"Describe the reactions involved in generation of ATP anaerobically. Discuss the advantages and disadvantages of generating ATP anaerobically in comparison to generating it from aerobic metabolism."

Adenosine triphosphate (ATP) is a molecule that is used as a form of energy currency throughout the cell. It is used so that mechanisms involved in digestion can produce a common intermediate that can be used to drive many of the energetically unfavourable chemical reactions. Its principal feature is the presence of a bond, between the two terminal phosphate groups, that can be readily hydrolysed releasing significant amounts of energy. The energy released can be coupled to further reactions that are vital for the cell to survive but will not occur spontaneously because of the need for an energy input. (A bond associated with an equal amount of energy lies between the first and second phosphate groups. This is used occasionally when the coupled reaction is so unfavourable that the energy released from hydrolysis of a single phosphate bond is not great enough.) Anaerobic generation of ATP is principally carried out by the stepwise oxidation of glucose. This is known as glycolysis. It occurs in the cytoplasm of the cell and is common to both prokaryotic and eukaryotic cells. This suggests that it was developed early in the evolutionary process. Later developing processes are thought to be confined to specific organelles. This would be consistent with the mechanism developing without the need for oxygen. In the earlier ages of the earth the atmosphere had virtually no oxygen in it. Glucose is transported into the cell along its concentration gradient. In the human body, glucose is either taken into the bloodstream directly from the food in the gut lumen, or formed in the gut from the breakdown of polysaccharides. Alternatively glucose can be released from the stores of glycogen in muscle and liver tissue. Other monosaccharides are The first stage in its oxidation is to activate the glucose molecule. This is done by addition of a phosphate group by hexokinase to make glucose-6-phosphate. Fructose 6-phosphate is then produced from the activated glucose and a second phosphate group is added. Both of these phosphorylations are accomplished by hydrolysis of ATP molecules. At this stage the process of glycolysis has used two ATP molecules. The enzyme that catalyses this second phosphorylation is known as phosphofructokinase. It is a major control point of glucose metabolism. High levels of AMP, ADP and inorganic phosphate activate the enzyme whereas high levels of ATP, citrate and fatty acids inactivate it. This works as a negative feedback mechanism. When the products of hydrolysis of ATP are in raised amounts, the enzyme activates the mechanisms of ATP production. When the products of ATP synthesis are raised, the enzyme shuts down and the reaction is halted. The enzyme fructose bisphosphatase works in the opposite manner to this and when ATP is abundant (from catabolism of other substrates) can synthesise glucose for later use. Fructose-1,6-bisphosphate is split into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. The dihydroxyacetone being subsequently converted to a second molecule of glyceraldehyde 3-phosphate. This prevents the need to find another set of enzymes to deal with the dihydroxyacetone separately. Both molecules of glyceraldehyde 3-phosphate are now phosphorylated by the addition of inorganic phosphate and the reduction of nicotinamide adenine dinucleotide (NAD+). The ATP debt is repaid at the next step as two molecules of ADP are phosphorylated to ATP (one ATP for each glyceraldehyde 3-phosphate molecule). The phosphates are then moved to new positions on the molecule forming glycerate-2-phosphate. A hydroxy group is removed to form phosphoenol pyruvate. A net 'profit' of ATP can then be generated by dephosphorylation of the two phosphoenol pyruvate molecules to pyruvate. Under aerobic conditions the pyruvate thus generated can then be converted to Acetyl Coenzyme A and fed into the citric acid cycle. The TCA cycle occurs in the mitochondria. It involves the breakdown of the acetyl part of acetyl CoA to carbon dioxide and water with the release of several molecules of ATP and the consumption of O2. In anaerobic conditions the pyruvate is reduced by the NADH of earlier steps to lactate. Anaerobic generation of ATP is very inefficient in its use of glucose and can generate only two molecules of ATP for each molecule of glucose oxidised. Aerobic oxidisation of the products of this glycolysis can provide up to 32 molecules of ATP in total. Aerobic methods of ATP production are clearly more efficient in their utilisation of glucose. Anaerobes will be at a disadvantage to aerobes in that aerobes will be able to survive in areas where there is a much lower amount of glucose or other appropriate substrate available. Anaerobic mechanisms are, however able to function when there is a shortage in oxygen supplied to the cell. In the original prokaryotic cells there was no free oxygen and so anaerobic respiration was their only energy supply. The same is true of yeasts and some other bacteria today that exists in low oxygen environments. Temporary shortage of oxygen often occurs in skeletal muscle during hard exercise when the blood supply is not able to cope with the oxygen demands of the muscle. This cannot go on indefinitely as, although lactate can diffuse quickly into the blood, it does not do this fast enough and the lactate builds up in the muscle. Eventually this will overwhelm the pH buffering mechanisms of the cell and denature the enzymes involved in glycolysis. This slows the process down until the lactate can be cleared into the bloodstream. (The lactate can then be used in the liver to synthesise glucose.) In the brain the sole energy substrate is glucose. The anaerobic mechanism serves to increase the lifespan of the brain briefly in cases of mild hypoxia or where there is a temporary loss of blood flow, as in cardiac arrest. The brain, however, uses a lot of energy and can remain undamaged for a mere three minutes without a constant flow of blood. A second advantage is that without the aerobic mechanisms the cells involved do not require mitochondria. Mitochondria (thought to be smaller organisms introduced into the cell by phagocytosis at an early stage of evolution) are the organelles responsible for oxidation of acetyl CoA (formed from pyruvate) to carbon dioxide and water. This is especially important in the cornea and lens of the eye where the presence of large numbers of organelles in the cells would cause diffraction of light and loss of visual acuity. The TCA cycle is also an important step in biosynthesis with many of the intermediate molecules in the cycle able to be diverted to alternative pathways. Any cell that is not able to use this pathway will be forced to find additional sources of the important amino acids and coenzymes that are released. If these have to be found outside of the cell then an extra strain will be placed on the energy resources of the cell Lack of molecular oxygen does not however completely exclude the use of the TCA cycle as the prime energy released is not used in synthesis of ATP. Initially the energy involved in produced by electron tranfer in oxidising NAD+ to NADH. In use of TCA as a biosynthetic pathway the reducing power stored in the NADH can be put to good use in the synthesis of important molecules. In aerobic metabolism this power forms ATP. Glysolysis is important to most cells and to some is their only source of energy. Cells that are able to synthesis ATP using molecular oxygen are however, at a significant selective advantage. They can survive in areas where amounts of glucose are much lower and so are able to capitalize on more areas or have increased populations in areas of high glucose availability, forcing anaerobic organisms to the few small niches on earth where oxygen is scarce.

©Nick Manville 1998