Lipids: Biochemical and Physiological Functions

Lipids

• Definition: Organic molecules characterized by their solubility in non-polar solvents such as ether, chloroform, and acetone
• Functions:
  • Energy storage
  • Protection of organs
  • Insulation
  • Absorption of vitamins
  • Energy sources
  • Hormones or vitamins
• Properties:
  • May be liquids or non-crystalline solids at room temperature
  • Colorless, odorless, and tasteless
  • Insoluble in water
  • Soluble in organic solvents like alcohol, chloroform, acetone, benzene, etc.
  • No ionic charge

Structure of Lipids

• Made up of the elements carbon, hydrogen, and oxygen
• Have a much lower proportion of water than other molecules such as carbohydrates
• Unlike polysaccharides and proteins, lipids are not polymers
• Made from two molecules: glycerol and fatty acids
• Glycerol molecule:
  • Made up of three carbon atoms with a hydroxyl group attached to it and hydrogen atoms occupying the remaining positions
• Fatty acids:
  • Consist of an acid group at one end of the molecule and a hydrocarbon chain
  • May be saturated or unsaturated
  • Saturated fatty acids have no C=C bonds
  • Unsaturated fatty acids have one or more C=C bonds

Classification of Lipids

• Simple Lipids:
  • Neutral fats (triacylglycerols, TG)
  • Waxes
  • Cholesterol esters
  • Vit A and Vit D esters
• Compound Lipids:
  • Phospholipids are a type of compound lipid that are crucial for maintaining the structure and function of biological membranes, including cell membranes and lipoproteins. They are amphipathic molecules, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions, which allows them to interact with both water and other lipids. The hydrophilic head of a phospholipid molecule is typically composed of a phosphate group and a glycerol moiety, while the hydrophobic tail is composed of two fatty acid chains. The structure of phospholipids permits them to form biomembranes by spontaneously aggregating to form a bilayer, with the hydrophobic tails facing inward and the hydrophilic heads facing outward. This unique structure is essential for maintaining the integrity and function of biological membranes, and phospholipids play a critical role in various cellular processes, including cell signaling, membrane trafficking, and cell adhesion.
  • Glycolipids
  • Aminolipids are a class of phospholipids containing amino groups in the head group, which is responsible for their unique chemical and biological properties. The addition of an amino group to the phospholipid backbone can enhance their binding affinity to proteins and other molecules, allowing them to play important roles in various cellular processes. For instance, aminolipids have been shown to participate in the formation of protein-lipid complexes, which can influence enzyme activity and membrane trafficking. Furthermore, aminolipids can also serve as ligands for proteins involved in signaling pathways, highlighting their importance in regulating cellular responses to environmental stimuli.
• Derived Lipids:
  • Hydrolysis products of simple and compound lipids
  • Include fatty acids, glycerol, sphingosine, and steroid derivatives

Triglycerides

• Lipids consisting of one glycerol molecule bonded with three fatty acid molecules
• Bonds between the molecules are covalent and are called ester bonds
• Formed during a condensation reaction
• Charges are evenly distributed around the molecule, so hydrogen bonds do not form with water molecules, making them insoluble in water

Fatty Acids

• Products of fat hydrolysis
• Occur in plant and animal foods, as well as in complex forms with other substances
• Fatty acids usually contain an even number of carbon atoms and are straight-chain derivatives

Nomenclature of Fatty Acids

• Saturated fatty acids:
  • Prefix: # of hydrocarbons
  • Suffix: -anoic
• Polyunsaturated fatty acids:
  • Prefix: # of hydrocarbons
  • Suffix: -enoic (1 double bond), dienoic (2 double bonds), etc.

Types of Fatty Acids

• Saturated fatty acids (SFAs)
• Monounsaturated fatty acids (MUFAs)
• Polyunsaturated fatty acids (PUFAs)

Omega-3 and Omega-6 Fatty Acids

• Omega-3 fatty acids:
  • Found in oil from certain types of fish, vegetables, and other plant sources
  • Not made by the body and must be consumed in the diet
  • Used to help lower triglyceride levels in the blood
• Omega-6 fatty acids:
  • Found in vegetable oils and other plant sources
  • Used to reduce the risk of heart disease
  • Lower total cholesterol levels, LDL cholesterol levels, and raise HDL cholesterol levels

Essential Fatty Acids

• Three polyunsaturated fatty acids (l Omega-6 fatty acids:

  • Found in vegetable oils such as sunflower, safflower, and corn oil, as well as in nuts and seeds like walnuts and flaxseeds, and in cell membranes of animals.

  Cannot be synthesized in the body and must be provided in the diet

• Lack of essential fatty acids in the diet can produce growth retardation and other deficiency manifestation symptoms

Hydrogenation

• Hydrogenation of liquid vegetable fats is done commercially to obtain solidified fats like margarine
• Creates trans fatty acids

Trans Fatty Acids

• Association between consumption of trans fats and heart disease
• No physiological need for the consumption of trans fats
• Avoid manufactured trans fats

Lipid Metabolism

• Role of the liver in lipid metabolism:
  • Excess protein and carbohydrates are converted into triglycerides
  • Elimination of cholesterol and phospholipids in bile
  • Synthesis of cholesterol and phospholipids
  • Modification of cholesterol and triglycerides to produce water-soluble lipoproteins for transport to other body tissues

Absorptive State and Post-resorptive State

• Absorptive state:
  • Liver converts glucose into fatty acids
  • Retrieves fatty acids from lipids supplied with chylomicrons
  • Fatty acids converted into neutral fats and phospholipids, VLDL formed
• Post-resorptive state:
  • Adipose tissue releases fatty acids
  • Fatty acids taken up by liver and oxidatively degraded to acetyl CoA
  • Acetyl CoA is converted into ketone bodies

Fatty Acid Activation and Transport

• Carnitine acyl transferase I (CAT I) and carnitine acyl transferase II (CAT II) are involved in the transport of fatty acids across the mitochondrial membrane
• Acyl group is transferred to carnitine, then transported across the membrane, and finally converted back into acyl CoA### Lipid Metabolism

Mobilization of Fats

• Glucagon, epinephrine, and norepinephrine are hormones that stimulate the mobilization of fats
• These hormones bind to G-protein coupled receptors, leading to the activation of adenylate cyclase, which converts ATP to cAMP
• cAMP activates protein kinase A, which phosphorylates and inactivates perilipin, allowing hormone-sensitive lipase to break down triglycerides into fatty acids and glycerol

Beta-Oxidation

• Beta-oxidation is the process by which fatty acids are broken down into acetyl-CoA
• It occurs in the mitochondria and is stimulated by the hormones mentioned above
• The process involves four main steps: activation, transport, beta-oxidation, and ketogenesis

Activation

• Fatty acids are converted to fatty acyl-CoA by fatty acyl-CoA synthetase, which requires ATP and CoA

Transport

• Fatty acyl-CoA is transported into the mitochondria by carnitine acyl transferase, which exchanges it with carnitine

Beta-Oxidation

• Fatty acyl-CoA is broken down into acetyl-CoA through a series of reactions involving fatty acyl-CoA dehydrogenase, trans-delta 2-enoyl-CoA, and beta-hydroxyacyl-CoA dehydrogenase
• These reactions produce NADH and FADH2, which can be used to generate ATP

Ketogenesis

• Ketogenesis is the process by which acetyl-CoA is converted into ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone)
• It occurs in the mitochondria of liver cells and is stimulated by low blood glucose levels, uncontrolled diabetes mellitus type 1, and carbohydrate-restricting diets
• Ketone bodies can enter the blood-brain barrier and be used as energy by the brain

Gluconeogenesis

• Gluconeogenesis is the process by which glucose is generated from non-carbohydrate sources (such as amino acids and lactate)
• It occurs in the mitochondria and cytosol of liver and kidney cells
• The process involves the conversion of pyruvate to glucose through a series of reactions involving malate, oxaloacetate, and phosphoenolpyruvate

Regulation of Beta-Oxidation

• Beta-oxidation is regulated by the availability of fatty acids and the energy needs of the body
• It is also influenced by hormones such as insulin, glucagon, and epinephrine

Test your knowledge on the biochemical and physiological functions of lipids, their metabolism in the human body, and the role of the liver in lipid metabolism. Learn about the characteristics, functions, and types of lipids.