Embden-Meyerhof glycolytic pathway

Mature red blood cells do not have nuclei, mitochondria, or microsomes therefore red blood cell function is supported through the most primitive and universal pathway. Glucose, the main metabolic substrate of red blood cells, is metabolized via two major pathways the Embden-Meyerhof glycolytic pathway and the hex-ose monophosphate pathway (Fig. 1). Under normal circumstances, about 90% of the glucose entering the red blood cell is metabolized by the glycolytic pathway and 10% by the hexose monophosphate pathway. 

From studies of eubacterial and eukaryotic metabolism, it has been previously argued that the Embden-Meyerhof glycolytic pathway is the ancient energy-conserving route of hexose catabolism [3]. However, a key enzyme of this catabolic route. 

The fermentation of glucose (reaction 1) proceeds by the Embden-Meyerhof glycolytic pathway. The electrons generated reduce COj formed in the fermentation with the formation of acetate (reaction 2), which occurs by the acetyl-CoA pathway described below. The result is the formation of three mol of acetate out of one mol of glucose (reaction 3). 

Again, in principle, alditols might be oxidized to the corresponding ketoses by NAD - or NADPe-linked dehydrogenases, or to the aldoses by NADP -linked dehydrogenases. Alternatively, the aldoses might be phosphorylated. Aldoses and ketoses may be phosphorylated, and alditol phosphates oxidized to aldose or ketose phosphates. These compounds are likely to be catabolized by the reactions of the Embden-Meyerhof glycolytic pathway or of the pentose cycle. 

Fig. 1. Pathways of glucose metabolism in eubacteria and eukaryotes. The three major catabolic pathways are the Embden-Meyerhof glycolytic sequence (solid lines), the Entner-Doudoroff pathway (heavy solid lines) and the pentose phosphate pathway (dashed lines). The sequence from glyceraldehyde 3-phosphate to pyruvate is common to all three pathways.

Fig. 17.2 The Embden-Meyerhof-Pamas pathway (glycolytic pathway).

Metabolic Functions. The formation of phosphate esters is the essential initial process in carbohydrate metaboHsm (see Carbohydrates). The glycolytic, ie, anaerobic or Embden-Meyerhof pathway comprises a series of nine such esters. The phosphogluconate pathway, starting with glucose, comprises a succession of 12 phosphate esters. 

All overview of the glycolytic pathway is presented in Figure 19.1. Most of the details of this pathway (the first metabolic pathway to be elucidated) were worked out in the first half of the 20th century by the German biochemists Otto Warburg, G. Embden, and O. Meyerhof. In fact, the sequence of reactions in Figure 19.1 is often referred to as the Embden-Meyerhof pathway. 

The oxidative pathway for the metabolism of D-glucose 6-phosphate (XLV), distinctive from the glycolytic, Embden-Meyerhof route (see p. 200) and known as the hexose monophosphate shunt, was suggested by certain experiments of Warburg,200 Gerischer,207 Lipmann,208 and Dickens209... 

It is difficult to realize that there were still unknown reactions in the glycolytic (Embden-Meyerhof) pathway until World War II. One of the then standard biochemistry texts (Thorpe) summarized the position in the 1938 edition as ... 

The glycolytic pathway was the first major metabolic sequence to be elucidated. Much of the definitive work was done in the 1930s by the German biochemists, Gustav Embden, Otto Meyerhof, and Otto Warburg. Because of their contributions the alternative name, Embden-Meyerhof pathway, is sometimes used for the glycolytic pathway. 

Embden and Meyerhof demonstrated the intermediates in the glycolytic pathway. 

But of all of nature s pathways, we sing the praise today Of Pamas, Embden, Meyerhof—the glycolytic way. 

Whilst the majority of investigations into halophilic hexose metabolism has been concerned with the catabolism of glucose, it has been recently reported [104,105] that Haloarcula vallismortis catabolises fructose via a modified Embden-Meyerhof pathway. Fructose is phosphorylated to fructose 1-phosphate via a ketokinase, and is then converted to fructose 1,6-bisphosphate via 1-phosphofructokinase. Aldol cleavage generates dihydroxyacetone-phosphate and glyceraldehyde 3-phosphate, both of which can be further metabolised via the glycolytic sequence described earlier. It remains to be established whether other halophilic archaebacteria can also catabolise fructose in this manner. 

Methanogens appear to interconvert glucose and pyruvate via the normal glycolytic pathway shown in Fig. 1. However, most methanogens are autotrophs and carbon is fixed into acetyl-CoA thus, much of the evidence for an Embden-Meyerhof route has been gained in the direction of carbohydrate synthesis and then mainly from the one organism, Methanobacterium thermoautotrophicum. 

The first metabolic pathway elucidated was the glycolytic pathway during the first half of the 20 century by Embden and Meyerhof. Otto Warburg, Cori and Parnas also made very important contributions relating to glycolytic pathway. Krebs established the citric acid and urea cycles during 1930-40. In 1940, Lipmarm described the central role of ATP in biological systems. 

All species of yeast can catabolize D-glucose, and the reaction sequences responsible for this metabolism are also those of the breakdown of other sugars. So, the breakdown of D-glucose will be considered first. Probably, most yeasts break D-glucose down to pyruvate by the glycolytic (or Embden-Meyerhof) pathway (see... 

Figure 21 -8 Major glycolytic pathways of the erythrocyte. Substrates are in uppercase type, and enzymes are in parentheses. EMP, The Embden-Meyerhof pathway HMP hexose monophosphate pathway or pentose shunt RLC, the Rapoport-Luebering cycle ADP, adenosine diphosphate ATP, adenosine triphosphate NAD, nicotinamide-adenine dinudeotide NADH, reduced nicotinamide-adenine dinucleotide NADP, nicotinamide-adenine dinucleotide phosphate NADPH, reduced nicotinamide-adenine dinucleotide phosphate.The step from ribulose-5-phosphate, which is shown as being catalyzed by transketolase and transaldolase, is an abbreviation of this portion of the HMR...

The enzyme endowment present at maturation of the RBC cannot be replaced, and only the glycolytic pathway (Embden-Meyerhof pathway) is available for energy production in the RBC. 

Similarly to other rhizosphere symbiotic bacteria, P. putida KT2440 lacks the enzyme 6-phophofructokinase (Pfk) [18] and therefore it exhibits a nonfunctional glycolytic Embden-Meyerhof-Parnas (EMP) pathway (Eigure 8.1). Accordingly,... 

Selig, M., Xavier, K.B., Santos, H., and Schonheit, P. (1997) Q)mparative analysis of Embden-Meyerhof and Entner-Doudoroff glycolytic pathways in hyperthermophilic archaea and the bacterium Ihermotoga. Arch. Microbiol, 167, 217-232. 

---