Fatty Acid Metabolism

Question 1

What is palmitate?

Answer 1

A physiological saturated fatty acid (C₁₆H₃₂O₂). Alkyl group + carboxyl group.

Question 2

How are long chain fatty acids removed from glycerol?

Answer 2

Via the activity of lipases.

Question 3

What happens to palmitate during β oxidation?

Answer 3

First, palmitate is activated by CoA binding to the carboxyl group. Following this the chain is cleaved at every second carbon 7 times (if there is 16 carbons). Each cleavage is a ‘round’ of β oxidation and produces 1 NADH and 1 FADH2 (for ATP synthesis) and an acetyl CoA. A 16 carbon fatty acid will produce 8 acetyl CoA, 8 NADH, 8 FADH.

Question 4

How are fatty acids activated?

Answer 4

Adenylate synthase catalyses the covalent attachment of phosphoryl group to an O on the carboxyl group forming acyl adenylate (the AMP is formed from ATP hydrolysis releasing 2 x Pi). Then the CoA binds by the catalytic activity acetyl CoA synthetase replacing the AMP and forming acyl CoA. The CoA interacts with the AMP via the sulfhydryl group within the CoA complex displacing the AMP.

Question 5

What is the first step of β oxidation?

Answer 5

Acyl dehydrogenase removes a hydrogen atom each from carbon 2/α and 3/β allowing a double bond to form between these carbons. (the β carbon is the carbon bound to the acyl CoA group to be cleaved that stays on the chain). The new structure is called trans-Δ2-enoyl CoA and FADH2 is formed from FAD harvesting the electrons from the C-H bonds to bond with hydrogen.

Question 6

What is the second step of β oxidation?

Answer 6

The double bond is hydrated (3’ CH = CH 2’ +H2O -> 3’ CHOH = CHH 2’) by enoyl CoA hydratase forming 3-L-hydroxyacyl-CoA.

Question 7

What is the third step of β oxidation?

Answer 7

3-L-hydroxyacyl-CoA dehydrogenase removes the hydrogens from the 3’ carbon forming 3-ketoacyl CoA and generating NADH from NAD+ harvesting the electrons and hydrogens from the bonds.

Question 8

What is the fourth step of β oxidation?

Answer 8

The terminal acyl-CoA group is cleaved in a thiolysis reaction by the CoA being catalysed by β keto thiolase, forming a new acyl Co-A and giving acetyl CoA from the cleaved group.
(acyl = has an R group on the end acetyl = CH3 on the end)

Question 9

What is the purpose of β oxidation / fatty acid catabolism?

Answer 9

The produced acetyl CoA can enter the TCA cycle. This response is usually when oxaloacetate is low.

Question 10

How are ketone bodies made?

Answer 10

Acetyl CoA x 3 condense -> acetoacetate
The product is then either converted to
⇌ D-3-hydroxybutyrate
or
-> acetone
Both of which are ketone bodies that can be used as fuels mostly by muscles and kidneys. The kidneys especially rely on acetone because if they relied on glucose it would be lost in urine.

Question 11

Explain the metabolic states / conditions when ketone bodies are made?

Answer 11

Under conditions of prolonged fasting or type II diabetes oxaloacetate is consumed to produce glucose. This reduces the availability of oxaloacetate for the TCA cycle which leads to the accumulation of acetyl-CoA (in the liver). When a threshold of accumulation of acetyl CoA is reached ketone bodies are made to be released into the blood to be used as fuel by other cells (especially cardiac muscle and renal cortex cells

Question 12

What is the atom economy of 1 β oxidation.

Answer 12

1 FADH2 ~ 1.5 ATP (first step)
1 NADH ~ 2.5 ATP (third step)
= 4 ATP -1 ATP invested at the start
= 3 ATP
(btw 1 GTP = 1 ATP)

Question 13

How does β oxidation in unsaturated fatty acids?

Answer 13

The double bond must be saturated for β oxidation to occur. This is done by transporting the unsaturated fatty acid into the mitochondria (like normal and the saturated parts undergo β oxidation) where it is isomerised and reduced to give a saturated cis enoyl.

Question 14

If the FA has an odd number of carbons what happens?

Answer 14

β oxidation occurs as normal but the remaining carbon group is converted to succinyl CoA to enter the TCA cycle.

Question 15

What is the first step of FA synthesis?

Answer 15

An acetyl CoA is carboxylated converting it to malonyl CoA by acetyl-CoA carboxylase (ACC), this requires 1 ATP. The substrates of FA are acetyl-CoA, malonyl-CoA and NAHPH

Question 16

What enzyme facilitates FA synthesis?

Answer 16

Fatty acid synthase: a homodimer, each protein has 3 domains and 7 catalytic sites. As its a homodimer each FA synthase can produce two FAs simultaneously. The reactions the enzyme catalyses are: condensation -> reduction -> dehydration -> reduction

Question 17

What does the cysteine part of FA synthase do?

Answer 17

It holds the elongating FA chain in place.

Question 18

What is required to make palmitate (C₁₆H₃₂O₂)?

Answer 18

1 x acetyl CoA
7 x malonyl-CoA (from 7 acetyl CoA, one ATP per carboxylation)
Each addition of malonyl- CoA requires 2 NADPH -> palmitate requires 14 NADPH

Question 19

Where does biosynthesis of FA occur?

Answer 19

In the cytosol, on/near the surface of the ER membrane as FAs are hydrophobic.
beta oxidation is in the mitochondria

Question 20

How are fatty acids further elongated after the activity of FA synthase?

Answer 20

Elongation occurs on/along the ER membrane by elongases attached to the membrane. the reactions are similar to the cytosolic FA synthesis but FA is attached to CoA instead of ACP

Question 21

Where does FA synthesis take place and what regulates it?

Answer 21

In the brain, liver and kidneys . regulated by hormones produced when mammary glands are activated (during and after pregnancy to make milk).

Question 22

What conditions cause FA synthesis?

Answer 22

The pathway is used when too much carbohydrate or glucose is ingested and causes an accumulation of citrate as a result of glycolysis -> acetyl CoA -> citrate (citrate can only be metabolised in the TCA cycle. Excess citrate is removed from the mitochondria to the cytoplasm where it will be digested to acetyl CoA (for FA synthesis) and oxaloacetate. Oxaloacetate will then be recycled -> reduced to malate -> oxidised to pyruvate -> mitochondria -> oxaloacetate to achieve equilibrium with acetyl CoA.

Question 23

Summarise fatty acid anabolism.

Answer 23

Cytosolic pathway: synthesis of saturated fatty acids - predominantly producing palmate.
Additional enzymes and reactions are required to produce the variations of fatty acids found in the body.
Further elongation of FAs occurs via enzyme found on the cytosolic face of the ER membrane.
Unsaturated fatty acids are produced at the ER via reduction reactions.

Question 24

How are FAs stored?

Answer 24

FAs are covalently bonded to glycerol to be stored as neutral fats. (mono-,di-, or tri- acylglycerol)

Question 25

How is a FA attached to glycerol?

Answer 25

The reactive carboxyl group of palmitate is activated by ATP > AMP -> acyl CoA (activated FA) allowing it to bind to one of the 3 hydroxyl groups on glycerol. This forms a neutral fat.
Only activated (CoA loaded) FAs can attach to glycerol as the carboxyl group of a FA needs to replaced with CoA (COO -> CO-AMP-> C-CoA) to react with the OH group of a glycerol

Question 26

Where and how are neutral fats stored?

Answer 26

Fatty acids are stored as lipid droplets within cell membranes between the phospholipids (neutral fats tend to consist mostly of triglycerides). The adipose cells (lipid bodies) are stable within the membrane. Low density lipo proteins coat the adipose cells. Adipose cells are energy sources for nearby tissues and serve as shock absorbers in soft tissues.

Question 27

How are neutral fats transported to muscle cells?

Answer 27

An activated hormone sensitive lipase will free the FAs from the glycerol allowing them to bind to a human serum albumin in the blood. FAs are unloaded into the muscle cell from the HSA and move into the outer membrane of the mitochondria where they are activated.

Question 28

How are fatty acids transported across the inner mitochondrial membrane?

Answer 28

First activated FAs will react a zwitterionic alcohol, carnitine, to form acyl carnitine displacing the CoA via carnitine acyltransferase I. Acyl carnitine is then transported by a acyl carnitine translocase embedded in the membrane to matrix. Once within the matrix the FA reattaches to a CoA that replaces carnitine via acylcarnitine transferase II to form acyl CoA

Question 29

What happens to neutral fats that are digested?

Answer 29

They are catabolised (β oxidation) in muscle cells for fuel and glycerol is recycled in the liver. Excess neutral fats may be stored by the liver and in adipose cells. Excess acetyl CoA produced via β oxidation is converted to ketone bodies for cardiac muscles and renal cortex.