but with a different ratio of the three enzyme activities to one another. In addition to the expression of BPGM in some non-erythropoietic tissues such as the placenta and the liver , this is a possible explanation for the occurrence of low levels of 2,3-BPG in non-erythroid cells. The reverse reactions from 2-PG via 3-PG to 1,3-BPG and thus the partial processes of glycolysis that run parallel to the Rapoport-Luebering cycle take place within the framework of gluconeogenesis .

Balance sheet

The first step of the Rapoport-Luebering cycle, the rearrangement from 1,3-BPG to 2,3-BPG, is an isomerization with a neutral material balance. The bisphosphoglycerate mutase as an enzyme for this reaction, however, requires the presence of magnesium ions . The hydrolytic cleavage of 2,3-BPG to 3-PG in the second step takes place with the consumption of a water molecule and the release of an inorganic phosphate . In the Rapoport-Luebering cycle - in contrast to the conversion of 1,3-BPG to 3-PG by the phosphoglycerate kinase in glycolysis - no adenosine triphosphate (ATP) is produced. The energy yield is therefore lower in the secondary route via the 2,3-BPG than in the direct route in glycolysis.

regulation

The compounds 2,3-BPG and 3-PG that arise in the Rapoport-Luebering cycle inhibit this secondary pathway, which is thus autoregulatory. 2,3-BPG also inhibits some of the enzymes in the glycolysis reaction sequence before the Rapoport-Luebering cycle, such as hexokinase and phosphofructokinase . It also acts as a cofactor for phosphoglycerate mutase in glycolysis. An increase in the amount of 1,3-BPG stimulates the production of 2,3-BPG. All glycolysis processes that lead to an increase in the 1,3-BPG concentration via activation or inhibition of enzymes thus accelerate the formation of 2,3-BPG.

An increase in the pH value also leads to an increase in 2,3-BPG, since the optimal pH value for the synthase activity of BPGM is around 7.2, while the phosphatase activity is optimal in the acidic range and then opposite the 2,3-BPG formation predominates. The hormones thyroxine , somatotropin , testosterone and erythropoietin also stimulate the formation of 2,3-BPG. By contrast , chloride , phosphate and, above all, the physiological phosphatase activator 2-phosphoglycolate lead to an increased cleavage of 2,3-BPG to 3-PG by the phosphatase function of BPGM.

meaning

Physiological function

Crystal structure of hemoglobin, the oxygen affinity of which is regulated by the 2,3-BPG formed in the Rapoport-Luebering cycle

Since the red blood cells of mammals, unlike most other body cells, do not have a nucleus or mitochondria , they have a specialized carbohydrate and energy metabolism without a citric acid cycle or respiratory chain . Glycolysis is the only way to generate energy in the erythrocytes besides the pentose phosphate pathway . About 20 percent of the 1,3-BPG produced in the erythrocytes in the glycolysis is converted via the Rapoport-Luebering cycle, the proportion of the 2,3-BPG formed is about 50 percent of all intermediates of the glycolysis in the erythrocytes and about two Third of the total erythrocyte phosphates. Under physiological conditions, 2,3-BPG is present in the red blood cells in about the same molar concentration as the blood pigment hemoglobin and around four times the ATP concentration. The amount of 2,3-BPG is determined by the ratio between the synthase and phosphatase activity of the BPGM.

Influence of the pH value, the 2,3-BPG concentration and the temperature on the oxygen binding curve of hemoglobin
green: left shift
red: right shift

The 2,3-BPG produced in the Rapoport-Luebering cycle acts primarily as an allosteric inhibitor of hemoglobin by stabilizing its non- oxygenated deoxy form, and thus regulating the binding capacity (affinity.) In inverse proportion to its concentration in the red blood cells ) of hemoglobin for oxygen. 2,3-BPG binds between the two beta subunits of hemoglobin in the pocket, which is formed in the unloaded state, also known as the T shape. The biophysical basis of the binding are interactions between the negatively charged groups of 2,3-BPG and positively charged amino acid residues in the binding pocket. An increase in the 2,3-BPG concentration shifts the oxygen binding curve of the hemoglobin to the right, which means that the bound oxygen is more easily released. Conversely, a decrease in the 2,3-BPG concentration leads to a left shift of the oxygen binding curve and thus a stronger binding of the oxygen to the hemoglobin.

Other factors that lead to an increase in the oxygen affinity of hemoglobin and in part also influence the 2,3-BPG level are a decrease in temperature , an increase in the pH value and a decrease in the carbon dioxide concentration . The combined effect of the pH value and the carbon dioxide partial pressure on the ability of hemoglobin to bind oxygen is also referred to as the Bohr effect and is the physicochemical basis for regulating gas exchange in the lungs and supplying metabolically active tissues with oxygen. Carbon monoxide , on the other hand, reduces the binding capacity of hemoglobin for oxygen, since it competes with oxygen for the same binding site in the hemoglobin molecule. An increase in the amount of 2,3-BPG improves the oxygen release in the body periphery and thus the oxygen supply in the tissues, especially under unfavorable conditions such as conditions that are associated with a lack of oxygen. For example, staying at higher altitudes leads to an increase in the 2,3-BPG concentration, which returns to normal values ​​about two days after returning to the starting altitude. Short-term or long-term physical exertion and endurance training also have an impact on the concentration of 2,3-BPG to varying degrees.

In addition to this function as a compensation mechanism, the Rapoport-Luebering cycle probably also plays a role in regulating the substance and energy balance of glycolysis. It enables an increased formation of the coenzyme nicotinamide adenine dinucleotide (NADH) in the glycolysis without a subsequent increase in the ATP concentration and the process of the glycolysis even with a low ATP requirement. In addition, 2,3-BPG represents an erythrocyte energy and phosphate store.

Medical relevance

Packed red blood cells in CPD / SAGM solution ( C Itrat, P calcium phosphate Product, D extrose / S alzlösung, A denin, G Lukose, M annitol)

Enzyme defects in those glycolysis reactions that take place after the formation of 2,3-BPG, due to an increase in its concentration, cause a decrease in the oxygen affinity of hemoglobin and thus an increased release of oxygen in the tissue . Conversely, defects in the reactions of glycolysis before the Rapoport-Luebering cycle lead to a decrease in the 2,3-BPG concentration and thus to a reduction in the release of oxygen in the tissue.

A targeted regulation of bisphosphoglycerate mutase to influence the 2,3-BPG concentration in the erythrocytes would, for example, be of therapeutic interest for the treatment of ischemia and sickle cell anemia . A reduction in BPGM activity due to glycation has been described in diabetic patients . A congenital deficiency of BPGM has only been documented in a few cases. Apart from secondary erythrocytosis (increased formation of red blood cells), the affected persons were largely symptom-free. A laboratory medical determination of the 2,3-BPG in erythrocytes and in serum is possible, however, due to the low diagnostic value, it is not common and is only of interest for special questions.

The 2,3-BPG in the erythrocytes, like the ATP, has an influence on the storability of blood products . Due to the increase in the lactate concentration as the storage period progresses, the pH value of the withdrawn blood shifts to the acidic range, whereby the 2,3-BPG is broken down more and its formation is inhibited. By adding additives such as dextrose and adenine , such as those contained in the currently used CPDA or CPD / SAGM blood bags, the decrease in the 2,3-BPG content can be delayed and thus the shelf life and function of stored blood improve.

Veterinary physiological aspects

The concentration of 2,3-BPG in erythrocytes and the extent of its effect on hemoglobin differs between different mammals. The hemoglobins of humans , horses , dogs , pigs , rabbits , guinea pigs , mice and rats , whose erythrocytes have a high 2,3-BPG concentration, react correspondingly strongly. In contrast, the effect of 2,3-BPG on hemoglobin, like the 2,3-BPG content in the red blood cells of sheep , goats and cattle , deer , antelopes and giraffes, as well as hyenas and cats, is less.

In birds , 2,3-BPG functions as a regulator of the oxygen affinity of hemoglobin only during embryonic development . A few days after hatching from the egg , it is then completely broken down and inositol phosphates such as inositol hexaphosphate (IHP) take over the function of 2,3-BPG in the rest of life . 2,3-BPG was only found in a few species in fish ; the dominant organophosphates in fish erythrocytes are ATP and guanosine triphosphate (GTP). The organophosphates in the erythrocytes of reptiles are mainly ATP, IHP and myo-inositol-5-phosphate (IP5).

The reason for the differences between mammals and other vertebrates is the particular energy metabolism of mammals' erythrocytes. In the nucleated erythrocytes of other vertebrates, the respiratory chain is the primary energy-supplying metabolic pathway and not, as in the red blood cells of mammals, glycolysis.

Discovery story

The 1950 article by Rapoport and Luebering in the Journal of Biological Chemistry on the formation of 2,3-BPG

2,3-BPG, the reaction product of the Rapoport-Luebering cycle, was first described and isolated in 1925, the starting substance 1,3-BPG by Erwin Negelein in 1939. The Austrian- born biochemist Samuel Mitja Rapoport and his technical assistant at the time, Janet Luebering discovered the reactions essential for 2,3-BPG formation in the USA in the 1940s and described them in several joint publications in the early 1950s. Research into this metabolic pathway led to the development of the ACD medium, containing citrate and dextrose , with which the shelf life of blood products could be increased from one to around three weeks. Because of the importance of this discovery for military medicine during the Second World War , Samuel Mitja Rapoport was honored with the "President's Certificate of Merit" by US President Harry S. Truman .

Because of his political convictions, Samuel Mitja Rapoport, who received a one-year scholarship at the Children's Hospital of the University of Cincinnati in 1937 and who had not returned to Europe after the German annexation of Austria because of his Jewish descent, went to the German Democratic Republic (GDR) in 1952 . Here he became one of the leading biochemists in the country and continued his research on the metabolism of red blood cells. Together with his wife Ingeborg Rapoport , who works as a pediatrician, and his son Tom Rapoport , who moved to Harvard University in 1995, he published articles on the pH dependence of 2,3-BPG formation and the regulation of glycolysis in the 1970s the erythrocytes.

The properties of bisphosphoglycerate mutase as the central enzyme of the Rapoport-Luebering cycle and its trifunctional activity were characterized in more detail in the 1960s and 1970s. In 1967 the effect of 2,3-BPG on hemoglobin was investigated, in 1978 the congenital occurrence of a complete BPGM deficiency in a patient was described. Ten years later, the isolation and characterization was carried out of on the human chromosome 7 lying gene for the enzyme. The molecular basis of the function of BPGM was examined in more detail in the 1990s, and in 2004 the crystal structure of the enzyme molecule was clarified. Four years later, it was described that the enzyme multiple inositol polyphosphate phosphatase (MIPP), which occurs in various tissues, also has activity as 2,3-BPG phosphatase. This discovery is important for the regulation of the release of oxygen from the hemoglobin and thus for the physiological role of the Rapoport-Luebering cycle.

Individual evidence

  1. ^ A b c d R. van Wijk, WW van Solinge: The energy-less red blood cell is lost: erythrocyte enzyme abnormalities of glycolysis. In: Blood . 106 (13 )/2005. American Society of Hematology, pp. 4034-4042.
  2. ↑ a b T. Fujita et al .: Human Erythrocyte Bisphosphoglycerate Mutase: Inactivation by Glycation In Vivo and In Vitro. In: Journal of Biochemistry . 124 (6 )/1998. Japanese Biochemical Society, pp. 1237-1244.
  3. ↑ a b c d e Y. Wang et al .: Crystal Structure of Human Bisphosphoglycerate Mutase. In: Journal of Biological Chemistry . 279/2004. The American Society for Biochemistry and Molecular Biology, pp. 39132-39138.
  4. ↑ a b R. Sasaki, K. Ikura, E. Sugimoto, H. Chiba: Purification of bisphosphoglycerate mutase, bisphosphoglycerate phosphatase and phosphoglycerate mutase from human erythrocytes: three enzyme activities in one protein. In: European Journal of Biochemistry . 50 (3) / 1975. Federation of European Biochemical Societies, pp. 581-593.
  5. ↑ a b V. Joulin et al .: Isolation and characterization of the human 2,3-bisphosphoglycerate mutase gene. In: Journal of Biological Chemistry . 263/1988. The American Society for Biochemistry and Molecular Biology, pp. 15785-15790.
  6. ↑ Gerhard Michal : Biochemical Pathways : Biochemistry Atlas. Spektrum Akademischer Verlag, Heidelberg 1999, ISBN 3-86-025239-9 , pp. 27/28.
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  8. ↑ H. Chiba, R. Sasaki: Functions of 2,3-bisphosphoglycerate and its metabolism. In: Current Topics in Cellular Regulation. 14/1978. Academic Press, pp. 75-116.
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  10. ^ Gerhard Thews , Ernst Mutschler , Peter Vaupel : Anatomie, Physiologie und Pathophysiologie des Menschen. Wissenschaftliche Verlagsgesellschaft, Stuttgart 1999, ISBN 3-8047-1616-4 , p. 117.
  11. ↑ John P. Greer, Maxwell Myer Wintrobe: Wintrobe's Clinical Hematology. Lippincott Williams & Wilkins, Philadelphia 2009, ISBN 0-78-176507-2 , p. 143.
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  14. ↑ a b P. Ravel, CT Craescu, N. Arous, J. Rosa, MC Gare: Critical Role of Human Bisphosphoglycerate Mutase Cys 22 in the Phosphatase Activator-binding Site. In: Journal of Biological Chemistry . 272/1997. The American Society for Biochemistry and Molecular Biology, pp. 14045-14050.
  15. ^ Bisphosphoglycerate Mutase Deficiency of Erythrocyte.  In: Online Mendelian Inheritance in Man . (English), OMIM entry on BPGM deficiency (English).
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  17. ^ Diphosphoglycerate Pathway. In: Jiro J. Kaneko, John W. Harvey, Michael Bruss: Clinical Biochemistry of domestic Animals. Fifth edition. Academic Press, San Diego 1997, ISBN 0-12-396305-2 , pp. 178-180.
  18. ^ RE Isaacks, LL Lai, PH Goldman, CY Kim: Studies on avian erythrocyte metabolism. XVI. Accumulation of 2,3-bisphosphoglycerate with shifts in oxygen affinity of chicken erythrocytes. In: Archives of Biochemistry and Biophysics . 257 (1) / 1987. Academic Press, pp. 177-185.
  19. ^ A b Organic Phosphate Effects on Oxygen Affinity. In: Stephen C. Wood, Claude Lenfant: Evolution of Respiratory Processes. A Comparative Approach. Informa Health Care, 1979, ISBN 0-82-476793-4 , pp. 212-214.
  20. ↑ R. Juel: 2,3-Diphosphoglycerate: its role in health and disease. In: CRC Critical Reviews in Clinical Laboratory Sciences. 10 (2) / 1979. CRC Press, pp. 113-146.
  21. ↑ Erwin Negelein, Heinz Brömel: R-Diphosphoglyceric acid, its isolation and properties. In: Biochemical Journal . 303/1939. Springer, pp. 132-144.
  22. ^ S. Rapoport, J. Luebering: The formation of 2,3-diphosphoglycerate in rabbit erythrocytes: The existence of a diphosphoglycerate mutase. In: Journal of Biological Chemistry . 183/1950. Pp. 507-516.
  23. ↑ S. Rapoport, J. Luebering: Glycerate-2,3-diphosphatase. In: Journal of Biological Chemistry . 189/1951. Pp. 683-694.
  24. ^ A. Tuffs: Samuel Mitja Rapoport. Obituary in: British Medical Journal . 329/2004. BMJ Group, p. 353.
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  26. ↑ Reinhold Benesch, Ruth Benesch: The effect of organic phosphates from the human erythrocyte on the allosteric properties of hemoglobin. In: Biochemical and Biophysical Research Communications . 26 (2) / 1967. Academic Press, pp. 162-167.
  27. ↑ R. Rosa, M.-O. Prthu, Y. Beuzard, J. Rosa: The first case of a complete deficiency of diphosphoglycerate mutase in human erythrocytes. In: Journal of Clinical Investigation . 62/1978. American Society for Clinical Investigation, pp. 907-915.
  28. ↑ MC Garel, V. Lemarchandel, MC Calvin, N. Arous, CT Craescu, MO Prehu, J. Rosa, R. Rosa: Amino acid residues involved in the catalytic site of human erythrocyte bisphosphoglycerate mutase. Functional consequences of substitutions of His10, His187 and Arg89. In: European Journal of Biochemistry . 213 (1 )/1993. Federation of European Biochemical Societies, pp. 493-500.
  29. ↑ J. Cho, JS King, X. Qian, AJ Harwood, SB Shears: Dephosphorylation of 2,3-bisphosphoglycerate by MIPP expands the regulatory capacity of the Rapoport-Luebering glycolytic shunt. In: Proceedings of the National Academy of Sciences . 105 (16) / 2008. United States National Academy of Sciences, pp. 5998-6003.

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This article was added to the list of excellent articles on December 5, 2008 in this version .