Digestion, Mobilization, and Transport of Fats

• Cells can obtain fatty acid fuels from:

  • Fats consumed in the diet
  • Fats stored in cells as lipid droplets
  • Fats synthesized in one organ for export to another
  • Fats obtained by autophagy
• Which answer choice is not a major source of fatty acid fuels in vertebrates?

  • Conversion in the liver of excess dietary amino acids to fats
  • Conversion in the liver of excess dietary carbohydrates to fats
  • Fats stored in adipocytes
  • Fatty acids in the diet


  Conversion in the liver of excess dietary amino acids to fats. Although vertebrates do have pathways to convert amino acids to fats, this is not a significant source of fatty acid fuels.
• Dietary fats are absorbed in the small intestine

  1. Bile salts emulsify dietary fats in the small intestine, forming mixed micelles
  2. Intestinal lipases degrade triacylglycerols
  3. Fatty acids and other breakdown products are taken up by the intestinal mucosa and converted to triacylglycerols
  4. Triacylglycerols are incorporated, with cholesterol and apolipoproteins, into chylomicrons
  5. Chylomicrons move through the lymphatic system and bloodstream to tissues
  6. Lipoprotein lipase, activated by apoC-II in the capillary, concerts triacylglycerols to fatty acids and monoacylglycerols
• Dietary acids are absorbed in the small intestine:

  7. Fatty acids enter the cells

  8. Fatty acids are oxidized as fuel or reesterified for storage
• Apolipoproteins are proteins in their lipid free form that binds lipids to form lipoproteins

  • Target triacylglycerols, phospholipids, cholesterol, and cholesteryl esters for transport between organs
• Chylomicrons are particles consisting of triacylglycerols, cholesterol, and apolipoproteins
• Lipoprotein particles are spherical aggregates of apolipoproteins and lipids

  • They are arranged with hydrophilic lipids at the core and hydrophilic protein side chains and lipid head groups at the surface
  • Various in densities depending on combinations of lipid and protein
  • Range from chylomicrons and very low density lipoproteins (VLDL) to very high density lipoproteins (VHDL)
• Apolipoprotein B-48 (apoB-48) is the primary protein component of chylomicrons
• Apolipoprotein C-II (apoC-II) is the protein picked up in the blood by chylomicrons from high density lipoprotein (HDL) particles
• Lipoprotein lipase is an extracellular enzyme in the capillaries of muscle and adipose tissue that hydrolyzes triacylglycerols to free fatty acids and monoacylglycerols.

  • Activated by apoC-II
• What does apolipoprotein C-II do?

  • It activates LCAT on HDL
  • It is a structural component of LDL
  • It is a ligand for the LDL receptor
  • It activates lipoprotein lipase activity on chylomicrons
  • It inhibits lipoprotein lipase activity on chylomicrons.

  It activates lipoprotein lipase activity on chylomicrons.
• In the blood, chylomicrons pick up apolipoprotein C-II (apoC-II) from high density lipoprotein (HDL) particles and are carried to muscle and adipose tissue. In the capillaries of these tissues, the extracellular enzyme lipoprotein lipase, activated by apoC-II, hydrolyzes triacylglycerols to free fatty acids and monoacylglycerols
• Which statement is false regarding the processes of dietary lipids in vertebrates?

  • Dietary lipids are emulsified by bile salts in the intestine
  • Triacylglycerols in mixed micelles in the intestine diffuse into cells of the intestinal mucosa
  • Ultimately, dietary lipids are oxidized as fuel by muscle or stored as triacylglycerols in adipose tissue
  • Dietary lipids are packaged in lipoprotein aggregates known as chylomicrons, which are then exported to the lymph system. 


  Triacylglycerols in mixed micelles in the intestine diffuse into cells of the intestinal mucosa.
• Water soluble lipases in the intestine convert triacylglycerols to monoacylglycerols, diacylglycerols, and free fatty acids. These products of lipase action diffuse or are transported into the epithelial cells lining the intestinal surface (the intestinal mucosa), where they are reconverted to triacylglycerols.
• Fatty acids are converted to triacylglycerols in the liver
• Triacylglycerols are packaged with specific apolipoproteins into VLDLs
• VLDLs are secreted and transported in the blood to adipose tissue
• Triacylglycerols are removed and stored in lipid droplets within adipocytes in the adipose tissue.
• Hormones trigger mobilization of stored triacylglycerols.
• Lipid droplets are organelles stored in adipocytes and steroid synthesizing cells that contain neutral lipids

  • Contain a core of triacylglycerols and sterol esters surrounded by a monolayer of phospholipids
• Perilipins are a family of proteins that coats the surface of lipid droplets to restrict access to lipid droplets

  • Prevent untimely lipid mobilization
• What are perilipins?

  • Major proteins of periwinkles
  • Apolipoproteins of chylomicrons
  • Coat proteins of lipid droplets
  • Cytosolic lipidated proteins
  • Enzymes with cholesteryl esterase activity


  Coat proteins of lipid droplets. Phosphorylated perilipins expose the lipids stored in a lipid droplet to the hormone sensitive lipase (HSL).
• Mobilization of stored triacylglycerols occurs when hormones (glucagon and epinephrine) signal the need for metabolic energy.
• PKA triggers changes that open the lipid droplet to the action of three cytosolic lipases
• Which factor would stimulate movement of fatty acids to muscle and the liver when blood glucose levels fall?

  • Insulin
  • Glucagon
  • An increase in protein kinase C in activity
  • An increase in phospholipase C activity
  • An increase in citric acid cycle activity in adipose

  Glucagon
• Low levels of glucose in the blood trigger the release of glucagon. Binding of this hormone to a G protein coupled receptor on the adipocyte plasma membrane triggers the mobilization of stored triacylglycerol.
• Free fatty acids, FFAs, are fatty acids released by lipases.
• Serum albumin is a blood protein that noncovalently binds and transports FFAs to target tissues.

  • Makes up about half of the total serum protein.
• Most of the biologically available energy of triacylglycerols resides in their three long chain fatty acids
• Glycerol kinase phosphorylates glycerol to form glycerol 3-phosphate.
• Glyceraldehyde 3-phosphate can enter glycolysis.
• Fatty acid conversion:

  1. Glycerol is converted to L-glycerol 3-phosphate by glycerol kinase
  2. L-glycerol 3-phosphate is converted to dihydroxyacetone phosphate by glycerol 3-phosphate dehydrogenase
  3. Dihydroxyacetone phosphate is converted to D-glyceraldehyde 3-phosphate by triose phosphate isomerase
  4. D-glyceraldehyde 3-phosphate enters glycolysis
• Which molecule can be produced rapidly from glycerol in only three steps, allowing an interaction between carbohydrate and lipid metabolism?

  • Acetyl CoA
  • Glucose
  • Pyruvate
  • Glyceraldehyde 3-phosphate
  • Phosphoenolpyruvate


  Glyceraldehyde 3-phosphate
• The glycerol released by lipase action is phosphorylated by glycerol kinase, and the resulting glycerol 3-phosphate is oxidized to dihydroxyacetone phosphate. The glycolytic enzyme triose phosphate isomerase converts this compound to glyceraldehyde 3-phosphate, which is oxidized via glycolysis.
• Small (less than 12 carbons) fatty acids diffuse freely across mitochondrial membranes
• Carnitine shuttle transports long chain fatty acids (containing 14 plus carbons) through the mitochondrial membrane

  • Requires activation to a fatty acyl CoA and attachment to carnitine.
• Where does beta oxidation occur?

  • In the cytosol
  • In the mitochondrial matrix
  • In the ER lumen
  • On the Golgi apparatus membrane
  • At the plasma membrane

  In the mitochondrial matrix. The enzymes of fatty acid oxidation in animal cells are located in the mitochondrial matrix.
• Why does beta oxidation occur in the mitochondrial matrix?

  • To allow coordinated regulation with fatty acid synthesis
  • To coordinate production of acetyl CoA with the introduction into the citric acid cycle.
  • To compartmentalize
  • Because necessary oxidative enzymes are present
  • All of the answers are correct

  All of the answers are correct
• Fatty acyl CoA synthetase are isozymes present in the outer mitochondrial membrane that activate fatty acids by conversion to fatty acyl CoA thioesters

  • Fatty acid + CoA + ATP --> fatty acyl-CoA + AMP + PPi
• Fatty acyl-CoA contains a thioester linkage between the fatty acid carboxyl group and the thiol group of coenzyme A

  • High energy compound
  • Formation is made favorable by the hydrolysis of two ATP bonds
  • Delta G'degree = -15 kJ/mol