In this video, In this video, we're going to talk about glycolysis, we're going to talk about glycolysis, its general aspects, its general aspects, the steps involved in glycolysis, the steps involved in glycolysis, and we're also going to talk about the routes that pyruvate can follow, and we're also going to talk about the routes that pyruvate can follow, such as fermentation. such as fermentation. Let's start by talking a little bit about the generalities. Let's start by talking a little bit about the generalities.
Glucose is a molecule that has six carbons. Glucose is a molecule that has six carbons. This molecule plays a very important role in the metabolism of plants, This molecule plays a very important role in the metabolism of plants, animals, animals, and microorganisms.
and microorganisms. In addition to being an excellent fuel, In addition to being an excellent fuel, glucose is an intermediary, glucose is an intermediary, let's say, let's say, that supplies other metabolic pathways, that supplies other metabolic pathways, such as the pentose phosphate pathway, such as the pentose phosphate pathway, the production of components of the extracellular and pericular matrix, the production of components of the extracellular and pericular matrix, the production of glycogen, the production of glycogen, among other very important metabolic pathways in our body. among other very important metabolic pathways in our body.
Glycolysis consists of a series of reactions catalyzed by enzymes that break down glucose in order to obtain mainly ATP, Glycolysis consists of a series of reactions catalyzed by enzymes that break down glucose in order to obtain mainly ATP, pyruvates, pyruvates, and NADH. and NADH. All glycolysis reactions occur in the cytosol. All glycolysis reactions occur in the cytosol. Now let's talk about each of the reactions that occur in glycolysis.
Now let's talk about each of the reactions that occur in glycolysis. First, First, glucose is converted to glucose phosphate by the enzyme exokinase. glucose is converted to glucose phosphate by the enzyme exokinase.
ATP is required for this reaction because a phosphate group is being donated to carbon-6 of glucose. ATP is required for this reaction because a phosphate group is being donated to carbon-6 of glucose. Let's keep in mind that exokinase is an enzyme that has a low Km.
Let's keep in mind that exokinase is an enzyme that has a low Km. meaning that it has a high affinity for its substrate. meaning that it has a high affinity for its substrate. This exokinase is widely distributed in many tissues and there is another enzyme that carries out this reaction called glucokinase.
This exokinase is widely distributed in many tissues and there is another enzyme that carries out this reaction called glucokinase. This glucokinase has a very high Km, This glucokinase has a very high Km, meaning that it has very low affinity and is present in hepatocytes and beta cells. meaning that it has very low affinity and is present in hepatocytes and beta cells. Another difference between these enzymes is that glucokinase has a higher maximum velocity and exokinase has a much lower maximum velocity. Another difference between these enzymes is that glucokinase has a higher maximum velocity and exokinase has a much lower maximum velocity.
These differences between Km and maximum velocity are justified by the fact that these enzymes are present in tissues that respond differently to changes in glucose levels. These differences between Km and maximum velocity are justified by the fact that these enzymes are present in tissues that respond differently to changes in glucose levels. For example, For example, glucokinase is found in tissues such as the liver and pancreas that have to respond quickly to changes in glucose levels. glucokinase is found in tissues such as the liver and pancreas that have to respond quickly to changes in glucose levels. Now, Now, glucose phosphate is converted into the substrate of phosphoglucose isomerase to form fructose phosphate.
glucose phosphate is converted into the substrate of phosphoglucose isomerase to form fructose phosphate. This reaction is reversible, This reaction is reversible, which is why you can see the direction of the arrows. which is why you can see the direction of the arrows. This means that fructose phosphate can be converted to glucose phosphate again by the same enzyme.
This means that fructose phosphate can be converted to glucose phosphate again by the same enzyme. The third step is the conversion of fructose phosphate to fructose-1, The third step is the conversion of fructose phosphate to fructose-1, 6-bisphosphate by the enzyme phosphofructokinase. 6-bisphosphate by the enzyme phosphofructokinase.
ATP is also used in this reaction. ATP is also used in this reaction. As you can see, As you can see, fructose-1, fructose-1, 6-bisphosphate contains 2 phosphates. 6-bisphosphate contains 2 phosphates.
one that was added in the first step of glycolysis and the other that is added in this step. one that was added in the first step of glycolysis and the other that is added in this step. In the next step, In the next step, aldolase comes into play. aldolase comes into play. This enzyme will convert fructose-1, This enzyme will convert fructose-1, 6-bisphosphate to dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.
6-bisphosphate to dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. In this case, In this case, we have that dihydroxyketone phosphate can be converted by means of the enzyme triis-phosphate isomerase to another glyceraldehyde. we have that dihydroxyketone phosphate can be converted by means of the enzyme triis-phosphate isomerase to another glyceraldehyde.
This means that in this step we could have two glyceral dihides, This means that in this step we could have two glyceral dihides, three phosphates. three phosphates. That is why from here on the reactions will be worth two. That is why from here on the reactions will be worth two. Because we are not only going to have one glyceraldehyde triphosphate, Because we are not only going to have one glyceraldehyde triphosphate, but two due to this conversion of dihydroxyacetone phosphate.
but two due to this conversion of dihydroxyacetone phosphate. As you may have noticed, As you may have noticed, two ATPs have been invested from glucose to glyceraldehyde triphosphate. two ATPs have been invested from glucose to glyceraldehyde triphosphate.
That is why these first five reactions are included in the phase called the preparatory phase or energy investment phase. That is why these first five reactions are included in the phase called the preparatory phase or energy investment phase. As a sixth step, As a sixth step, we have that glyceraldehyde 3-phosphate is going to be converted to we have that glyceraldehyde 3-phosphate is going to be converted to 1,3-bisphosphoglycerate thanks to the enzyme glyceraldehyde 3-phosphate hydrogenase.
1,3-bisphosphoglycerate thanks to the enzyme glyceraldehyde 3-phosphate hydrogenase. In this reaction, In this reaction, the oxidized NAT is going to be converted into reduced NAT. the oxidized NAT is going to be converted into reduced NAT. Note the direction of the arrows.
Note the direction of the arrows. This reaction is reversible. This reaction is reversible. Let's also consider that here we are not talking about a single glyceraldehyde triphosphate, Let's also consider that here we are not talking about a single glyceraldehyde triphosphate, but two.
but two. Let's remember this step because reduced NAT will be very important later for fermentation. Let's remember this step because reduced NAT will be very important later for fermentation. What happens under anaerobic conditions?
What happens under anaerobic conditions? In the seventh step, In the seventh step, the enzyme phosphoglycerate kinase comes into play. the enzyme phosphoglycerate kinase comes into play. This enzyme converts This enzyme converts 1,3-bisphosphoglycerate to 1,3-bisphosphoglycerate to 3-phosphoglycerate.
3-phosphoglycerate. Let's note that ATP is being formed in this reaction. Let's note that ATP is being formed in this reaction.
But let's remember that 2 to 1, But let's remember that 2 to 1, 3-bisphosphoglycerate are being formed here, 3-bisphosphoglycerate are being formed here, therefore 2 ATPs are being formed. therefore 2 ATPs are being formed. Then, Then, 3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycerate mutase.
3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycerate mutase. Then, Then, 2-phosphoglycerate is converted to phosphoenolpyruvate thanks to the enzyme enolase. 2-phosphoglycerate is converted to phosphoenolpyruvate thanks to the enzyme analyase. Water is released in this reaction. Water is released in this reaction.
Finally, Finally, the tenth step consists of the conversion of phosphoenolpyruvate to pyruvate. the 10th step consists of the conversion of phosphoenolpyruvate to pyruvate. ATP is formed in this step.
ATP is formed in this step. This reaction is catalyzed by the enzyme pyruvate kinase. This reaction is catalyzed by the enzyme pyruvate kinase.
But let's remember that it is not But let's remember that it is not 1-phosphopyruvate, but 2. 1-phosphopyruvate, but 2. Therefore, Therefore, In this step we consider two ATPs that are being gained. in this step we consider 2 ATPs that are being gained. So, So, we had said that from glucose to glyceraldehyde 3-phosphate 2 ATPs are invested.
we had said that from glucose to glyceraldehyde 3-phosphate two ATPs are invested. But from glyceraldehyde 3-phosphate to pyruvate four ATPs are being obtained. But from glyceraldehyde 3-phosphate to pyruvate 4 ATPs are being obtained. So we could say that the reactions from glyceraldehyde 3-phosphate to pyruvate are considered in the energy gain phase because 4 ATPs are being obtained.
So we could say that the reactions from glyceraldehyde 3-phosphate to pyruvate are considered in the energy gain phase because four ATPs are being obtained. However, However, let's remember the ATPs that were invested. let's remember the ATPs that were invested.
Therefore, Therefore, we could say that the net ATPs are 2 because 4 in total minus 2 invested, we could say that the net ATPs are 2 because 4 in total minus 2 invested, 2 net ATPs. 2 net ATPs. To summarize, To summarize, we have that in glycolysis glucose is catabolized, we have that in glycolysis glucose is catabolized, it is degraded to form 2 pyruvates, it is degraded to form 2 pyrobates, 2 NH and 2 ATPs. 2 NH and 2 ATPs.
Now let's talk about the pathways that pyrobates can follow. Now let's talk about the pathways that pyruvates can follow. Each pyruvate formed in glycolysis can follow two pathways mainly depending on the context of the cell. Each pyrobate formed in glycolysis can follow 2 pathways mainly depending on the context of the cell.
If the cell is in aerobic conditions, If the cell is in aerobic conditions, meaning there is oxygen, meaning there is oxygen, these pyruvates will be used by the mitochondria to finally form 32 ATPs. these pyruvates will be used by the mitochondria to finally form 32 ATPs. However, However, when the cell is in anaerobic conditions, when the cell is in anaerobic conditions, meaning without oxygen, meaning without oxygen, or the cells do not have mitochondria, or the cells do not have mitochondria, such as erythrocytes.
such as erythrocytes, or there is also a high energy demand, or there is also a high energy demand, such as in muscle cells. such as in muscle cells. These pyrobates will enter fermentation and in total we will have to produce only 2 ATPs as a product of the degradation of glucose.
These pyruvates will enter fermentation and in total we will have to produce only 2 ATPs as a product of the degradation of glucose. Do you remember the step of glycolysis when glyceraldehyde 3-phosphate was converted to Do you remember the step of glycolysis when glyceraldehyde 3-phosphate was converted to 1,3-bisphosphoglycerate? 1,3-bisphosphoglycerate? Yes, this step is very important because reduced NAT is being formed here.
Yes, this step is very important because reduced NAT is being formed here. This reduced NAT is very important in fermentation. This reduced NAT is very important in fermentation.
For example, For example, a lactic fermentation. a lactic fermentation. Here, Here, pyruvate is converted to lactate by the enzyme lactate cyruginase.
pyrobate is converted to lactate by the enzyme lactate syruginase. This enzyme requires the reduced N-AT that was formed in that step of glycolysis. This enzyme requires the reduced NAD that was formed in that step of glycolysis.
Note that lactate cyruginase releases oxidized NAT. Note that lactate cyruginase releases oxidized NAT. This oxidized NAT is used again in the seventh step of glycolysis to form reduced NAT again.
This oxidized NAT is used again in the seventh step of glycolysis to form reduced NAT again. Under anaerobic conditions, Under anaerobic conditions, This cycle is repeated constantly in order to recycle the oxidized NAT and the reduced NAT is used to feed the fermentation. this cycle is repeated constantly in order to recycle the oxidized NAT and the reduced NAT is used to feed the fermentation. Another example of fermentation is alcoholic fermentation. Another example of fermentation is alcoholic fermentation.
Here, Here, pyruvate, pyruvate, by means of pyruvate decarboxylase, by means of pyruvate decarboxylase, undergoes decarboxylation to form acetaldehyde. undergoes decarboxylation to form acetaldehyde. This enzyme requires thiamine pyrophosphate and magnesium cations as cofactors. This enzyme requires thiamine pyrophosphate and magnesium cations as cofactors. Since it is a decarboxylase, Since it is a decarboxylase, what is going to happen here is decarboxylation, what is going to happen here is decarboxylation.
meaning carbon dioxide is going to be formed. meaning carbon dioxide is going to be formed. The acetaldehyde that is formed will be converted to ethanol by the enzyme alcohol dehydrogenase.
The acetaldehyde that is formed will be converted to ethanol by the enzyme alcohol dehydrogenase. Here we note again that reduced NAT is being used to release oxidized NAT. Here we note again that reduced NAT is being used to release oxidized NAT. This oxidized NAT will be recycled, This oxidized NAT will be recycled, meaning it will be used in the seventh step of glycolysis in the conversion of glyceraldehyde triphosphate to meaning it will be used in the seventh step of glycolysis in the conversion of glyceraldehyde triphosphate to 1,3-bisphosphoglycerate.
1,3-bisphosphoglycerate. To summarize, To summarize, glycolysis is divided into two phases. glycolysis is divided into two phases the investment phase where two atps are used and the gain phase where four atps are obtained we could say that the products of glucose catabolism by glycolysis are two pyribates two nadh and two net atps The investment phase, where two ATPs are used, and the gain phase, where four ATPs are obtained.
We could say that the products of glucose catabolism by glycolysis are 2 pyruvates, 2 NADH, and 2 net ATPs.