Case 5 BIOCHEM - Fatty acid metabolism, synthesis; oxidative phosphorylation

Question 1

Outline how dietary triacylglycerols are degraded and absorbed by the body

Answer 1

• Dietary triacylglycerols are degraded by a small extent by lipases and the low pH in the stomach, but mostly bypass untouched into the duodenum. The duodenum signals to the pancreas upon the arrival of fat.
• Alkaline pancreatic juice is secreted into the duodenum which raises the pH of the digestive mixture, allowing for the hydrolysis triacylglycerols by pancreatic lipase.
• The hydrolysis of ester bonds in triacylglycerols depend upon the presence of bile salts (cholesterol derivative). These agents act as detergents to emulsify triacylglycerols.
• Short chain fatty acids released in this way are absorbed directly into the villi of the intestinal mucosa, whereas long-chain fatty acids form mixed micelles with bile salts and are carried in this fashion to the surfaces of the epithelial cells that cover the villi.
• The fatty acids enter the intestinal cells, where they are condensed with glycerol to form new triacylglycerols inside the smooth ER. These triacylglycerols aggregate with lipoproteins to form particles called chylomicrons, which are then secreted at the basal side of the intercellular space, and are transported into the lymphatic system (lacteals) and on to the bloodstream.
• Triacylglycerols are hydrolysed to release fatty acids by lipoprotein lipase at the target tissues, which can then be oxidised via B-oxidation.

Question 2

Outline the general process of B-oxidation

Answer 2

fatty acids are degraded by oxidation of the B carbon followed by cleavage between the alpha- and beta- carbons. Repetition of this process yield 2 carbon units of acetyl-CoA.

Question 3

Explain how fatty acid is activated for B-oxidation

Answer 3

The process of B oxidation begins with the formation of a thiol ester bond between the fatty acid and coenzyme A. This condensation of coA activates fatty acid for reaction in the B-oxidation pathway. This process is accompanied by the hydrolysis of ATP to form AMP.

Question 4

Explain how the activated fatty acids (fatty acetyl-CoA) are transported across mitochondrial membranes

Answer 4

• fatty acyl-coA are transported into the inter membrane space by fatty acyl-coA carrier, and subsequently converted to fatty acyl-carnitine by enzyme Carnitine acyltransferase I associated with the mitochondrial outer membrane
• fatty acylcarnitine is transported across the inner membrane by protein carriers, then associated with carnitine acyltransferase II on the matrix side of the inner membrane, which transfes the fatty acyl group back to coA to form fatty acyl-coA, leaving free carnitine.

Question 5

Products of fatty acid oxidation

Answer 5

1. FADH2
2. NADH
3. Acetyl-coA

= all are sources of ATP

Question 6

Outline the steps of B-oxidation

Answer 6

1. dehydrogenation (removal of 1 hydrogen from a-Carbon and 1 from b-Carbon)
2. hydration (replace with OH and H groups)
3. dehydrogenation (remove 2 H’s)
4. thiolytic cleavage (cleavage at b-Carbon, releasing 2 carbons as acetyl coA)

products: fatty acyl CoA* + acetyl coA
* repeats the 4 steps until entire fatty acid is converted acetyl coA

Question 7

Explain what happens to the product of lipid hydrolysis

Answer 7

hydrolysis of lipids produce glycerol and fatty acids.

• glycerol can be converted to pyruvate, which undergoes glycolysis to generate acetyl coA
• fatty acids undergo B-oxidation, producing acetyl coA and NADH and FADH2
• acetyl coA then enters the CAC and produces NADH and FADH2
• NADH and FADH2 enters the electron transport chain and facilitates oxidative phosphorylation, which yields free energy.

Question 8

The role of ketone bodies in metabolism

Answer 8

• Most acetyl coA produced by fatty acid oxidation are further oxidised by the CAC, however some of the acetyl coA are converted to ketone bodies (acetone, acetoacetate, B-hydroxybutyrate) by a process called ketogenesis. These metabolites are synthesised in mitochondria matrix in the liver, and are important sources of fuel and energy for the brain, heart, and skeletal muscle during prolonged starvation.
• ketone bodies enter the target tissues via blood, where they are recovered to acetyl-coA and are used for energy synthesis in the CAC.

Question 9

Outline the control of ketogensis

Answer 9

• during starvation, oxaloacetate is used for gluconeogeneis, this slows down the CAC. Acetyl coA, unable to enter the CAC, are then converted to ketone bodies.
• ketogenesis occurs when there is a large amount of acetyl coA in the liver.

Question 10

The citrate shuttle

Answer 10

• fatty acid synthesis occurs in the cytosol, and requires acetyl coA
• acetyl coA is transported through the mitochondrial matrix by condensing with oxaloacetate to form citrate
• citrate is hydrolysed in the cytosol by an enzyme citrate lyase, releasing oxaloacetate and acetyl coA

Question 11

Stage I of fatty acid synthesis

Answer 11

• irreversible activation of acetyl coA to malonyl coA (using CO2 and ATP)
• catalysed by acetyl coA carboxylase
• control point

Question 12

Stage II of fatty acid synthesis

Answer 12

• fatty acid synthase: multisubunit protein that bring together the building blocks of fatty acid (acetyl coA). Synthesis occurs with all reaction parents bound to the protein
  1. Acetyl-CoA is immobilised on acyl carrier protein (ACP)
  2. acetyl is transferred to the condensing enzyme (CE)
  3. malonyl coA is also immobilised on ACP
  4. malony coA + Acetyl generates acetoacetyl and CO2
  5. reduction at B carbon by addition of 2H-atoms
  6. dehydration at a-b bond
  7. second reduction step and transfer of fully reduced 4C chain

steps 3 - 7 are repeated after butyryl is transferred to CE and a new malonyl coA reacts with ACP.
§ after 7 cycles, palmitate is generated

Question 13

Outline the functions of Fatty Acid Synthase

Answer 13

1. catalyses reaction of fatty acid synthesis
2. growing fatty acid chain is synthesised on acetyl, which is bound to acyl carrier protein (ACP)
3. the fatty acid synthase builds up the saturated fatty acid molecule up to 16C length

Question 14

Differences between the synthesis and oxidation of fatty acids

Answer 14

SYNTHESIS
1. occurs in cytoplasm

2. one multifunctional protein complex
3. 2-C added from malonyl coA
4. requires NADPH and NADP (at reduction steps)

OXIDATION
1. mitochondrial matrix

2. separate enzymes
3. 2 C cut as acetyl coA
4. FAD and NAD

Question 15

Outline the hormonal regulation of fatty acid synthesis

Answer 15

INSULIN - positive regulator of fatty acid synthesis, negative regulator of fatty acid oxidation

GLUCAGON - negative regulator of fatty acid synthesis, positive regulator of fatty acid oxidation