Catabolism of Fats: Glycerol Metabolism: Study with Video Lessons, Practice Problems & Examples

Glycerol metabolism involves the catabolism of fats, primarily for ATP production through glycolysis. It begins with the hydrolysis of triacylglycerol into glycerol and fatty acids. Glycerol is then phosphorylated to glycerol 3-phosphate using ATP, followed by oxidation to dihydroxyacetone phosphate (DHAP) via dehydrogenase, reducing NAD⁺ to NADH. DHAP can enter glycolysis, leading to pyruvate formation, which can further undergo aerobic respiration to produce Acetyl CoA, NADH, FADH₂, and ATP.

Glycerol Metabolism Concept 1

Everyone. In this video, let's talk about glycerol metabolism. Now here we're going to say that this is involved in the catabolism of fats. We're going to say the main purpose of glycerol metabolism is ATP production through glycolysis. If we take a look here, we have stages 1 and 2.

We're going to hydrolyze our lipids here, which gives us our fatty acid plus glycerol. Paying attention to glycerol, we then go into glycerol metabolism, where it's converted to DHAP. That can undergo glycolysis in order to form pyruvate. Remember, in glycolysis here, we will make our high energy molecules of NADH and ATP. DHAP is oxidized through the use of NAD⁺.

NAD⁺, as a result of this oxidizing of DHAP, is reduced to NADH. Right? So just remember, those are cornerstones of glycolysis. We know that we're using NAD⁺ on our substrate. That NAD⁺ becomes NADH.

We also make ATP. And then here, we make our pyruvate. This pyruvate, if we have oxygen available, would undergo aerobic respiration going to Acetyl CoA formation, and then stages 34 to make NADH, FADH₂, and ATP at the end. Alright. So just keep that in mind when we're talking about glycerol metabolism.

Glycerol Metabolism Concept 2

Everyone. Let's take a look at the stages of glycerol metabolism. Here, we're going to say that glycerol metabolism begins with the hydrolysis of triacylglycerol (TAG) as a molecule, and it's hydrolyzed into glycerol and 3 fatty acids. The newly released Glycerol then travels to the liver to undergo 2 biochemical reactions. Now, reaction 1, we're gonna say is irreversible and consumes 1 ATP for energy.

Here, we consume 1 ATP, so glycerol is transitioned into Glycerol 3-Phosphate. Here, we've added a phosphate group to the 3rd carbon. This phosphate group is inorganic phosphate. So this will just represent inorganic phosphate.

We're gonna say reaction 2 then changes that Glycerol 3-Phosphate into DHAP. So reaction 2 results in dihydroxyacetone phosphate, which is DHAP. This one is reversible in nature. In this one, we have NAD⁺ being reduced to NADH, and as a result, that's what helps to create our DHAP at the end. Alright.

So just remember, glycerol metabolism, this is an overview, a big view of it. It's 2 stages, 1 and 2. The first one is irreversible. The second one is just transferring or changing of glycerol 3-phosphate into DHAP. Alright.

So just keep that big picture in your mind as you delve a little bit deeper into each one of these steps.

Glycerol Metabolism Example 1

Take a look at this example question. Here it says, "Which of the following statements about glycerol metabolism is true? It doesn't involve a phosphorylation step." Well, step 1, we convert glycerol to Glycerol 3-phosphate. We're adding a phosphate group, so that is a phosphorylation step.

The first step is energy-producing in nature. No, the first step is energy-consuming. We have to use 1 ATP for energy. A source of glycerol is from the triacylglycerol molecule that must first undergo hydrolysis.

That is true. Triacylglycerol or TAG has to undergo hydrolysis so that it breaks up into glycerol and our three fatty acids. So this is true. The second step is energy-consuming in nature. No.

The second step is not energy consuming. It doesn't require an ATP molecule for energy. That's step 1 or stage 1. So the only statement here that's true is option C.

Glycerol Metabolism Concept 3

Hey everyone. Let's take a look more deeply at the 2 stages or steps of Glycerol Metabolism. In 1, we have phosphorylation. We're going to say the enzyme Glycerol Kinase catalyzes the phosphorylation of Glycerol. Now remember, kinases are just helping to transfer a phosphate group from one molecule to another.

Here it uses ATP as a source of energy and inorganic phosphate, which we're going to show as this symbol. Now, here we have our glycerol. We're going to utilize ATP, we need to consume 1 ATP. Doing that changes ATP to ADP. We'd also use here magnesium ion to help facilitate this change.

And as a result, the hydrogen here on glycerol will get changed into our inorganic phosphate. So that becomes Glycerol 3 Phosphate. Right? So again, we're going to say here that our reaction 1 is a phosphorylation. This is just the first step or stage of our Glycerol Metabolism.

Glycerol Metabolism Concept 4

Now the second reaction of glycerol metabolism is an oxidation reaction. Here, we're going to say that dehydrogenase enzyme is what oxidizes glycerol 3-phosphate into dihydroxyacetone phosphate or DHAP. In this reaction, 1 NAD⁺ is reduced to 1 NADH. If we take a look here, we have glycerol 3-phosphate, so here's our inorganic phosphate on carbon 3. We're going to utilize this enzyme.

NAD⁺ is reduced to NADH. We also use our zinc ion to help with this reaction. Here, what happens is we're going to oxidize this alcohol group of Glycerol 3-Phosphate to a carbonyl group, and in doing so, we create DHAP. Now, recall that DHAP can also then directly enter reaction 5 of glycolysis. Right?

So just remember, when it comes to glycerol metabolism, reaction 1 is a phosphorylation reaction and this reaction 2 represents an oxidation reaction.

Glycerol Metabolism Example 2

We take a look here at this example question that says, which of the following statements is true in regard to Glycerol Metabolism? Here, the first step involves the use of a kinase that phosphorylates a secondary alcohol group. Remember, we're changing glycerol to glycerol 3-phosphate; that alcohol represents not a secondary alcohol, but a primary alcohol. So this would not be true. NAD⁺ represents a high-energy molecule produced during step 2.

No, it's not NAD⁺ that represents the high-energy molecule produced; instead, it's NADH. An isomerase enzyme could be used for the conversion of Glycerol 3-Phosphate to DHAP. Now remember, an isomerase would just create another isomer of glycerol from Glycerol 3-Phosphate. That would mean that we have the same number of elements in both compounds, but we're not using an isomerase.

We're doing an oxidation, so we're using a dehydrogenase. We're actually losing elements. DHAP and Glycerol 3-Phosphate do not have the same molecular formula; therefore, they're not isomers of each other. So an isomerase would not have been used. So this leaves "d" as our answer.

Here, the ATP molecule serves as a phosphate source to phosphorylate a primary alcohol group. That's correct. Remember, we're going from glycerol to glycerol 3-phosphate. We are adding an inorganic phosphate to a primary carbon. It was a primary alcohol when we phosphorylated it.

So here, the only answer that's correct would have to be option "d".

Which of the following outlines the overall pathway of glycerol metabolism? 

• A. (1) Phosphorylation (2) Decarboxylation (3) Reduction
• B. (1) Cleavage of a triacylglycerol (2) Phosphorylation (3) Oxidation
• C. (1) Carboxylation (2) Phosphorylation (3) Decarboxylation
• D. (1) Oxidation & Decarboxylation (2) Isomerization (3) Cleavage of a triacylglycerol (4) Hydration

Which of the following represents the complete chemical reaction for the two stages of glycerol metabolism?

• A. Glycerol  +  NAD⁺  +  ADP ⟶ Glycerol-3-phosphate  +  NADH  +  ATP
• B. Glycerol  +  NAD⁺  +  ATP ⟶ DHAP  +  NADH  +  H⁺  +  ADP
• C. Glycerol-3-phosphate  +  NADH  +  H⁺ ⟶ Glycerol  +  ADP  +  P_i
• D. Fatty acid + NaOH + H₂O ⟶ Glycerol  +  NADH  +  H₂