glycolysis

Aldolase enzyme

- An enzyme that will cut fructose1,6-bisphosphate into DHAP and GAP - In the fourth reaction of glycolysis that produces two different products.

possible fates of pyruvate depending in availability of oxygen

1) anaerobic conditions (oxygen is not sufficient) - pyruvate is fermented (cytoplam) to alcohol (in yeast) or lactate (eg. in mammalian skeletal muscles ) in thhe cytoplasm 2) aerobic conditions ( oxygen sufficient) - pyruvate enters krebs cylce in mitochondria where it will be oxidise in the presence of oxygen to become carbon dioxide and water - generating an additional ~30 ATP molecules - also known as cellular respirarion - cellular respiration consists of kerbs cycle and oxidative phosphorylation

2 stages of glucose metabolism

1. glycolysis 2.(a) fermentation - occurs in the absence of oxygen in the cytoplasm - partial breakdown of glucose OR (b) cellular respiration - occurs in the presence of oxygen in the mitochondria -complete breakdown of glucose

3 regulatory points of glycolysis

1. hexokinase 2. Phosphofrusctokinase-1 (PFK-1) 3. pyruvate kinase

How many reactions are there in glycolysis

10

Stage 1 or investment (Reaction 1-5)

A preparatory stage in which the hexose glucose is phosphorylated and cleaved to yield two molecules of the triose glyceraldehyde-3-phosphate. Stage I consumes two ATPs. This process utilizes two ATPs in a kind of energy investment.

Overall reaction of glycolysis

Glucose + 2NAD + 2ADP + 2Pi -> 2NADH + 2 pyruvate + 2 ATP + 2H2O + 4H+

Important points to remember in glycolysis

It is important to remember... 1.The names of the reactants, products and enzymes involved in each step 2.Specifically the 2 ATP-utilizing reactions, the NAD+- utilizing reaction and the 2 ATP-generating reactions 3.That NAD+ provides the oxidizing power for glycolysis to occur in the absence of O2. As such, in order for glycolysis to occur continuously, NAD+ must be continuously be regenerated -In the presence of O2, NAD+ would be regenerated via oxidative phosphorylation (later part of cellular respiration) -In the absence of O2, NAD+ would be regenerated via fermentation

How is energy in Glucose harvested in Glycolysis

Lets look at energy investment and harvest. In the first reaction is the phosphorylation of glucose which is the addition of phosphate to glucose. For that reaction we need ATP to jumpstart the pathway. For the second reaction we don't need ATP. For the third reaction we need ATP to make fructose 1,6-biphosphate through phosphorylation. Reaction 1 and 3 in the investment stage require ATP and that is why 2 ATP is used up. From reaction 3, the molecule will change to glycerladehyde 3-phosphate and dihydroxyacetone phosphate. In the harvest stage, from glyceraldehyde 3-phosphate we will start to get back our ATP. We get back a total of 2 ATP from reaction 7 and 10. We will go through reaction 6-10 two times because in 1 mol of glucose we make 2 glyceraldehyde 3-phosphate and therefore the process runs twice. In reaction 7 and 10 there is a single ATP generator but because the process runs twice then we get 4 ATPs. That is why we have a net gain of 2 ATP because in the investment stage we use 2 ATP and in harvest stage we gain 4 atps.

Phosphofructokinase regulation point

Phosphofructokinase gets turns off when there is a build up of citrate and atp. If we have a lot of citrate that automatically implies that we have a lot energy and it is one of the intermediate products of CAC. If there is too much of citrate it will feedback to phosphofructokinase-1. if we have a high level of ATP it will also feedback to phosphofructokinase to switch it off. We also have ways to turn on phosphofructokinase and that will be dependent on the level of AMP and the level of fructose 1,6 bisphosphate. Those two molecules will switch on phosphofructokinase-1.

phosphoglucose isomerase enzyme

Phosphoglucose isomerase enzyme is a isomerase that does not change the chemical formula but it changes the shape and structure of the substrate. It will take glucose 6 phosphate and change it to fructose 6 phosphate. The formula is the same, it is just that in glucose 6 phosphate we have a six sided ring and in fructose 6 phosphate it becomes 5 sided ring but the number of carbons, oxygen and everything it remains the same. First is the binding of the substrate in the active site. glucose 6-phosphate will bind with phosphoglucose isomerase and it will take out hydrogen and go through some arrangement with the bonds of the substrate. Then it will have an exchange of H+ with medium. Then it goes through another change and finally at the 5th reaction it puts back the hydrogen

pyruvate kinase regulation point of glycolysis

Pyruvate kinase catalyses the last reaction of glycolysis. It also has feedback loops and ATP can tell the pyruvate kinase to switch off and fructose 1,6 biphosphate can tell the pyruvate kinase to be switched on.

Reactions of the investment stage (reaction 1-5)

Reaction 1 : Glucose → Glucose 6-phosphate Glucose is phosphorylated by the enzyme hexokinase to form glucose 6- phosphate. Glucose gains energy by being phosphorylated at the expense of one ATP. Reaction 2 : Glucose 6-phosphate → Fructose 6-phosphate (F6P) Glucose 6-phosphate is converted into its isomer, fructose 6-phosphate, by phophoglucose isomerase (PGI) enzyme. Reaction 3: Fructose 6-phosphate → Fructose 1,6-bisphosphate A phosphate group is transferred from ATP to fructose-6-phosphate, producing fructose-1,6-bisphosphate. This step is catalyzed by the enzyme phosphofructokinase (PFK) , which can be regulated to speed up or slow down the glycolysis pathway. In this reaction we use 1 ATP. Reaction 4: Fructose 1,6-bisphosphate → Glyceraldehyde 3-phosphate (GAP) + Dihydroxyacetone phosphate (DHAP) Fructose-1,6-bisphosphate splits to form two three-carbon sugars,DHAP and GAP. This reaction is catalysed by aldolase enzyme. They are isomers of each other, but only GAP can directly continue through the next steps of glycolysis. Reaction 5 : DHAP ⇌ GAP Dihydroxyacetone phosphate is converted into glyceraldehyde 3- phosphate. This reaction runs in both directions, but because the glyceraldehyde 3-phosphate is converted further in glycolysis, the equilibrium favors the formation of glyceraldehyde 3-phosphate. This reaction is catalysed by triose phosphate isomerase.

Reactions of the harvest stage

Reaction 6: GAP → 1,3-Bisphosphoglycerate (1,3-BPG) Two half reactions occur simultaneously: 1) Glyceraldehyde-3-phosphate is oxidized, and 2) NAD⁺ is reduced to NADH and H⁺ . The overall reaction is exergonic, releasing energy that is then used to phosphorylate the molecule, forming 1,3-bisphosphoglycerate. This reaction is catalysed by glyceraldehy 3-phosphate dehydrogenase (GAPDH). Reaction 7: 1,3-BPG → 3-Phosphoglycerate (3PG) . A phosphate group is removed from each 1,3-bisphosphoglycerate to make two ATP and 3-phosphoglycerate. This reaction is mediated by the enzyme phospho-glycerokinase. Reaction 8: 3PG → 2-Phosphoglycerate 3PG is converted into its isomer, 2-phosphoglycerate. This reaction is catalysed by phosphoglycerate mutase Reaction 9 : 2-Phosphoglycerate → Phosphoenolpyruvate 2-phosphoglycerate loses a molecule of water, becoming phosphoenolpyruvate . PEP is an unstable molecule, poised to lose its phosphate group in the final step of glycolysis. This reaction is catalysed by enolase Reaction 10: PEP → Pyruvate PEP readily donates its phosphate group to ADP, making a second molecule of ATP. As it loses its phosphate, PEP is converted to pyruvate, the end product of glycolysis. This reaction is catalysed by pyruvate kinase.

Stage 2 or harvest (reaction 6-10)

The two molecules of glyceraldehyde-3-phosphate are converted to pyruvate, with concomitant generation of four ATPs. Stage II produces four ATPs. Glycolysis therefore has a net profit of two ATPs per glucose.

glucose metabolic pathway simplified

The white bubble is a summary of glycolysis. We start with glucose and it will go through breakdown reactions and we will get 2 NADH molecules. NADH is a carrier for high energy electron. We will look at the generation of 2 ATP molecule in a single glycolysis pathway and the end result of glycolysis is 2 pyruvate generated from a single molecule of glucose. The reason we that we get two molecules of pyruvate Is because in glucose there are six carbons and pyruvate has 3 carbon. So basically we break down glucose into half at the end and we get 2 molecules of pyruvate. From there, we will move on to citric acid cycle(CAC). Pyruvate is used to jumpstart the citric acid cycle. Pyruvate has been converted into acetyl coA and that will jump start CAC and it occurs in the presence of oxygen. CAC is an aerobic oxidation. In the absence of oxygen, we will do the fermentation pathway. For us, we will carry out homolactic fermentation in which we produce 2 lactate molecules If it is yeast, then it will go through alcoholic fermentation in which they produce 2 carbon dioxide and 2 ethanol which is enjoyed by people in the form of wine or beer.

hexokinase regulation point of glycolysis

When the level of glucose 6 phosphate builds up it will go back to hexokinase and tell it to stop. And that you have already seen is describe as feedback inhibition.

definition of glycolysis

a process by whcih a glucose molecule is broken down (under anaerobic condictions ) into 2 pyruvate molecules, producing a net yield of 2 ATP molecules in the process

Where does glycolysis occur?

it occurs in the cytoplasm

Regulation of Glycolysis

there is a built in feedback for regulation of glycolysis . If we have a lot of ATP, we don't need to make more. The level of ATP will give a feedback to the enzymes in glycolysis and will tell the enzymes to shut down and that will inhibit glycolysis. This is a feedback inhibition. When we are energy poor, which means that we have used up a lot of energy and we have a lot of AMP. AMP will go to enzymes in glycolysis and it will tell the enzymes to switch on or work faster and it will activate the glycolysis pathway.

Glycolysis is separated into

two stages

How hexokinase enzyme works

•Conformational change upon glucose binding •'Closing' of two lobes draws the substrates glucose and ATP together •After the product glucose- 6-phosphate is formed, the conformation goes back to starting state When the glucose goes and fits into the active site, we have a conformational change. Hexokinase has a open layout before the glucose comes in. After the glucose comes in, the hexokinase wraps itself around, bends over and enfold the glucose. After the reaction is done, it also has bound to ATPand the phosphate from atp will be transferred to glucose. After glucose 6 phosphate is formed it will leave the active site and then the whole enzyme will go back to starting point.

disease association with glycolysis defects

•For patients with deficient levels of hexokinase and pyruvate kinase enzymes, the oxygen binding to their haemoglobin is altered. •For hexokinase deficient cases, the O2 binding curve is shifted to the left of normal. That results in too tight association, so O2 cannot be sufficiently released. •For pyruvate kinase deficient cases, the O2 binding curve is shifted to the right of normal. That results in too loose association, so O2 cannot be sufficiently taken up. Because of these defects, and these are enzyme in glycolysis, they will be affected in terms of anaemia. Glycolysis has a effect on anaemia For hexokinase deffiency what happens is the binding of oxygen becomes too tight. The RBC will pick the oxygen in the lungs and try to go to the tissues to deliver the oxygen but because the hameoglobin holding too tightly it cannot release the oxygen enough and therefore the tissue don't get enough oxygen that it needs For patients that don't have enough pyruvate, we have a binding that is too loose so when the rbc goes to the lungs even the lungs is under high condition of oxygen, it can only load up less than 40 percent of oxygen and with that less than 40 percent in haemoglobin, it goes to the tissues & release the oxygen, but only a little oxygen is released. The normal haemoglobin has 70 percent bound so it can release a lot of oxygen to the cells and the tissues

why is extra ATP produced by cellular respiration is important?

•The extra ATP produced by cellular respiration is so important to organisms that anything which interferes with its production, such as cyanide/arsenate poisoning or lack of O2, quickly results in the death of the organism. Cellular respiration is very important to us because the vast majority of the energy needed comes from aerobic respiration (28-30 ATP is produced) and only about two ATP come from glycolysis .There is a very large difference and anything that interferes with aerobic respiration will potentially kill us. For example, cyanide. Cyanide is a poison and it shuts down our aerobic respiration very quickly and we will not be able to make the ATP molecules and people can die very quickly if they take sufficient amount of cyanide