embden mayerhof paranas pathway
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EMBDEN–MEYERHOF–PARNAS PATHWAY
Embden–Meyerhof–Parnas pathway (EMP pathway; Embden-Meyerhof pathway; hexose bis phosphate pathway; glycolysis) A sequence of reactions in which glucose is broken down to pyruvate∗. The EMP pathway occurs in a wide range of organisms, both prokaryotic and eukaryotic, and can operate under both aerobic and anaerobic conditions.
By this pathway one molecule of glucose yields two molecules each of pyruvate and NADH and two (net) of ATP (from SUBSTRATE-LEVEL PHOSPHORYLATION); if glucose 6-phosphate is derived from GLYCOGEN, the net yield of ATP is 3 molecules.
In respiratory modes of metabolism the pyruvate is commonly converted to acetyl-CoA which enters the TCA CYCLE; NADH is oxidized via a respiratory chain. In fermentative modes the fate of pyruvate depends on species and/or conditions, but must always involve regeneration of NAD+ from NADH by reduction of an endogenous substrate. The initial step of the EMP pathway, the formation of glucose 6-phosphate from glucose, is catalysed by hexokinase in e.g. yeasts and some bacteria, while in other bacteria glucose is phosphorylated during its uptake by a PTS.
PTS (phosphoenolpyruvate-dependent phosphotransferase system) A TRANSPORT SYSTEM, found in both Gram-negative and Grampositive bacteria, in which the substrate (e.g. a hexose, disaccharide) is phosphorylated in a reaction which is an essential part of the transport process. The source of energy and phosphate is normally phosphoenolpyruvate (PEP). Certain enzymes can generate PEP from ATP or GTP.
In members of the Enterobacteriaceae, sugars whose uptake is PTS-dependent (‘PTS sugars’) include glucose, fructose, mannose, mannitol, sorbitol, glucosamine, N-acetylglucosamine and N- cetylmannosamine. Lactose is a PTS sugar e.g. in Staphylococcus aureus. Because sugars are often metabolized via their phosphate derivatives – e.g. glucose 6-phosphate in EMP, phosphorylation during uptake is a positive aspect of PTS transport. However, a non-metabolizable substrate may be phosphorylated and transported by a PTS – e.g. the sugar analogue methyl -glucoside can be transported by the glucose- PTS of Escherichia coli and of Clostridium pasteurianum.
Phosphofructokinase is a key enzyme in the EMP pathway, and the presence of this enzyme in an organism is generally taken as evidence that the EMP pathway occurs in that organism. In yeasts (and mammals) phosphofructokinase is inhibited by high levels of ATP and citrate, and is stimulated by high levels of AMP; in e.g. Escherichia coli phosphofructokinase is inhibited by phosphoenolpyruvate (but not by citrate), and is stimulated by ADP and GDP (but not by AMP).
The EMP pathway functions not only in energy-yielding metabolism but also in supplying intermediates for biosynthesis: For example:
Dihydroxyacetone phosphate can be reduced to glycerol phosphate for use in the biosynthesis of lipids.
Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI [email protected]
Phosphoenolpyruvate may be drawn off for aromatic amino acid biosynthesis.
3-phosphoglycerate for cysteine, glycine and serine biosynthesis. Pyruvate for alanine, leucine and valine biosynthesis.
The terms ‘EMP pathway’ and ‘glycolysis’ are often used for the complete pathway for glucose degradation: e.g. glucose → lactate (i.e., HOMOLACTIC FERMENTATION) or glucose → ethanol (i.e., ALCOHOLIC FERMENTATION).
Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI [email protected]