fatty acids FOCUS

Describe steps to breakdown fatty acids

1. glycerol → g3p for glycolysis (requires ATP, get NADH)

2. activate fa: fatty acid + ATP + coA → fatty acyl coA + AMP + PPi

3. B oxidation: produce 1 NADH, 1 FADH, 1 acetyl coA per cycle

   1. dehydrogenation: fatty acyl coA → trans delta² enoyl coA

   2. hydration: trans delta² enoyl coA → L-B hydroxy acyl coA

   3. dehydration: L-B hydroxyl acyl coA →B ketoacyl coA

   4. cleavage: B ketoacyl coA→ acetyl coA + (c ) n-2

Net ATP lost/gained w steps of fatty acid breakdown

Lipolysis: 0

Glycerol to pyruvate: + 2 NADH, +1 ATP

pyruvate to acetyl coA: +1 NADH

CAC: glycerol’s acetyl coA: + 3 NADH, + FADH2, + GTP

activation of fatty acid: - 2ATP

• fa + ATP + coA→ fatty acyl coA + AMP + PPi

B oxidation (1 round): + FADH2, + NADH

Equation for fatty acid activation

fatty acid + ATP + coA → fatty acyl coA + AMP + PPi

PPi → 2 Pi

What enzyme is used to activate fatty acids

fatty acyl coA synthetase

Equation for 1 pass through B oxidation (c16-c14)

(c16) coA + CoA + FAD + NAD+ H2O → (c14) coA + acetyl coA + FADH2 + NADH + H+

Equation for complete B oxidation of palmitoyl CoA

palmitoyl coA + 7 NAD+ + 7FAD + 7 CoA + 7 H2O → 8 acetyl coA + 7 NADH + 7H+ + 7 FADH2

→ palmitoyl CoA + 7 coA + 28 ADP + 28 Pi + 7 O2 → 8 acetyl coA + 28 ATP + 7 H2O

• 7 NADH + 7H+ + 7/2 O2 → 7 NAD+ + 7H2O (makes 7×2.5 ATP)

• 7 FADH2 + 7/2 O2 → 7 FAD + 7 H2O (makes 7×1.5 ATP)

8 acetyl coA → 3(8) NADH + 8 FADH2 + 2(8)CO2 + 8 GTP

• 24 NADH + 24 H+ + 12 O2 →24 NAD+ + 24 H2O (makes 60 ATP)

• 8 FADH2 + 4O2 →8 FAD + 8 H2O (makes 12 ATP) + 8 from GTP

= palmitoyl CoA + 108 ADP + 108 Pi + 23 O2 → coA + 108 ATP + 23 H2O + 16 CO2

What is step 1 of B oxidation w enzyme?

1. dehydrogenation: fatty acyl coa + FAD → trans Δ² enoyl coA + FADH2

Enzyme: Acyl-CoA dehydrogenase

What is the CAC analog to step 1 of B oxidation

succinate dehydrogenase

Step 2 of B oxidation w enzyme

2. Hydration: trans Δ² enoyl acyl coA → L -B hydroxyacyl coA

Enzyme: enoyl coA hydratase

What is the CAC analog to step 2 of B oxidation

fumarase

Step 3 of b oxidation w enzyme

3. Dehydration: L-B hydroxyacyl-coA + NAD+ + H+ → B ketoacyl coA + NADH

Enzyme: B hydroxyacyl coA dehydrogenase

What is step 3 of B oxidation analogous to

malate dehydrogenase

what is step 4 of B oxidation w enzyme

Cleavage: B-ketoacyl coA → acetyl coA + (c )n-2

Enzyme: thiolase

After a triacylglycerol is hydrolyzed, explain how its glycerol is used, and why most of its energy is in the fatty acids

glycerol kinase phosphorylates glycerol to glycerol 3P, then oxidized to DHAP

then isomerized to G3P, enters glycolysis

• glycerol is only 3/50+ carbons in TAG, already partially oxidized

• the 3 FA contain most of carbons, highly reduced carbons release far more energy when ox

Describe the activation of a fatty acid to fatty acyl-CoA, and explain why the reaction costs the equivalent of two ATP.

Fatty acid + coA + ATP → fatty acyl coA + AMP + 2Pi

Fatty acyl-CoA synthetase, in the outer mitochondrial membrane, links the fatty acid

carboxyl group to the thiol of CoA, forming a high-energy thioester.

• 1. fatty acid reacts w ATP → fatty acyl adenylate and PPi

• 2. CoA displaces AMP

• inorganic pyrophosphatase hydrolyzes PPi to 2Pi, makes highly favorable

• Since ATP → AMP, spends equiv of 2 ATP

Describe the carnitine shuttle, and explain why long-chain fatty acids need it but short-chain fatty acids do not.

• Short and medium chain diffuse freely, while long chain cannot—>carnitine shuttle

• Transfer acyl from coA to carnitine → fatty acyl carnitine

• acyl-carnitine/carnitine cotransporter moves fa carnitine to matrix, while moves carnitine out

• in matrix, acyl group transferred back to mitochondrial coA, regen fatty acyl coA

Explain why the cell keeps separate cytosolic and mitochondrial pools of CoA and fatty acyl-CoA, and how malonyl-CoA uses this separation to prevent a futile cycle.

Mitochondrial coA: used mainly for oxidative degradation of pyruvate, FA, AA

cytosolic coA: FA synth, membrane lipid synth

• conversin of carnitine ester commits it to oxidation

Malonyl coA (1st intermed of FA synth) blocks entry of FA into mitochondria

• when cell makes fatty acids, newly made FA not immediately oxidized

** blocks FA from entering carnitine shuttle, prevents oxidation

Describe the four reactions of β oxidation, naming each enzyme and product, and compare them with the citric acid cycle reactions they resemble

1. fatty acyl coA + FAD → trans Δ²-enoyl coA. (acyl coA dehydrogenase)

   1. introduces trans double bond bw α and β carbons

   2. Resembles succinate dehydrogenase

2. trans Δ²-enoyl coA + H2O → L-B-hydroxyacyl-coA (enoyl coa hydratase)

   1. adds water across double bond

   2. Resembles fumarase

3. L-B-hydroxyacyl-coA + NAD+ + H+ → B ketoacyl coA + NADH

   1. (B hydroxyacyl coa dehydrogenase) oxidizes hydroxyl to ketone

4. B ketoacyl coA + coA→ acetyl coA + (c ) n-2

   1. thiolase cleaves in reverse claisen

Stearic acid is a saturated C18 fatty acid. Calculate the number of passes through β oxidation and the number of acetyl-CoA, NADH, and FADH₂ produced from stearoyl-CoA.

9 acetyl-CoA, 8 NADH, and 8 FADH₂

Calculate the total ATP yield from the complete oxidation of stearoyl-CoA, and from free stearate.

From β oxidation: 8 NADH × 2.5 = 20 and 8 FADH₂ × 1.5 = 12, for 32 ATP (4 per pass).

From the citric acid cycle: 9 acetyl-CoA × 10 = 90 ATP. The total from stearoyl-CoA is 122

ATP. Activating free stearate costs two ATP equivalents, so the net yield from stearate is

120 ATP.

Explain what metabolic water is and why it is important to hibernating animals and camels

NADH + H+ + ½ O2 → NAD+ + H2O

→ each pair of e- that pass from NADH or FADH2 to O2 = 1 H2O (metabolic water)

• complete ox of palmitoyl-coA yields 23 H2O

• because fats highly reduced, oxidizing produces large amount of water

• Hibernating animals rely on it to meet needs

Name the three ketone bodies, and describe the reactions in liver mitochondria that make them from acetyl-CoA.

Acetoacetate, D-β-hydroxybutyrate, and acetone.

1. Thiolase condenses 2 acetyl coA to make acetoacetyl coA (reverses last step of B ox)

2. HMG-CoA synthase adds 3rd acetyl coA to make HMG- CoA

3. HMG-CoA lyase cleaves HMG-CoA to free acetoacetate and acetyl coA

Acetoacetate

• can be reduced to form D-B-hydroxybutyrate (D-B hydroxybutyrate dehydrogenase) using NADH

• Can be decarboxylated to form acetone (acetoacetate decarboxylase)

Describe how D-β-hydroxybutyrate is used as fuel in an extrahepatic tissue such as heart or skeletal muscle.

D-β-hydroxybutyrate dehydrogenase oxidizes D-β-hydroxybutyrate back to acetoacetate, producing NADH

• B-ketoacyl-coA transferase activates acetoacetate to acetoacetyl-coA

• Thiolase cleaves acetoacetyl-coA to 2 acetyl coA → CAC

Explain why the liver produces ketone bodies but does not use them, and why this arrangement is useful to the body

• Liver lacks B-ketoacyl-coA transferase (can’t activate acetoacetate →acetyl coA)

• producer, not consumer of ketone bodies

• allows liver to oxidize FA for own energy, exports excess acetyl-coA as glucose

What enzyme converts acetoacetyl coA to 2 acetyl coA

thiolase

what enzyme converts acetoacetate to acetoacetyl coA

b-ketoacyl coA transferase

Starting from acetyl coA, how is acetoacetate formed?

2 acetyl coA ←> acetoacetyl coA (enzyme: thiolase)

acetoacetyl coA → HMG-coA (HMG-coA synthase)

HMG-coA →acetoacetate (HMG-CoA lyase)

How is D-B-hydroxybutyrate forrmed from acetoacetate

Reduced by NADH

acetoacetate + NADH → D-B-hydroxybutyrate + NAD (enzyme: D-B hydroxybutyrate dehydrogenase)

How is acetone formed from acetoacetate

Decarboxylated

acetoacetate → acetone + CO2 (enzyme: acetoacetate decarboxylase)

Person w untreated T1 diabetes has high blood glucose, but can’t take it up into muscle, adiapose

• why FA oxidation in liver increases? Why acetyl-coA accumulates rather than entering CAC?

Without insulin, tissues behave as if starving even though high blood glucose

• fat stores mobilized, liver oxidizes more FA

• FA synth is low, so malonyl-coA is low, carnitine shuttle open

At same time, liver does gluconeogenesis, draws OAA away

• without enough OAA, acetyl coA from B-ox can’t enter CAC, accumulates

Explain why ketone bodies are overproduced and why extrahepatic tissues cannot use them all.

Accumulating acetyl-CoA accelerates ketone body formation in the liver.

• Production > capacity of extrahepatic tissues to oxidize, so ketone bodies build up

Define ketosis, acidosis, and ketoacidosis, and explain why ketoacidosis

can be dangerous

Ketosis: high levels ketone bodies in blood, urine

acidosis: lowered blood pH (can be caused by inc acetoacetate, D-B-hydroxybutyrate)

ketoacidosis: both ketosis and acidosis

• severe ketoacidosis overwhelms blood buffering capacity, can disrupt organ func —> coma/death

Explain how the same pathway helps a person survive prolonged starvation

In prolonged starvation, the liver converts fatty acids into ketone bodies, which the

brain and other tissues use instead of glucose.

• reduces need for glucose from AA (spares muscle protein)

• ketone production matched to use —> ketosis w/o dangerous acidosis helps survive