Lipid Metabolism in Exercise

Question 1

What are lipids?

Answer 1

• diverse biological compounds, characterised by low solubility in water
• fatty acids
• triacylglycerol’s
  
  • these two are more important energy sources with high ATP yield but at a slow rate of oxidation
• steroid
  
  • cell signalling and membrane function
• phospholipids
  
  • form membranes - present at the interface between lipid and water

Question 2

How do lipids differ from carbohydrates?

Answer 2

• carbohydrates are hydrophilic, whereas lipids are largely hydrophobic
• the longer the fatty acid chain and few the double bonds, the lower the solubility in water
• the differences in structural properties between carbohydrates and lipids alters the manner by which lipids are digested, absorbed and metabolised for energy

Question 3

What are phospholipids?

Answer 3

• major component of cell membranes that effect cell signaling
• consist of a glycerol unit connected by two fatty acids and a phosphoric group via ester bonds
• membrane lipids are amphipathic meaning one end is hydrophobic and the other is hydrophilic

Question 4

What are triacyglycerols?

Answer 4

• majority of lipids from dietry fat of which around 90-95% are triacylglycerol’s
• serve as an energy store in adipose tissue and muscle
• one glycerol unit and 3 fatty acids connected by ester bonds
• triacylglycerol cannot pass the membrane of gastrointestinal cells and must be broken down

Question 5

How are hydrophobic molecules processed?

Answer 5

1) ingestion: large coarse lipid droplets are reduced in size to fine lipid droplets by bile acids, made in liver and released by gallbladder
2) intestinal lumen: pancreatic lipase hydrolyses the ester bonds to yield 2 fatty acids and 2 monoacylglycerol
3) enterocytes: these products freely enter the intestinal cells where TAG is reformed
4) bloodstream: all lipids are packaged into chylomicrons and exported into lymph nodes and then the blood where the lipids can be distributed after each meal

Question 6

What is the process that triacylglycerols undertake?

Answer 6

triacylglycerols -> 2 monoacylglycerol + 2 fatty acids -> the reverse reaction isn’t favoured so fatty acids combine with acyl coA -> 2 monoacylglycerol + 2 acyl coA -> triacylglycerol

Question 7

What are chylomicron?

Answer 7

• chylomicrons are a class of lipoprotein
• single layer of phospholipids with hydrophobic core
• once formed, chylomicrons pass into extracellular space, then into lymphatic
• this process happens for a number of hours after a meal

Question 8

What is the fat content within the human body?

Answer 8

• lipid is mainly stored in a specialised tissue called adipose as well as muscle (0.2-0.8%)
• adipose tissue is found both under the skin and around our internal organs (~15% men and ~23% women)
• the cytoplasm of an adipocyte is dominated by a large lipid droplet filled with triacylglycerol (~80%)
• this serves as a huge energy reserve which can be utilised by working muscle for fuel during exercise
• changes in triacylglycerol breakdown and synthesis occur in adipose tissue and muscle during exercise

Question 9

What lipolysis occurs in skeletal muscle?

Answer 9

• triacylglycerol stores are contained in lipid droplets within adipocytes or muscle fibres
• the proximity of lipid droplets to mitochondria minimises the distance fatty acids move for degradation and ATP production
• three key enzymes are all located in type 1 muscle fibres, making lipolysis optimal during aerobic exercise
• exercise speeds up FFA degradation in adipose tissue and muscle by making a process called beta oxidation
• the product of beta oxidation enters the TCA cycle

Question 10

How does the breakdown and synthesis of triacylglycerol happen?

Answer 10

• adipose tissue and muscle contain the necessary enzymes to drive lipolysis and synthesis of triacylglycerol
• triacylglycerol breakdown occurs in the cytosol via 3 enzymes
  
  • adipose TAG lipase
  • hormone sensitive lipase
  • monoacylglycerol acyltransferase
• triacylglycerol synthesis also occurs in the cytosol:
  
  • glycerol generated from dietry glucose forms the TAG glycerol backbone
  • 3 fatty acids are then added in 2 steps via the enzyme glycerol phosphate acyltransferase
• in a fed state: synthesis>breakdown
• during exercise: breakdown> synthesis

Question 11

What is the link between exercise and lipolysis?

Answer 11

• the rate of lipolysis in adipose tissue increases within 5-10 minutes of exercise onset
• the rate of lipolysis in adipose tissue is influenced by:
  
  • epinephrine: increases lipolysis
  • epinephrine also decreases lipolysis
  • insulin reduces lipolysis
• low moderate intensity exercise
  
  • increase in epinephrine concentration and decrease in insulin concentration drive the AMP pathway favourbly to activate AGTL, HSL and MGL
• high intensity exercise
  
  • increase in epinephrine concentration and stable or increase in insulin concentration suppresses the AMP pathway to inhibit AGTL, HSL and MGL

Question 12

What is the fate of lipolytic products during exercise?

Answer 12

• exercise speeds up lipolysis of triacylglycerol in both adipose tissue and skeletal muscle
• adipocytes: the glycerol and FFA’s formed by lipolysis in adipocytes leave the tissue and enter the blood carried by albumin
• FFA’s from lipolysis in muscle remain and FFA’s from adipose tissue are imported into muscle via fatty acid binding protein at the plasma membrane
• muscle - FFA are primarily used to beta oxidation and ATP provision
• liver - glycerol is used for gluconeogenesis and some FFA may enter for triacylglycerol synthesis

Question 13

Why does fatty acid degradation occur in the muscle?

Answer 13

• fatty acids are degraded through the pathway of beta oxidation in the mitochondria
• over a 3 step process, FFA’s are activated and transported across two mitochondrial membranes for degradation via a cycle with carnitine
  
  • activation: FFA’s are activated by a reaction with CoA forming acyl-coA. This can then pass to the inter membrane space
  • carnitine binding: carnitine takes the acyl group, allowing transport into the mitochondrial matrix
  • CoA restoration: the acyl chain is then taken from carnitine to reform acyl-coa inside the mitochondrial matrix
• acyl-coa enters the beta oxidation pathway in the mitochondrial matrix
• a single cycle of beta oxidation involves 4 reactions that degrade acyl-coA:
  
  • x1 acetyl-coA
  • x1 FADH2
  • x1 NADH
  • acyl-coA (minus 2 carbons)
• acyl-CoA then begins another cycle of beta oxidation
• acyl denotes any fatty acid chain and therefore some fatty acids undergo more cycles of beta oxidation

Question 14

What is the energy yield from degrading fatty acids?

Answer 14

• energy yield of beta oxidation
  
  • this is not energetically favoured and produces no ATP, but 8 acetyl coA, 7 NADH that enter TCA cycle and ETC
• energy yield of fatty acid oxidation
  
  • TCA and ETC ‘pull’ beta oxidation and yield a huge amount of energy from fatty acid oxidation