Ketone Bodies

This is the Outline of the pathway for keto-body formation and usage.

we are going to use this throughout the lecture to organize our thoughts.

The lecture is going to be divided into 5 subsections.

Etymology (helps memory)

• Ketone → from German “Keton” (related to acetone)

• Acetoacetate

  • “Aceto” = acetyl group

  • “acetate” = derived from acetic acid

• β-hydroxybutyrate

  • “hydroxy” = OH group

  • “butyrate” = 4-carbon chain (butyric acid = butter acid)

• Acetone

  • simplest ketone (volatile)

Ketone bodies = alternative fuel molecules made by the liver when glucose is low

ketone bodies are an alternative fuel for cells.

1. acetyl-CoA is transformed into ketone bodies.

2. ketone bodies are converted back into acetyl-CoA

3. acetyl-CoA is able to go back into the Kreb’s Cycle.

Ketone bodies have several properties that make them a perfect ideal alternative fuel for the cells when there is no glucose available (during fasting metabolic states).

Important Properties

1. water soluble

2. freely transported in blood

3. produced by the liver

4. used in the proportion of the concentration they have in the blood

5. Ketone bodies serve as energy sources for most tissues during carbohydrate deprivation

During Starvation

Acetyl-CoA comes from 3 places:

1. Amino Acid Catabolism (protein catabolism)

2. Glycolysis (carbohydrate catabolism)

3. Fatty Acid Oxidation (fatty acid catabolism)

All three of these lead to the production of Acetyl-CoA

During carbohydrate starvation, the liver is flooded with acetyl-CoA from the amino acid catabolism, from glycolysis, from fatty acid oxidation.

However, during the starvation (no glucose coming in) the oxaloacetate in the liver depleted due to gluconeogenesis. (gluconeogenesis uses oxaloacetate, OAA: “entry ticket” for acetyl-CoA into Krebs cycle. Problem: Oxaloacetate gets used up, LOW OAA)

5. Meanwhile… fat is being burned like crazy

• Fat → fatty acids

• Fatty acids → β-oxidation → lots of acetyl-CoA

6. Acetyl-CoA tries to enter Krebs cycle…

But:

• No OAA = no entry

• Krebs cycle slows down

Think: Acetyl-CoA + OAA → citrate (this step is blocked)

7. So acetyl-CoA builds up

Now the liver is like: I have too much acetyl-CoA and nowhere to put it.

8. Solution: convert to ketone bodies

Liver turns excess acetyl-CoA into:

• Acetoacetate

• β-hydroxybutyrate

• Acetone

9. Send ketones to the body

• Brain uses ketone bodies

• Muscles use ketone bodies

This saves glucose

Therefore, Acetyl-CoA cannot enter into the Kreb’s cycle and therefore is converted into ketone bodies.

What is Ketogenesis?
Ketogenesis is the production of Ketone bodies and it occurs strictly in the liver.

Here we have the overall diagram but we are going to break it down one by one.

1. During Starvation

   Acetyl-CoA comes from 3 places:

   1. amino acid catabolism (protein catabolism)

   2. Glycolysis (carbohydrate catabolism)

   3. Fatty Acid Oxidation (fatty acid catabolism)

   all three of these lead to the production of Acetyl-CoA

2. 2 acetyl-CoA → 1 Aceto-Acetyl-CoA (thiolase enzyme)

If there is a large amount of acetyl-CoA, the cell begins to condense 2-carbon units together.

Think of acetyl-CoA as many loose 2-carbon pieces.
The first logical step is to join two of them into a 4-carbon intermediate.

Why is thiolase used? “Thio” Sulfur, it takes the SH-CoA off of Acetyl-CoA

Thiolase is an enzyme that can work in reversible carbon-carbon bond formation/cleavage involving acetyl-CoA units.

So the reasoning is: “We have excess 2-carbon acetyl groups. Let’s combine two of them to begin building a ketone body precursor.

3. Aceto-acetyl-CoA + another acetyl-CoA → HMG-CoA

Enzyme: HMG-CoA synthase

Reaction idea:

Acetoacetyl-CoA + acetyl-CoA → HMG-CoA
(3-hydroxy-3-methylglutaryl-CoA)

Why add another acetyl-CoA?

Now the pathway is building a larger intermediate that can be rearranged and split into the first true ketone body.

This step takes the 4-carbon acetoacetyl-CoA and adds one more 2-carbon unit, producing a 6-carbon intermediate.

Why is HMG-CoA important?

HMG-CoA is the committed ketogenesis intermediate in liver mitochondria.

This step is basically: “Let’s convert excess acetyl-CoA into a specialized molecule designed to be split into ketone bodies.”

Clinical/high-yield note

HMG-CoA synthase is the key regulated enzyme of ketogenesis.

Again, here is a different type of diagram for you to have the chemical structures of the compounds that go into this ketone body formation.

thiolase (I think this is step 1 of ketone body formation)?

1. 2 acetyl-CoA → 1 Aceto-Acetyl-CoA (thiolase enzyme)

Thiolase takes 2-Acetyl-CoA and converts them and combines them to form aceto-acetyl-CoA.

This is a reversible reaction.

(step 2)

The next step in ketogenesis is carried out by the HMG-CoA synthase

it takes the aceto-acetyl-CoA, binds it to another molecule of acetyl-CoA to form HMG-CoA.

-step 2 is the rate limiting step of Ketogenesis and this only present in significant quantities in the liver, therefore, the liver is the only organ that can produce these ketone bodies.

Next step (step 3)

The next step takes the HMG-CoA and converts it into aceto-acetate. This is done by the enzyme called HMG-CoA lyase.

Lyase = enzyme that breaks a bond WITHOUT using water or ATP

The aceto-acetate is the first ketone body that is produced by the liver and can already be exported into the bloodstream for the usage by peripheral tissues.

Ketone Bodies Interconversion

Aceto-Acetate (first ketone body produced) however, can be converted into another ketone body and into acetone, and we will see how that happens in the next slide.

Aceto-acetate into acetone

Aceto-acetate can be converted into D-B-hydroxybutyrate and into acetone by B-hydrocarboxybutarate dehydrogenase .

B-hydrocarboxybutarate dehydrogenase

-makes D-B-hydroxybutyrate

-dehydrogenates NADH

The equilibrium between aceto-acetate and D-B-hydroxybutyrate is determined by the NAD+/NADH equilibrium in the cell.

NADH drives reduction → makes β-hydroxybutyrate
NAD⁺ drives oxidation → makes acetoacetate

Aceto-acetate released into blood

Acetone is formed spontaneously and it’s a molecule that is formed through the breathe of the patient.

Acetoacetate is unstable because it is a β-keto acid, and at physiological conditions it can spontaneously decarboxylate to form acetone.

CO₂ is released in this reaction and is transported in the blood to the lungs, where it is exhaled.

Acetone is also exhaled, because it is is volatile and cannot be used for energy, so it is released into the bloodstream and excreted through the lungs, causing a characteristic fruity breath.

Ketolysis

Next is ketolysis. (peripheral tissues metabolizing ketone bodies, turning them BACK into acetyl-CoA)

1. The ketone bodies have been produced in the liver

2. The ketone bodies have been exported into the bloodstream

3. The peripheral tissues (skeletal muscles, renal cortex of the kidney, and brain) can use those ketone bodies as an alternative source of energy

The main tissues that use ketone bodies are skeletal muscles, renal cortex, and the brain (if the brain really needs to).

Ketolysis continued

This process is depicted here. depending on what ketone body enters the cell, if it’s 3-hydrobutarate or aceto-acetate that have to be interconverted as we saw in the previous slide.

2. Then the aceto-acetate has to be converted into acetoacetyl-CoA (though the enzyme Succinyl-CoA acetoacetate CoA transferase) and it’s helped by the Succinyl-CoA molecule.

Succinyl-CoA ketolysis is a crucial metabolic pathway in peripheral tissues (brain, kidney, muscle) that converts ketone bodies (acetoacetate) back into acetyl-CoA for ATP energy production, using succinyl-CoA as a CoA donor.

2. acetoacetyl-CoA gets converted into 2 molecules of acetyl-CoA through thiolase

3. 2 acetyl-CoA goes into the Kreb’s Cycle in the mitochondria of the peripheral tissues

-Therefore, organs and cells that don’t have mitochondria cannot do the kreb’s cycle and therefore cannot use ketone bodies as a source of energy.

Just to break it down into smaller steps, here is the conversion of acetoacetate into 3-hydroxybutarate

-this was the first reaction on the bottom of the last slide.

Here is the conversion from aceto-acetyl-CoA to aceto-acetate, and this is a reversible reaction.

But the good thing about this reaction in the cells is the aceto-acetyl-CoA is actively removed to the next reaction very quickly.

question

the correct answer is C) in most cells in the mitochondria for the Kreb’s cycle

-this is a Kreb’s cycle enzyme

after the aceto-acetyl-CoA is converted into the two molecules of acetyl-CoA through the enzyme thiolase, these acetyl-CoAs can then go into the Kreb’s Cycle for energy production.

question

name two types of cells that cannot use ketone bodies as an energy source:

1. red blood cells (because they don’t have mitochondria)

2. hepatocytes (because they don’t have the succinyl-coa-acetoacetate CoA transferase enzyme)

metabolic state during starvation and diabetes

very important in disease processes, it’s important for you to know these three concepts.

1. Ketonemia: the rate of ketone body formation is greater than the use, therefore the levels begin to rise.

2. ketonuria: ketone bodies are found in urine.

3. Ketoacidosis: severe acidosis in the blood of the patient due to increased circulating bodies.

untreated diabetes

during uncontrolled diabetes or prolonged states of fasting what happens is decrease in insulin and an increase in glucagon and there is an increase of fatty acid synthesis in plasma, and so the hepatic output of ketone bodies is increased and therefore the patient suffers from ketoacidosis.

-this is something that you need to be aware of for untreated diabetes.