Anaerobic Metabolism (Glycolysis)

• Glucose is a 6-carbon monosaccharide. There's approximately 10g of it in the blood plasma. Glucose is osmotically active (a high glucose concentration will draw water towards it), and acts as an immediate energy source. Glucose can be synthesised from non-carbohydrate sources (amino acids, or glycerol) in the process of gluconeogenesis.
• Glycogen is a branched polysaccharide made from alpha-glucose monomers. Linear chains of glycogen are formed through alpha-1-4-linkages, while branches stem from alpha-1-6-linkages.

  There's approximately 400g of glycogen within the blood plasma, and other stores of glycogen include the muscles, and liver. Glycogen has a low osmolarity (has minimal osmotic effect), and isn't an immediate fuel source.
• Glycolysis is the first stage of aerobic and anaerobic respiration. Glycolysis is the conversion of 1 molecule of glucose (6-carbon molecule) into 2 molecules of pyruvate (3-carbon molecule).

  Glycolysis occurs in the cytosol of all body tissues, although, not every body tissue will be doing glycolysis at the same time.

  The yield of glycolysis are ATP and NADH.
• Sources of glucose for glycolysis include:

  • sugars and starch gained from the diet.
  • the breakdown of glycogen in the liver.
  • gluconeogenesis of amino acids or glycerol. (glycerol isn't obtained from free fatty acids).
• There are 10 reactions to glycolysis and they are split into 4 stages:

  • Activation (reactions 1-3)
  • Splitting of the 6-carbon sugar in half (reactions 4 and 5)
  • Oxidation (reaction 6)
  • Synthesis of ATP (reactions 7-10).
• Reaction 1 is converting glucose into glucose-6-phosphate. This reaction is catalysed by the enzyme hexokinase in all body tissues except the liver (in the liver, reaction 1 is catalysed by glucokinase - an isozyme of hexokinase), and this reaction also uses 1 molecule of ATP. The phosphate group that gets added onto glucose comes from this ATP molecule.

  Reaction 1 is an irreversible reaction.
• Reaction 1 is an important reaction in glycolysis, as it keeps the glucose molecule within the cell. Normal glucose can exit a cell down it's concentration gradient through its protein transporters. However, being converted into glucose-6-phosphate prevent glucose from going through the glucose transporters, and given that glucose-6-phosphate is a negatively charged/polar molecule, it cannot pass through the cell membrane directly. As a result, the molecule cannot leave the cell and is able to undergo the appropriate metabolic pathways.
• Hexokinase has a low Km (Km is the substrate concentration at which the enzyme will achieve half its Vmax), meaning that it has a very high affinity for glucose, and glucose concentration doesn't have to be high for hexokinase to bind to its substrate (glucose).

  On the other hand, glucokinase has a higher Km, meaning that glucokinase has a lower affinity for glucose, and a higher substrate concentration is necessary for glucokinase to bind to glucose. This is significant because the liver can perform gluconeogenesis.
• After reaction 1, glucose-6-phosphate can go down the glycolysis pathway, or the glycogen-synthesis pathway.
• Reaction 2 converts glucose-6-phosphate into fructose-6-phosphate. Reaction 2 is an isomerisation reaction (glucose-6-phosphate - an aldehyde - becomes a ketone - fructose-6-phosphate), catalysed by the enzyme phosphohexose isomerase. Reaction 2 is a reversible reaction.
• Reaction 3 converts fructose-6-phosphate into fructose-1,6-bisphosphate. Reaction 3 is catalysed by the enzyme phosphofructokinase (PFK), and it also uses a molecule of ATP - the phosphate group added into fructose-6-phosphate comes from this molecule of ATP. Reaction 3 is an irreversible reaction.

  Reaction 3 is important because it results in a phosphate group bound to each end of the 6-carbon molecule.
• Reaction 4 is the splitting of fructose-1,6-bisphosphate into 2 3-carbon molecules: glyceraldehyde-3-phosphate (GAP), and dihydroxyacetone phosphate (DHAP). Reaction 4 is catalysed by the enzyme aldolase. Reaction 4 is a reversible reaction.
• Reaction 5 converts dihydroxyacetone phosphate (DHAP) into glyceraldehyde-3-phosphate (GAP). Reaction 5 is catalysed by the enzyme triose phosphate isomerase. Reaction 5 is a reversible reaction.

  DHAP itself can be used into the synthesis of fats, but it can also be converted into GAP for glycolysis.
• Reaction 6 converts glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate. 1 hydrogen atom is removed from GAP, and a phosphate group is added to the other end of the 3-carbon molecule.

  
  

  Reaction 6 is an oxidation reaction, and it's catalysed by the enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Reaction 6 is a reversible reaction, and it also NADH (and H+).
• Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is also known as a house-keeping protein. It's an enzyme whose concentration can be measured and used as an index because this enzyme will be present in all cells, as all cells do glycolysis.
• Reaction 7 converts 1,3-bisphosphoglycerate into 3-phosphoglycerate. The phosphate group added in reaction 6 is removed from 1,3-bisphophoglycerate and is added onto a molecule of ADP. Hence, ADP is converted into ATP. The transfer of a phosphate from one molecule to another (by a kinase enzyme) is substrate level phosphorylation.

  Reaction 7 is catalysed by the enzyme phosphoglycerate kinase. Reaction 7 is a reversible reaction.

  In reaction 7, substrate level phosphorylation occurs.
• Reaction 8 is the conversion of 3-phosphoglycerate to 2-phosphoglycerate. The phosphate group on the 3rd carbon is moved to the 2nd carbon, and the hydrogen atom is moved from the 2nd carbon to the 3rd carbon.

  Reaction 8 is an isomerisation reaction, catalysed by the enzyme phosphoglycerate mutase. Reaction 8 is a reversible reaction.
• Reaction 9 converts 2-phosphoglycerate into phosphoenolpyruvate. A water molecule is removed from 2-phosphoglycerate. Reaction 9 is catalysed by the enzyme enolase, and it is a reversible reaction.
• Reaction 10 is the conversion of phosphoenolpyruvate into pyruvate. The remaining phosphate on phosphoenolpyruvate is removed and added onto ADP. Hence, ADP turns into ATP.

  Reaction 10 is catalysed by the enzyme pyruvate kinase.

  Reaction 10 is an irreversible reaction.

  In reaction 10, there is substrate level phosphorylation.
• In glycolysis, there are 2 reactions (reaction 7 and reaction 10) that produce ATP, both of which do so by substrate level phosphorylation.

  This results in a production of 4 ATP molecules from 1 glucose molecule. But given that 2 ATP molecules were used in the activation stage of glycolysis (in reaction 1 and reaction 3), the overall net production of glycolysis is 2 molecules of ATP and 4 molecules of NADH.
• All cells, whether mitochondria is present or oxygen is sufficient, will undergo glycolysis. This is because glycolysis doesn't require (additional) oxygen, and glycolysis occurs in the cytosol, not the mitochondria.
• If oxygen in tissues is limited, then pyruvate won't enter the TCA Cycle and be metabolised into carbon dioxide. Instead pyruvate is converted into lactate in order to convert the cofactor NADH back to NAD+. This is so that NAD+ can re-enter glycolysis and keep the glycolysis pathway going to produce ATP.

  The reaction to convert pyruvate to lactate is catalysed by the enzyme lactate dehydrogenase. 2 hydrogens are removed from NADH (NADH is oxidised) and added to pyruvate to produce lactate.
• In the muscles, the reaction converting pyruvate into lactate is irreversible - resulting in the build up of lactic acid in the muscles. In the liver however, the reaction converting pyruvate into lactate is reversible. Lactate can be converted back into pyruvate in the liver.
• If oxygen in the tissues is sufficient, then pyruvate will be converted into acetyl CoA, and acetyl CoA will then enter the TCA cycle. Carbon dioxide will be produced from the TCA cycle.

  Acetyl CoA can also be used in the synthesis of fatty acids.
• In microorganisms, pyruvate can be converted into ethanol in anaerobic respiration via alcohol fermentation.
• Glycolysis is regulated by 2 ways: allosteric control and hormonal control.
• The enzyme phosphofructokinase (enzyme that catalyses reaction 3 of glycolysis) is under allosteric control by ATP, citrate, and AMP.

  ATP can act as an allosteric inhibitor when ATP is plentiful. ATP will bind to the allosteric site on phosphofructokinase, and in doing so, reduce the functioning of the enzyme, and prevent fructose-1,6-bisphosphate from forming than then subsequently splitting.

  Citrate acts as an allosteric inhibitor when it is in high concentration.

  AMP (produced during protein synthesis) will act as an allosteric activator when it's in high concentration.
• The only source of ATP in red blood cells is from glycolysis, since they have no mitochondria.

  Glycolysis is a major source of ATP in the brain, since the brain lacks the enzymes necessary to use fats as fuels.