Lecture 18: Metabolism of Fat & Ketone Bodies Flashcards by Emma Djabali

What is the purpose of β oxidation of FAs?

Acetyl-CoA synthesis for ATP generation

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What is the most common FA synthesized by our body? How many Cs?

Palmitic acid: 16 Cs

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How many acetyl-CoAs generate per 1 palmitic acid? How many ATPs?

8 acetyl CoAs

108 ATPs

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What is the net equation of the beta oxidation of palmitic acid?

Palmitic acid + 7 NAD+ + 7 FADH + 7 H2O + 7CoA-SH = 8 acetyl CoA + 7NADH + 7H+ + 7FADH2

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Saturated FA β Oxidation Step 1: Enzyme?

Acyl-CoA dehydrogenase

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Saturated FA β Oxidation Step 1: Substrates/Products?

Palmitoyl-CoA + FAD = trans-Δ^2-enoyl-CoA + FADH2

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Saturated FA β Oxidation Step 1: Coenzyme?

FAD

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Saturated FA β Oxidation Step 1: Where is acyl-CoA dehydrogenase located?

Mito inner membrane

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Which saturated FA β Oxidation enzyme has 3 isozymes? Why?

Acyl-CoA dehydrogenase: for different FA chain lengths: very long, medium, and short

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Saturated FA β Oxidation Step 1: describe the oxidation of palmitoyl-CoA in detail.

Double bond formed between alpha and beta C: point of unsaturation is now formed

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Saturated FA β Oxidation: what nomenclature change occurs during this reaction? Which step?

Alpha and beta Cs are now called Δ2 and Δ3

Steps 1 and 3

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Saturated FA β Oxidation Step 2: Enzyme? What is this enzyme similar to?

Enoyl-CoA hydratase, similar to fumarase in TCA

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Saturated FA β Oxidation Step 2: Substrates/Products?

trans-Δ^2-enoyl-CoA + H2O = L-β-Hydroxy-Acyl-CoA

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Saturated FA Beta Oxidation Step 2: Coenzyme?

Water

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Saturated FA Beta Oxidation Step 1: Rxn type?

Oxidation reaction

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Saturated FA Beta Oxidation Step 2: Rxn type?

Hydration

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Saturated FA β Oxidation Step 2: describe the hydration of trans-Δ^2-enoyl-CoA in detail.

OH added to Δ3 C = βC
H added to Δ2 C = αC
(no more double bond)

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Saturated FA Beta Oxidation Step 2: main purpose?

Setting the stage for ketone group addition and departure of acety-CoA in step 3

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Saturated FA Beta Oxidation Step 3: Enzyme?

β-Hydroxyl-Acyl-CoA dehydrogenase

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Saturated FA Beta Oxidation Step 3: Substrates/Products?

L-β-Hydroxy-Acyl-CoA + NAD+ = β-Ketoacyl-CoA + NADH + H+

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Saturated FA Beta Oxidation Step 3: Coenzyme?

NAD

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Saturated FA Beta Oxidation Step 3: Rxn type?

Oxidation

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Saturated FA Beta Oxidation Step 3: describe the oxidation of L-β-Hydroxy-Acyl-CoA in detail.

β C-OH => β C=O (1 electron taken from the β C and 1 from the O)

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Saturated FA Beta Oxidation Step 4: Enzyme? 2 names

Acyl-CoA Acetyl Transferase = Thiolase

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Saturated FA Beta Oxidation Step 4: Substrates/Products?

β-Ketoacyl-CoA +CoA-SH = Acetyl-CoA + myristoyl-CoA

Saturated FA Beta Oxidation Step 4: Coenzyme?

None

Saturated FA Beta Oxidation Step 4: Rxn type?

Nucleophilic substitution

What is the result of 1 round of FA beta oxidation?

FA chain is reduced in length by 2Cs and 1 acetyl-CoA is expelled

How does the the β oxidation of unsaturated cis FAs occur?

1. Initial β oxidation steps occur up until the cis double bond 2. Δ3,Δ2-enoyl-CoA isomerase moves the double bond 1 C closer to C=O so it becomes trans 3. β oxidation resumes

How does the the β oxidation of odd number FAs occur?

1. Usual β oxidation steps | 2. Last step: propionyl-CoA (3 Cs) is released with acetyl-CoA

What kind of FA is oleic acid (aka olive oil)?

Mono-unsaturated FA

What happens to the propionyl-CoA released through the β oxidation of odd number FAs?

1. Propionyl-CoA carboxylase converts it to methylmalonyl-CoA (4 Cs) using carbonic acid, biotin, and ATP 2. Methylmalonyl rearrangement to convert it to succinyl-CoA (using B12) 3. Succinyl-CoA feeds into the TCA

Are all FAs non-anaplerotic and CANNOT contribute to gluconeogenesis (do not form intermediates of a metabolic pathway)?

NOPE, odd numbered ones are

Where are ketone bodies synthesized?

Liver mitochondria exclusively

What happens to the ketone bodies produced by the liver once they arrive at target tissues?

Converted back to acetyl-CoA to enter TCA

What are the 3 types of ketone bodies? Which one is predominant in the blood?

1. Acetoacetate 2. β-hydroxybutarate (predominant in blood) 3. Acetone

Which 2 ketone bodies are produced from the 3 one?

β-hydroxybutarate and acetone are reduced forms of acetoacetate

Which ketone body cannot be used as metabolic fuel?

Acetone

How do ketone bodies affect pH? Why?

Very negative so reduce pH

Under what conditions does ketogenesis occur?

Fasting = TCA cycle intermediates are being used for gluconeogenesis and FA β-oxidation is high = acetyl CoA builds up

Describe the 4 steps of ketogenesis to generate all 3 types of ketone bodies.

1. Thiolase: condenses 2 acetyl-CoA together (reverse of last step of FA β-oxidation) = acetoacetyl-CoA 2. HMG-CoA synthase activated: acetoacetyl-CoA + acetyl-CoA = HMG-CoA 3. HMG-CoA lyase: HMG-CoA = acetyl-CoA + acetoacetate 4a. β-hydroxybutarate dehydrogenase: NADH + H+ + acetoacetate = β-hydroxybutarate 4b. Acetoacetate decarboxylase: acetoacetate = acetone + CO2 (also can happen spontaneously)

Does the first acetyl-CoA added during ketogenesis end up in the final product?

NOPE

Can the brain use FAs for energy? Why/Why not?

NOPE because they are bound to albumin in plasma and so do not traverse the blood-brain barrier

How to calculate the # of rounds of beta oxidation needed?

of acetyl-CoAs produced - 1

From what macromolecule does most of the acetyl CoA used in ketogenesis come?

FAs

Describe the 3 steps of ketone bodies breakdown to generate acetyl-CoA.

1. β-hydroxybutyrate dehydrogenase: ketone body + NAD+ = acetoacetate + NADH 2. β-ketoacyl-CoA transferase: succinyl-CoA + acetoacetate = acetoacetyl-CoA + succinate 3. Thiolase: acetoacetyl-CoA = 2 acetyl-CoA

What is the only organ that can perform ketogenesis? Why?

Liver because it has the enzymes

How does the brain switch from using glucose to ketone bodies?

Starvation activates β-ketoacyl-CoA transferase

What portion of brain energy demands can be met by ketone bodies during starvation?

1/3 to 2/3

Other than for starvation energy, why else are ketone bodies generated?

Detoxification of too many acetyl-CoAs as ketone bodies can be: 1. Excreted by kidneys 2. Sweated out 3. Released in small intestine via bile salts 4. Exhaled

Where are FAs synthesized?

Cytoplasm

Where does the acetyl-CoA used for FA synthesis come from?

Glycolysis and PDC: from mito ONLY

Can acetyl-CoA cross the mito inner membrane?

NOPE

How is acetyl-CoA transferred form the mito to the cytoplasm? Describe the 3 steps.

Citrate shuttle: 1. Citrate synthase: acetyl CoA + oxaloacetate = citrate (TCA step 1) 2. Citrate transported out of matrix via citrate transporter 3. Citrate lyase: citrate + ATP = oxaloacetate + acetyl-CoA

After transporting acetyl-CoA out of the mito matrix, how does oxaloacetate go back in? 2 ways.

1. Converted to malate and then via malate-alpha-keto-glutarate antiporter 2. Conversion to malate and then to pyruvate by malic enzyme and then via pyruvate transporter

What do malic enzymes use to convert pyruvate to malate and vice versa?

NADP/NADPH

What is the main enzyme used in FA synthesis?

FA synthase

What are the 2 main parts of FA synthase?

1. Acyl carrier protein (ACP) | 2. 4'-phosphopantetheine chain (same as CoA but it has ACP instead of AMP nucleotide)

What is the role of the acyl carrier protein (ACP) in FA synthase?

Point of attachment of acyl chain

What are the 3 main difference between FA oxidation and synthesis?

1. Oxidation vs reduction 2. Different enzymes 3. Acetyl-CoA released vs malonyl-CoA added

Describe the steps of FA synthesis.

1. Acetyl-CoA carboxylase: acetyl CoA = malonyl CoA 1a. Biotin carboxylase: biotin arm + ATP + HCO3- = carboxybiotin 1b. Transcarboxylase: carboxybiotin + acetyl-CoA = malonyl-CoA 2. Acetyl-CoA + ACP = Acetyl-ACP (KS subunit) + CoA-SH 3. Malonyl CoA + ACP = Malonyl-ACP + CoA-SH (phosphopantetheine chain) 4. Malocyl-ACP + acetyl-ACP = CO2 + β-ketoacyl-ACP 5. β-ketoacyl-ACP + NADPH = hydroxyacyl-ACP + NADP 6. hydroxyacyl-ACP = enoyl-ACP + H2O 7. enoyl-ACP + NADPH = 4C FA chain + NADP REPEAT!

What FA synthesis step is the one regulated by hormones?

Step 1

What hormones is step 1 of FA synthesis regulated by?

Adrenaline and insulin

From what end of the FA do we take off Acetyl-CoAs in beta oxidation?

Delta end = where the CoA is

What are the 2 ways in which TAGs can enter glycolysis?

1. Odd numbered FAs make succinyl-CoA which can feed into gluconeo 2. Glycerol can be converted to glycerol-3-phosphate which can be converted to DHAP

Why can't the liver use ketone bodies?

It lacks the enzyme β-ketoacyl transferase (step 2 of ketone body breakdown)

What kind of people exhale extra acetone?

People on a diet (like Atkins) who do not eat carbs so are lacking TCA intermediates and create a lot of acetone to get rid of the excess acetyl-CoA

Do people with favism lack enough NADPH to perform biosynthesis?

No, because they can create NADPH through the conversion of malate to pyruvate (but not from the PPP)