Discuss the formation of ATP during glycolysis
Introduction
Glycolysis is a metabolic pathway that is found in the cytosol of cells in all living organisms it breaks down glucose, a simple sugar to pyruvate. This unique process can take place when there is oxygen available and also when there is no oxygen available under aerobic and anaerobic conditions.
In 1987 Hans Buchner and Eduard Buchner accidently came across something new. What they were interested in was manufacturing cell-free extracts of yeasts or clinical use. Sucrose was added to preserve the extracts. This is where they discovered something un-usual. The cell free extract converted the sucrose to ethanol. This showed that metabolism can happen outside of living cells. This investigation had led to several scientists to inspect the breakdown of glucose more thoroughly. In the 1930s, Gustav Embden, Otto Meyerhof and Jacob Parnes concluded that the breakdown of glucose consists of ten steps. Each one of these steps is broken down by another enzyme. Now researchers have concluded that glycolysis is the preferred way of or the breakdown of glucose in; archea, bacteria and eukaryotes. These steps of glycolysis are all the same in mostly all living organisms. This says that glycolysis was involved in the evolution of life on our planet.
Glycolysis is separated in to three phases. The first section in known as endergonic reaction that needs ATP which is also known as energy investment. In the first phase, glucose is very stable and not broken down easily. It consists for three steps. These steps are when two ATP molecules are hydrolyzed to form fructose-1, 6-biphosphate. Step one is where glycolysis starts with a reaction where glucose receives a phosphate group from an ATP molecule. The ATP acts as both a phosphate and also the energy needed to attach the phosphate to the molecule of glucose. ATP is converted to ADP and becomes the ADP of the cell until it is converted into ATP again. The phosphorylated glucose is called glucose-6-phosphate which in turn makes it more reactive. (It is more easily trapped in the cell compared to glucose). Step two is where the structure of glucose-6-phosphate undergoes another reaction where the hydrogen and the oxygen atoms are rearranged. The reaction is where glucose-6-phosphate is converted to its isomer which is fructose-6-phosphate. The third step is where another ATP donates a phosphate to the molecule. This forms fructose-1, 6-bisphosphate. The phosphate groups are now bound at carbon one and six, this means that the molecule is ready to split.
The second phases is known as the cleavage phase which consists of two steps. This is where a six carbon sugar is broken down into two molecules of glyceraldehyde-3-phosphate. At step four, fructose-1, 6-biphosphate is then split into two 3 carbon sugars. These are glyceraldehydes-3-phosphate (G3P) and dihydroxyacetone phosphate. Step five is where dihydroxyacetone phosphate is converted to its isomer which is glyceraldehyde-3-phosphate to increase the metabolism in glycolysis. This means that the products in glycolysis at this point are two molecules of G3P. This can be summarised by:
Glucose (six carbon compound) + 2 ATP a 2 G3P (three carbon compound) + 2ADP
The third phase is known as exergonic reaction or the liberation phase where ATP and NADH are released. This is where the two glyceraldehydes-3-phosphate molecules are catalyzed to form two pyruvate molecules, which produces two molecules of NADH and four molecules of ATP, because the two molecules of ATP are used up in the endergonic reaction which needs ATP, the net yield of ATP is two molecules. The net reaction of glycolysis can be shown below:
C6H12O6 + 2 NAD+ + 2 ADP2- + 2 Pi2 a 2 CH3 (C=O) COO– + 2 H+ + 2 NADH + 2 ATP4- + 2 H2O
GlucosePyruvate
In the third phase there are five steps. The first step is where, each glyceraldehyde-3-phosphate encounters dehydrogenation with NAD+ as the hydrogen acceptor. The resulting factor of this reaction is phosphoglycerate. This then reacts positively with inorganic phosphate present in the cytosol to produce 1,3-biphosphoglycerate. NADH is produced. In 1,3-biphosphoglycerate a phosphate group is de-activated (upper left) which means that the bond will break in a very high exergonic reaction. The next step is where a phosphate is removed from 1,3-biphosphoglycerate to produce 3-phosphoglycerate. The phosphate that is removed is transferred to ADP to produce ATP. The phosphate group in 3-phosphoglycerate is transferred to produce 2-phosphoglycerate. This is done by the enzymatic shift enzymatic shift of the phosphate group. This is known as a preparation reaction. Next a water molecule is removed from the 2-phosphoglycerate which forms phosphoenolpyruvate (PEP). This product has a phosphate group attached by a bond that is not stable which means that the, bond will break in a high exergonic reaction. Then a phosphate is removed from phosphoenolpyruvate to produce pyruvate. The phosphate that is removed is transported to ADP to form ATP. In a cell when there is enough ATP feedback inhibition takes place. When the concentration levels are really high, ATP joins to an allosteric site in phosphofructokinase, this then breaks down the third step in glycolysis. When ATP is joined to the allosteric site, a change in structure takes place that forms the enzyme to be inactive. This then stops glucose from breaking down more which then inhibits excessive amounts of ATP.
Conclusion
During the investment phase of glycolysis two molecules of ATP are taken up, but then in the energy liberation phase four molecules of ATP are produced. This suggests that glycolysis produces a net profit of two ATP’s per glucose. The energy liberation phase can be summarized by the following:
2 G3P + 2 NAD+ + 4 ADP a 2 pyruvate + 2 NADH + 4 ATP.
