BABY YØU LIGHT UP OUR WORLD WITH YOUR ATP
This week we will be focusing on Glycolysis
Let us break it up:
Glyco-meaning sugar (glucose) and lysis-meaning breaking
Tadaaaa: It is the breaking down of glucose
Glycolysis is the first step in cellular respiration. This is where enzyme reactions convert glucose to pyruvate (2) and it is also important for the production of ATP (Adenosine Triphosphate).
Something you should know:
Glucose is one of the most crucial substances that our body require to function properly and it supplies organisms with the correct amount of energy needed.
So, where does all the excitement occurs?
Glycolysis takes place in the cytoplasm, this is where all the enzymes are present to break down glucose.
Who were the scientists involved in discovering Glycolysis???
The scientists who were involved in discovering Glycolysis were:
Gustav Embden Otto Meyerhof Jakub Karol Parnas
What enzymes are involved in the reactions? How many enzymes are used?
Well, there are ten (10) enzymes involve in Glycolysis, they are as follows:
2) Phosphohexose isomerase
5) Triose phosphate isomerase
6) Glyceraldehyde 3-phosphate dehydrogenase
7) Phospho-glycerate kinase
8) Phospho-glycerate mutase
10) Pyruvate Kinase
The diagram below outlines each step involve in Glycolysis:
Okay, so don’t get a huge headache by trying to understand the picture. Below has an outline on what is occurring in each step, hopefully you all should understand.
Glycolysis consists of ten steps which are divided into two sections which includes the preparatory phase and payoff phase where pyruvate, ATP and NADH are formed.
Step1: This is where phosphorylation occurs and the reaction is irreversible. The enzyme Hexokinase is responsible for glucose to react with ATP to produce glucose-6-phosphate, where the adding of the phosphate group to the glucose molecule makes it unstable, hence promoting the reaction.
Step 2: glucose-6-phosphate is converted to fructose-6-phosphate with the help of the enzyme Phosphohexose isomerase.
Step 3: Another enzyme, phospho-fructokinase-1 removes the phosphate group from ATP giving it to fructose-6-phosphate to form fructose 1, 6-bisphosphate. Phospho-fructokinase-1 is the most important regulating enzyme and this reaction is also irreversible.
Step4: An Aldolase enzyme reaction split the fructose 1, 6-bisphosphate into Glyceraldehyde 3-phosphate and Dihydroxyacetone phosphate.
Step5: This is where the enzyme Triose phosphate isomerase makes its presence and rearranges the Dihydroxyacetone phosphate to form a second Glyceraldehyde 3-phosphate. At this stage in Glycolysis glucose has been metabolised to Glyceraldehyde 3-phosphate (2) and two ATPs have been consumed.
We have completed the Preparatory Phase
Let us see what happens in the Payoff Phase
Step6: The two Glyceraldehyde 3-phosphates are oxidised to 1, 3-Bisphosphoglycerate (2) by the enzyme Glyceraldehyde 3-phosphate dehydrogenase. This step produces one NADH (Nicotinamide adenine dinucleotide) for each oxidise 1, 3-Bisphosphate, giving a total of two NADH. These NADHS are used later on to produce more ATP for the cell.
Step7: A Phospho-glycerate kinase transfers a phosphate group from the 1, 3-Bisphosphoglycerate to ADP to for ATP and 3-Phosphoglycerate (2). This step results in two ATPS and two 3-Phosphoglycerate molecules.
Step8: This involves a Phospho-glycerate mutase reaction which moves the phosphate group in the third carbon of 3-phosphoglycerate to the second carbon to form 2-phosphoglycerate.
Step9: An Enolase enzyme reaction removes the water from the 2-phosphoglycerate to form two Phosphoenolpyruvate. This step occurs for the two molecules of 2-phosphoglycerate
Step10: Pyruvate kinase enzyme reaction removes a phosphate group from Phosphoenolpyruvate and donates it to ADP to produce ATP and Pyruvate. This step is similar to reactions 1and 3 where it is irreversible. At this stage two pyruvate molecules, four ATPs and Two NADH are formed for each glucose that was broken down for Glycolysis.
ATP IS AN IMPORTANT ENERGY MOLECULE REQUIRED FOR MANY BIOCHEMICAL PATHWAYS, ESPECIALLY FOR LIFE!
While I was researching I came across some harmful effects of Glycolysis which alarmed me. I never knew Glycolysis could be a bad thing but I guess in life there will always be good and bad characteristics in everything that exist, hence proving the point that no person or thing is perfect.
This is just a brief insight on two of the effects caused by Glycolysis.
It is known that malignant tumour cells can be caused when Glycolysis levels are way higher than those of normal tissues. Otto Warburg was the scientist who discovered this in the 1930s and it is referred to as the Warburg effect.
The second disease caused by an imbalance in Glycolysis is Alzheimer. This is a very unsafe disease which affects the nervous tissues of our brain and also it interferes with its functions.
So next time you think of sitting down with a bag of chocolates or candies Glycolysis will punish you!!!
Harvey, Denise R. Ferrier.2011.Lippincott’s Illustrated Reviews Biochemistry 5th Edition.
(Harvey and Ferrier 2011, 90-92)
Nelson, D.L.and Cox, M.M.2008.Lehninger Principles of Biochemistry 5th Edition.
(Nelson and Cox 2008,529)
Mathur, Meenakshi Mehta.2002.Bio-Chemistry 1st Edition.
(Mathur and Mehta 2002, 126-130)
“10 Steps of Glycolysis.” About.com Biology. (accessed March 2, 2014). http://biology.about.com/od/cellularprocesses/a/aa082704a.htm
“Things to Know About Glycolysis | Glycolysis.” Things to Know About Glycolysis | Glycolysis. (accessed March 2, 2014) http://www.glycolysis.org/things-to-know-about-glycolysis.html.
PART 2 OF GLYCOLYSIS
In case you missed some information on EACH STEP OF THE GLYCOLYTIC PATHWAY HERE IS A LITTLE MORE INFORMATION FOR YOU GUYS:
Steps in the Metabolic Pathway of Glycolysis which are Enzyme-Controlled Reactions:
The glucose is formed from the hydrolysis of glycogen and starch. The glucose formed is kept in low concentrations to favor the conditions of Le Chatelier’s Principle (Equilibrium Principle), which states that,
If there is a change in the equilibrium of a chemical system, may it be in concentration, temperature, pressure or volume, the position of equilibrium will ‘shift’ or will be changed to accommodate a new established equilibrium.
There are ten steps in glycolysis. The first five reactions are the first phase which consumes energy. In the other five reactions, there is a net production of ATP and hence, energy. The reactions are outlined as follows:
Step 1: The first step in the glycolytic pathway is the phosphorylation (addition of a phosphate group) of glucose to glucose-6-phosphate (charged to avoid diffusion through the cellular membrane) by an ATP molecule. The enzyme that catalyses this reaction is from a family of enzymes known as hexokinases with the cofactor* being Mg2+. The concentrations of glucose are kept low to allow the shift of the reaction to the right (formation of glucose-6-phosphate).
Step 2: The second step involves the reaction involving the conversion of glucose-6-phosphate (pyranose ring) to its isomer fructose-6-phosphate (furanose ring). The enzyme which catalyses this reaction is phosphoglucoisomerase which is an isomerase. This reaction is reversible, but due to the low concentrations of fructose-6-phosphate, the position of equilibrium shifts to the right allowing more production of fructose-6-phosphate.
Step 3: The third step involves the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate. This consumes one molecule of ATP which is used for two purposes: to add a phosphate group to the fructose-6-phosphate molecule and to provide energy for this reaction. This reaction is catalysed by the enzyme phosphofructokinase-1 which is a kinase with the coenzyme being Mg2+ and this reaction is irreversible. This step is necessary since the fructose-1,6-bisphosphate is a very unstable, charged molecule that will form two charged molecules which cannot diffuse across the non-polar cellular membrane.
Step4: The fourth step is where the destabilization of the fructose-6-phosphate to fructose-1,6-bisphosphate causes the molecule to split roughly down the middle into two triose sugars. The enzyme catalysing this reaction is aldolase which is a lyase and is a reversible reaction. The two products or triose sugars formed are: D-glyceraldehyde-3-phosphate (GADP/GAP) and Dihydroxyacetone phosphate (DHAP), which are both carboanions.
Step 5: In the fifth step, dihydroxyacetone phosphate (DHAP) is immediately interconverted to D-glyceraldehyde-3-phosphate (GADP/GAP). The enzyme that catalyses this reversible reaction is triose phosphate isomerase which is an isomerase. This gives simplicity to the reactions that follow since one specific triose sugar is used (D-glyceraldehyde-3-phosphate/GADP/GAP). Therefore the net gain of steps 4 and 5 are two molecules of D-glyceraldehyde-3-phosphate (GADP/GAP).
Step 6: The sixth step involves the dehydrogenation of the D-glyceraldehyde-3-phosphate (GADP/GAP) and the addition of a phosphate group from the cytosol in the form of H3PO4 (aq.). Both of these reactions are catalysed by glyceraldehyde phosphate dehydrogenase, an oxidoreductase, which is reversible. First, the glyceraldehyde phosphate dehydrogenase is used to reduce 2NAD+ molecules (a hydrogen carrier) to NADH+H+ (reduced NAD+), then the inorganic phosphate group is added (phosphorylation). The product formed is D-1,3-Bisphosphoglycerate which is a stable molecule.
Step 7: The seventh step involves the conversion of D-1,3-Bisphosphoglycerate to 3-phosphoglycerate by the removal of an inorganic phosphate group and the generation of two ATP molecules (from the two D-glyceraldehyde-3-phosphate/GADP/GAP molecules formed in Step 5). The enzyme involved in this reversible reaction is phosphoglycerate kinase which is a kinase with the cofactor being Mg2+.
Step 8: The eighth step involves the conversion of 3-phosphoglycerate to 2-phosphoglycerate. The enzyme catalysing this reversible reaction is phosphoglycerate mutase.
Step 9: In the ninth step there is a conversion of the two molecules of 2-phosphoglycerate to two molecules of phosphoenolpyruvate by the removal of water. The enzyme catalysing this reaction is enolase which is a lyase with the cofactor being 2Mg2+.
Step 10: This step is the tenth and final step where the two molecules of phosphoenolpyruvate are converted to two molecules of pyruvate by removal of two inorganic phosphate groups and generation of two ATP molecules. This irreversible reaction is catalysed by pyruvate kinase with the cofactor being Mg2+.
- *Cofactor: a chemical compound which is non-protein that is necessary for the proper functioning of (in most cases) proteins which are enzymes. Cofactors also balance the charges in a molecule (for example ATP) and they assist conformational proteins.
- From Steps 1-5 there is the consumption of two ATP molecules for phosphate groups and for energy.
- From Steps 6-10 there is the production of four ATP molecules and two NADH+H+ (reduced NAD+) molecules.
- Therefore there is a net gain of two ATP molecules and two NADH+H+ (reduced NAD+) molecules
- All products from Steps 6-10 are multiplied by two since two D-glyceraldehyde-3-phosphate/GADP/GAP molecules are produced in Step 5.
- From the two pyruvate molecules formed, their fate may be either the TCA cycle in the presence of oxygen or formation of lactate or alcoholic fermentation (formation of ethanol).
E.C. NUMBER OF EACH ENZYME:
- Hexokinase: 188.8.131.52. (Transferase)
- Phosphoglucoisomerase: 184.108.40.206. (Isomerase)
- Phosphofructokinase-1: 220.127.116.11. (Transferase)
- Aldolase: 18.104.22.168. (Lyase)
- Triose Phosphate Isomerase: 22.214.171.124. (Isomerase)
- Glyceraldehyde phosphate dehydrogenase: 126.96.36.199. (Oxidoreductase)
- Phosphoglycerate Kinase: 188.8.131.52. (Transferase)
- Phosphoglycerate Mutase: 184.108.40.206. (Isomerase)
- Enolase: 220.127.116.11. (Lyase)
- Pyruvate Kinase: 18.104.22.168. (Transferase)
Fates of the pyruvate (5 different possibilities):
In the absence of oxygen, two main forward reactions can occur in the presence of certain enzymes. The reactions are separately classified due to the difference in the metabolism of the pyruvate (pyruvic acid). Both processes are known as Fermentation Reactions. These are processes which produce ATP under anaerobic conditions (the absence of oxygen). But, more importantly, fermentation reactions are those that regenerate NAD+ from NADH+H+. Fermentation was first discovered by Louis Pasteur who referred to it as “a life without air”.
- The first possibility is the conversion of pyruvate to L-lactate by reduction of the pyruvate molecule by NADH+H+ in the presence of the enzyme lactate dehydrogenase (EC: 22.214.171.124.) in a reversible reaction.
- This is an important reaction in Erythrocytes which are mature red blood cells. They depend only on glycolysis as their main source of energy.
- The purpose of production of lactate on the erythrocytes? Firstly, they are red blood cells which do not contain organelles within their structure, which includes mitochondria. Therefore the only means of production of ATP is through glycolysis.
If we look back at the sixth reaction of glycolysis, there is the conversion of two molecules of glyceraldehyde-3-phosphate to two molecules of 1,3-bisphosphate. The enzyme catalyzing this reaction is glyceraldehyde-3-phosphate dehydrogenase. In this reaction an inorganic phosphate will bond to the glyceraldehyde-3-phosphate molecule, with the phosphate group being removed from the cytosol in the form of H3PO4 (aq.). This reaction alone is not energetically feasible. This explains the coupled oxidation reaction occurring with the phosphorylation, in which NAD+ is converted to NADH+H+ (reduced NAD+). NAD+ is a hydrogen carrier, which is present in low concentrations in the cell and once it is reduced, it cannot revert back to its oxidized state. This is where the conversion of pyruvate to lactate comes into play. The reaction regenerates NAD+ for use in glycolysis. If this is the only source of energy, if all NAD+ is used up, the glycolytic pathway stops. Again, erythrocytes do not contain mitochondria and therefore, there is no use of the TCA cycle or ETC for synthesis of ATP.
- Lactate is produced under specific conditions in the body. Mostly during vigorous exercise where the muscle tissue is deficient of a steady supply of oxygen to it, there is a rampant increase in the lactate production. Lactate in its ionized form is acidic and it is toxic in large quantities to the body, which causes muscle cramps signaling the body to stop vigorous activity.
- The second possibility, under anaerobic conditions, is the conversion of pyruvate to ethanol through two metabolic reactions. The first reaction involves the removal of carbon in the form of carbon dioxide from the pyruvate molecule. It is converted to acetaldehyde/ethanal + CO2 by the enzyme pyruvate decarboxylase with the cofactors being TPP (Thiamine Pyrophosohate) and Mg2+. This reaction is irreversible.
The second reaction involves the conversion of the acetaldehyde/ethanal to ethanol in a reversible reaction. This reaction is where the NAD+ is regenerated from the NADH+H+ (reduced NAD+) in a reduction reaction. The enzyme catalyzing the reaction is alcohol dehydrogenase.
- The third possibility is under aerobic conditions, where pyruvate is converted into Acetyl Coenzyme A (CoA) in an irreversible reaction. The enzyme and cofactors are pyruvate dehydrogenase complex (E1 + E2 + E3) and TPP (thiamine pyrophosphate), Coenzyme A (CoA with a sulphide linkage -SH), lipoate and FAD respectively. NAD+ is also converted to NADH+H+ (reduced NAD+), in an oxidation reaction. This is a high energy reaction which produces a ΔG˚= -33.4 KJmol-1 (very negative) and therefore is not energetically feasible to be a reversible reaction.
- The fourth possibility is the conversion of pyruvate back to glucose in a process known as gluconeogenesis/glucogenesis.
- The fifth possibility is the transamination of the pyruvate to the amino acid alanine. Transamination is the process whereby an amine group is transferred from one molecule (usually glutamate) to α-keto acid in an enzyme catalyzed reaction by enzymes known as aminotransferases. This is where glutamate transfers an amine group to pyruvate in a reversible reaction to form alanine and α-ketoglutarate. The enzyme catalysing this reaction is alanine transaminase.
Glutamate + Pyruvate ↔ Alanine + α-Ketoglutarate (catalyzed by alanine transaminase)
Campbell, Mary, Farrell, Shawn. Biochemistry 7th edition. Cengage Learning, 2011.
Conn, Eric E. Outlines of Biochemistry. John Wiley and Sons, Inc., 1976.
Fruton, Joseph S., Simmonds Sophia. General Biochemistry. John Wiley and Sons, Inc., 1953.
Gonzaga.edu. “Biochemistry Dictionary” http://guweb2.gonzaga.edu/faculty/cronk/biochem/P-index.cfm?definition=pyruvate
MCQ FOR YOU TO TRY:
What is the importance of fermentation?
- To break down pyruvate due to its toxicity.
- Since there is a negative value of ΔG˚ the energy is lost as heat which is needed by the organism.
- To regenerate the NAD+, since is not present in high concentrations in the cell and is needed for the catalysis of the sixth reaction in the glycolytic pathway.
- To produce alcohol, needed by yeast cells.
- The provision of ATP for use of metabolic reactions in the cell.
Hope you all enjoyed