lipids

Lipids

Learning Objectives

• List the five subclassifications for lipids based on biochemical function.
• Classify fatty acids based on their degree of unsaturation, chain length, and the number and position of multiple bonds using the structure or delta and/or omega notations.
• Differentiate between simple and mixed triacylglycerols, fats, and oils.
• Describe the human body's response to dietary fats based on the degree of unsaturation and omega-fatty acid classification of fatty acid residues in dietary fats.
• Describe the structural characteristics and the "head and two tails" model for glycerophospholipids, sphingophospholipids, and sphingoglycolipids.
• Describe the structural characteristics of steroid nuclei, cholesterol, and steroid hormones.
• Describe the role of the lipoproteins HDL and LDL in cholesterol transportation within the body.
• Describe cell membrane structure in terms of structural arrangement associated with a lipid bilayer.
• Describe the structure of bile acids and their biochemical function.
• List the two major classes of steroid hormones and their biochemical functions.
• Describe the general structural features of eicosanoids and biological waxes.
• List the three principal subclasses of eicosanoids and the main physiological effects associated with each subclass.

Five Lipid Subclassifications

• Definition: A lipid is an organic compound found in living organisms that is insoluble (or only sparingly soluble) in water but soluble in nonpolar organic solvents; also known as "fats."
• Classification by Biochemical Function: Lipids can be classified into five categories:
  1. Energy-storage lipids: Triacylglycerols (TAG)
  2. Membrane lipids: Phospholipids, sphingoglycolipids, and cholesterol
  3. Emulsification lipids: Bile acids
  4. Messenger lipids: Steroid hormones and eicosanoids
  5. Protective-coating lipids: Biological waxes

Structural Diversity of Lipids

• A key feature of all lipids is that they contain a water-soluble "head group" and a water-insoluble "tail."
• The tail can consist of more than one group.

Fatty Acids

• Classification by Number of Carbons:
  • Long-chain fatty acids: C12 to C26
  • Medium-chain fatty acids: C8 to C10
  • Short-chain fatty acids: C4 to C6
• Classification by Degree of Unsaturation:
  • Saturated fatty acid: No double bonds (e.g., 16:0)
  • Monounsaturated fatty acid: One double bond (e.g., 18:1(∆9))
  • Polyunsaturated fatty acid: More than one double bond (e.g., 18:2(∆6,9))
• Definition: A fatty acid is a naturally occurring monocarboxylic acid that serves as a building block for many lipids, tends to contain an even number of carbons, and is often part of a more complex lipid molecule.

Numerical Shorthand

• Notation: Two numbers are used: the first represents the number of carbons, while the second indicates the number of carbon–carbon double bonds.
• Examples include:
  • 18:0 (18 carbons, zero double bonds)
  • 16:1 (16 carbons, one double bond)
• Delta Notation: Uses the Greek capital letter delta (Δ) to denote the location of double bonds relative to the carboxyl carbon (C1). Example: 18:3(Δ9,12,15) indicates three double bonds at carbons 9, 12, and 15.
• Monounsaturated fatty acids (MUFAs) generally have double bonds at Δ9, while polyunsaturated fatty acids (PUFAs) typically feature double bonds at Δ9,12, and optionally Δ9,12,15.
• Arachidonic Acid Exception: Notation 20:4(Δ5,8,11,14) describes arachidonic acid.

Omega Notation

• Omega-3 Fatty Acids (ω-3): Unsaturated fatty acids with the endmost double bond three carbons away from the methyl end.
• Examples of ω-3 Fatty Acids:
  • Omega-6 Fatty Acids (ω-6): Unsaturated fatty acids with the endmost double bond six carbons away from the methyl end.
• Omega notation utilizes the Greek lowercase letter omega (ω) followed by a number to indicate double-bond position relative to the methyl end of the fatty acid chain.

Classification Example

• Determine whether the fatty acid is:
  • Saturated or unsaturated?
  • If unsaturated, is it monounsaturated or polyunsaturated?
  • Is it an ω-3 or ω-6 fatty acid, or neither?
  • Write shorthand notation and delta notation if applicable.

Triacylglycerides (Triglycerides)

• Definition: A triacylglycerol is a lipid formed by the esterification of three fatty acids to a glycerol molecule.
• Functions:
  • Serves as an energy-storage molecule.
  • Stored in adipocytes (fat cells).
  • Exhibits high packing efficiency compared to glycogen.
  • The most abundant type of lipid in the human body.
  • Composed of one glycerol molecule and three fatty acids linked via ester linkages.

Triglycerides: Simple vs. Complex

• Simple Triglyceride: All three fatty acids are the same.
• Complex Triglyceride: Contains at least two different types of fatty acids.

Fats and Oils

• Oils:
  • Mixtures of TAGs that are liquid at room temperature.
  • Generally sourced from plants and fish.
  • Higher composition of unsaturated fats.
• Fats:
  • TAG mixtures that are solid at room temperature.
  • Primarily derived from animals.
  • Higher concentration of saturated fats leading to tighter packing.
• Shared Characteristics:
  • Both fats and oils have varying fatty acid mixtures depending on the source (plant or animal).
  • Individual profiles are also affected by diet and climate.
• Pure Fats and Oils: Colorless, odorless, and tasteless.

Profile of Dietary Fats

“Good” and “Bad” Fats

• Omega-3 (ω-3) and omega-6 (ω-6) fatty acids influence heart disease risk.
• Diets high in ω-3 and low in ω-6 correlate with decreased heart disease (e.g., Inuit diet).
• Fatty acids from cold-water fish (e.g., Albacore tuna, salmon, mackerel) are rich in ω-3.
• Diets high in ω-3 and low in ω-6 show low incidence of heart disease.
• Diets low in ω-3 and high in ω-6 increase heart disease risk; typical U.S. diet is high in ω-6 from plants.
• Classification of Fats:
  • Saturated Fats: Considered bad and linked to increased heart disease risk.
  • Monounsaturated Fats: Considered good and associated with decreased heart disease and cancer risk.
  • Polyunsaturated Fats: Can be both good and bad; decrease heart disease risk but may increase certain cancer risks.

ω-3 Fatty Acids in Fish (Per 3.5-oz. Serving, Raw)

Essential Fatty Acids

• Definition: Essential fatty acids must be obtained from dietary sources as the body cannot synthesize them adequately.
• Two Essential Fatty Acids:
  1. Linoleic acid (18:2), an ω-6 fatty acid.
  2. Linolenic acid (18:3), an ω-3 fatty acid.

Phospholipids

• Types of Phospholipids:
  1. Glycerophospholipids: Glycerol as the platform molecule.
  2. Sphingophospholipids: Sphingosine as the platform molecule.
• Definition: A phospholipid contains one or more fatty acids, a phosphate group, and a platform molecule to which they are attached, along with an alcohol that is linked to the phosphate group.
• Function: Major lipid found in cell membranes.

Structural Features of Glycerophospholipids

• Hydrophilic Group: The polar portion attracted to water, allowing it to mix.
• Hydrophobic Group: The nonpolar portion that does not attract water and cannot mix with it.

Sphingosine Structure

• The fatty acid is linked via an amide bond.
• Phosphate is bonded to the terminal —OH group via an ester linkage.
• An amino alcohol is attached to the phosphate via another ester linkage.
• A sphingophospholipid contains a fatty acid, a phosphate group attached to a sphingosine, and an alcohol linked to the phosphate.

Sphingoglycolipids

• Cerebrosides: Carbohydrate is a monosaccharide (usually glucose or galactose) found in the brain and myelin sheath.
• Gangliosides: Carbohydrate is an oligosaccharide with up to seven carbohydrate units, found in the brain's gray matter and in the myelin sheath.
• A sphingoglycolipid consists of a fatty acid and a carbohydrate component attached to a sphingosine with a glycosidic linkage.

Lipid Classification Charts

• Chemistry at a Glance: Summarizes the structural relationships among various types of fatty-acid-containing lipids, categorized by biochemical function.
  • Energy-storage Lipids: Triacylglycerols (fats and oils)
  • Membrane Lipids: Glycerophospholipids and sphingolipids
  • Emulsification Lipids: Bile acids
  • Messenger Lipids: Steroid hormones and eicosanoids
  • Protective-coating Lipids: Biological waxes

Steroid Structures

• Cholesterol: A component of cell membranes and a precursor to other steroids.
• Steroid Structure: Four fused rings (three six-membered and one five-membered), with specific numbers assigned to carbon positions. Common features include:
  • An =O and —OH on C3
  • A double bond between C5 and C6
  • A carbon chain attached to C17

Cholesterol in the Body

• Synthesis and Sources: Synthesized in the liver (800-1000 mg daily) and obtained primarily from animal-origin foods. Requires lipoproteins for blood transport due to water insolubility.
• Lipoproteins:
  • LDLs (Low-Density Lipoproteins): Transport cholesterol from the liver to cells; often termed "bad cholesterol."
  • HDLs (High-Density Lipoproteins): Transport excess cholesterol back to the liver; termed "good cholesterol."
• Health Implications: High cholesterol levels are linked to atherosclerosis characterized by plaque buildup in arteries, restricting blood flow due to imbalances between LDL and HDL levels.

Cell Membranes and Lipid Bilayers

• Typical thickness: 6 to 9 nanometers, about 10% of the width of a human hair.
• Structure:
  • Three Distinct Regions: Two exterior polar heads and one interior nonpolar tail.
  • Movement: Lipids can move freely laterally within the bilayer.
• Composition: Cell membranes consist of 80% lipids and 20% proteins and carbohydrates, primarily phospholipids, glycolipids, and cholesterol.
• Lipid Bilayer: A structure where the nonpolar tails are oriented inward and polar heads face outward towards the aqueous environment.

Phospholipids and the Lipid Bilayer

• Tail Configuration: Bends in the molecule prevent tight packing of nonpolar tails, affecting the partition of lipids in the bilayer.
• Chirality: The glycerol and sphingosine components exist as L-isomers. C2 is a chiral center, adding to structural complexity.
• Solid vs. Liquid Fat: The solid nature of butter results from the high concentration of saturated fats that fit tightly together, while unsaturated fatty acids in oils make them liquid due to kinks in their structure.

Cell Membrane Components

1. Integral Membrane Proteins:
   • Penetrate the cell membrane, either partially or fully spanning it.
2. Peripheral Membrane Proteins:
   • Do not penetrate the membrane and are located on its surface, possibly connected to carbohydrates (glycoproteins) acting as markers.

• Cholesterol's Role: Stabilizes membrane rigidity; its —OH group points outward toward water.

Bile Acids

• Function: Act as emulsifiers to disperse and stabilize water-insoluble substances in aqueous solutions.
• Synthesis: Produced by the liver, stored in the gallbladder, and released into the small intestine during digestion.
• Types: Simple bile acids (di- or trihydroxy derivatives) and complexed bile acids that bond with amino acids (glycine or taurine) to enhance their emulsifying capability.

Steroid Hormones

• Definition: Hormones derived from cholesterol, produced by ductless glands to send messages between various body cells.
• Classes:
  1. Sex Hormones: Regulate reproduction and secondary sex characteristics.
  2. Adrenocorticosteroids: Regulate various biochemical processes.

Eicosanoids

• Definition: Oxygenated derivatives of C20 fatty acids, function as messenger lipids but are not true hormones. Synthesized by tissues exerting effects.
• Classes:
  1. Prostaglandins
  2. Leukotrienes
  3. Thromboxanes

Physiological Effects of Eicosanoids

• Functions Include:
  1. Mediation of the inflammatory response to tissue damage.
  2. Production of pain and fever.
  3. Regulation of blood pressure.
  4. Induction of blood clotting.
  5. Control of reproductive functions, such as labor induction.
  6. Regulation of the sleep/wake cycle.

Biological Waxes

• Components: Typically contain a saturated fatty acid (14 to 36 carbons) and an alcohol component (16 to 30 carbons).
• Definition: Biological waxes are monoesters of long-chain fatty acids and long-chain alcohols, insoluble in water due to long carbon chains.
• Notable Functions: Provide water-repellent properties, serving as protective coatings in various contexts.

Lipid Classifications Based on Function

• Chemistry at a Glance: Lipids are classified by their biochemical function:
  • Energy Storage Lipids: Triacylglycerols (fats & oils) used for energy during high demand.
  • Membrane Storage Lipids: Structural components of cell membranes, including sphingoglycolipids and cholesterol.
  • Emulsification Lipids: Bile acids help solubilize water-insoluble substances.
  • Messenger Lipids: Steroid hormones and eicosanoids regulate functions within tissues.
  • Protective-coating Lipids: Waxes and other lipids fulfill protective roles in organisms.