Biochemistry- fat digestion and metabolism

Question 1

Describe the role, release and reasorption of bile

Answer 1

The gall bladder of the liver contains bile, a liquid comprised mostly of bile salts, cholesterol and phospholipids.

 These compounds (especially bile salts) are amphiphilic and contribute to the emulsification of TAG into smaller lipid
droplets, providing an increased surface area for hydrophilic enzymes to access and breakdown lipid molecules.

 The resulting lipid droplets also contain the fat-soluble vitamins (A, D, E and K).

 Bile is released into the small intestine during digestion, if this is prevented, lipids are
mostly unabsorbed, resulting in fatty stools (steatorrhea).

 The reabsorption of bile (made from cholesterol) is called enterohepatic recirculation.

Question 2

Describe the emulsification and hydrolysis of TAG in digestion

Answer 2

TAG is firstly emulsified and digested. It is absorbed as MAG and FFA, and converted back into TAG. Converted into a
chylomicron.
 During the fed state, adipose tissue releases lipoprotein lipase to break down the TAG.

Hydrolysis of TAG is essential for absorption
to occur.
 The amphipathic nature of the products and
the cone-shape of 2-monoacylglycerol
facilitate their emulsion into micelles

Question 3

Describe the ‘absorption of lipids’ stage

Answer 3

The micelles (containing 2-MAG, LCFAs) diffuse into
the enterocytes, and are then recombined into
TAG.
 The chylomicron made in the enterocyte is too
large to enter the capillaries, so it enters the lacteal
instead.
 Chylomicrons enter the blood via the thoracic duct,
about 3-5 hours after the end of a meal (coincides
with peak LPL activity).

Question 4

Describe the activation of fatty acids

Answer 4

Free fatty acids can be toxic to a cell, and must be esterified to coenzyme A before either fatty acid oxidation, or
incorporation into triacylglycerol.
 The thioester bond is more reactive than a normal ester bond.
 The coenzyme A provides a large compound for enzymes to bind to, the binding energy contributes to stabilising transition
forms during catalysis.
 Acyl CoA synthases are found within membranes, and are specific for different chain lengths.

Question 5

Describe the release of fatty acids from lipoproteins

Answer 5

Chylomicrons and very low density lipoproteins (VLDLs) are both sources of circulating TAG (chylomicrons are only
available 2 hours after digestion).
 After digestion, adipose tissue secretes lipoprotein lipase (LPL), which migrates to local capillaries and releases fatty acids
that may be taken up and stored by the nearby adipocytes.
 Other tissues (especially muscle) also secretes lipoprotein lipase, but they are either using them as a fuel, or storing them
for later use as a fuel.
 In addition, these tissues tend to secrete LPL during fasting, as during the fed state, they are focused on increasing their
uptake of glucose.
 Lipoprotein lipase is found on the luminal surface of capillaries near the site of secretion, attached to proteoglycans.

Question 6

Describe the entry of fatty acids into the mitochondria

Answer 6

No transporter for fatty acid CoA on the
mitochondrial membrane.
 Fatty acid oxidation occurs in the mitochondrial
matrix, but the inner mitochondrial membrane is
impermeable to fatty acyl CoA.
 CPT1 is inhibited in the fed state, and active in the
fasting state.

Question 7

Describe the process of fatty acid oxidation

Answer 7

Fatty acid oxidation produces energy for many
tissues, the process requires oxygen and occurs in the matrix of the mitochondria.
 Fatty acid oxidation is a step-by-step removal of 2 carbon units of acetyl CoA.
 It produces energy in the form of reduced coenzymes, FADH2 and ADH.
 Fatty acid oxidation produces no ATP.
 The reduced coenzyme can be converted into other energy currencies (i.e. concentration gradients or ATP) in the
mitochondrial matrix.
 The acetyl CoA can be further oxidised in the matrix, to produce more energy.
 The breakdown of fatty acids always begins at the carboxylic end.

Question 8

How is fatty acid oxidation regulated

Answer 8

For most tissues, fatty acid oxidation is most important during the fasting state, and starvation, as this avoids using the
limited glucose.
 This regulation is mostly due to control of adipose tissue, which stores fatty acids in the fed state, and releases them during
fasting (and exercise).
 In the liver, as fatty acid oxidation increases, and the acetyl CoA produced increases, not all of the acetyl CoA can be
oxidised. So the liver combined the 2 carbon acetyl molecules into 4 carbon ketone bodies. The brain can use 4 carbon
ketone bodies.
 These can be taken up by other tissues (even the brain) converted back into acetyl CoA and then oxidised as a fuel, further
decreasing their need for glucose.

Question 9

What are the stages of ketone body synthesis

Answer 9

For most tissues, fatty acid oxidation is most important during the fasting state, and starvation, as this avoids using the
limited glucose.

 This regulation is mostly due to control of adipose tissue, which stores fatty acids in the fed state, and releases them during
fasting (and exercise).

 In the liver, as fatty acid oxidation increases, and the acetyl CoA produced increases, not all of the acetyl CoA can be
oxidised. So the liver combined the 2 carbon acetyl molecules into 4 carbon ketone bodies. The brain can use 4 carbon ketone bodies.

 These can be taken up by other tissues (even the brain) converted back into acetyl CoA and then oxidised as a fuel, further
decreasing their need for glucose.
Synthesis occurs in the liver mitochondria

Catabolism: Occurs in extra-hepatic mitochondria.
 Acts as an alternative fuel to glucose.

Question 10

What are the downsides of ketones

Answer 10

Acetoacetate spontaneously decarboxylates to acetone, which is highly volatile, and evaporates from the mouth.
 This represents a significant loss of fuel.
 Besides this, much of the ketone bodies are also lost in the urine.

 Apart from acetone, ketone bodies are acidic. The very high concentration of ketone bodies produced in starvation and
untreated type I diabetes mellitus can cause metabolic acidosis. This can result in respiratory compensation and vomiting.

 D-β-hydroxybutyrate and acetoacetate are also both highly soluble, so significant amounts are lost in the urine. This is still preferable to running out of glucose prematurely, or breaking down too much protein for gluconeogenesis.

 Acetone is difficult to metabolise, it is also highly volatile and evaporates on the breath.
 Acetone has a characteristic fruity smell, which can be used in diagnosis of ketoacidosis.

Question 11

What is MCAD deficiency (beta oxidation defect)

Answer 11

The most frequently diagnosed defect in β-oxidation.
 In fasting hypoketotic (no ketone bodies) hypoglycaemia (unable to metabolise fats so use more glucose), lethargy, coma,
seizures, death.
 Responsible for 2% of sudden infant death cases in which infants and 20% people die in their first crisis.
 Once diagnosed, advice is to avoid fasting, have a low fat diet and take carnitine supplements. If followed, there are
normally no further incidents.
 Leads to an accumulation of fatty acid oxidation intermediates in plasma and urine, such as medium-chain acyl-carnitines.
Tandem mass-spectrometry of blood spots allow early diagnosis.
 Now screened for the products in the blood (not the gene). Tandem mass spectrometry is used.