Lec 8 & 9

Lipids and Membranes: Learning Goals

  • Goals:

    1. Understand the names and structures of fatty acids (including double bonds) and phospholipids.

    2. Analyze the impact of structure on physical properties.

    3. Explore structure and composition of membranes.

    4. Identify types and properties of membrane proteins.

  • Reading Assignments:

    • Chapter 10: Sections 10.1, 10.2

    • Chapter 11: Section 11.1

    • Note: Skip figures not covered in lectures.

  • Textbook Problems:

    • Chapter 10: Questions 2, 3, 8, 10, 15, 16

    • Chapter 11: Questions 4, 5, 15

Structure and Nomenclature of Fatty Acids, Membranes, and Storage Lipids

Lipids

  • Definition: Organic molecules characterized by low solubility in water, relatively hydrophobic.

  • Functions of Lipids:

    • Storage of Energy: Fats serve as a long-term energy source.

    • Formation of Membranes: Essential for constructing cellular boundaries.

    • Hormones and Intracellular Messengers: Involved in signaling processes.

Biological Functions of Lipids

  • Energy Storage:

    • They exist in reduced forms which store a large amount of energy.

  • Hydrophobic Nature:

    • Provides good packing, insulation, low thermal conductivity, and mechanical protection.

    • The hydrophobic characteristic aids in repelling water, keeping the organism's surface dry.

Biological Functions of Lipids (cont'd)

  • Membrane Structure: Provides structural integrity in cells.

  • Cofactors for Enzymes:

    • Vitamin K: Involved in blood clot formation.

    • Coenzyme Q: Plays a role in ATP synthesis in mitochondria.

  • Signaling Molecules:

    • Paracrine Hormones: Act locally.

    • Steroid Hormones: Act throughout the body.

    • Growth factors and vitamin precursors (e.g., vitamins A and D) are included.

  • Vitamins and Antioxidants:

    • Vitamin E acts as an antioxidant.

  • Pigments:

    • Contribute to the color of fruits and vegetables (e.g., carotenoids in tomatoes, carrots, and pumpkins).

Types of Lipids

  • Two Major Categories:

    1. Lipids with Fatty Acids (Complex Lipids):

    • Include storage lipids and membrane lipids.

    1. Lipids without Fatty Acids:

    • Include cholesterol, vitamins, and pigments.

Classes of Membrane Lipids

  • Phospholipids:

    • Major constituents, including phosphoglycerides and sphingomyelin.

  • Glycolipids:

    • Such as cerebrosides, gangliosides (sphingolipids), and plant galactolipids.

  • Cholesterol: Adds stability and fluidity to membranes.

Fatty Acids

  • Definition: Carboxylic acids with hydrocarbon chains containing 4 to 36 carbons (12 to 24 are most common).

  • Characteristics of Natural Fatty Acids:

    • Usually have an even number of carbons.

    • Types of Fatty Acids:

    • Saturated: No double bonds between carbon atoms in the chain.

    • Monounsaturated: One double bond between carbons.

    • Polyunsaturated: More than one double bond in the alkyl chain.

Examples of Important Fatty Acids

  • Palmitate (16:0):

    • Ionized form (palmitic acid) with a pK of approximately 4.5, thus COO- at physiological pH.

    • Most abundant saturated fatty acid in humans.

  • Oleate (18:1):

    • Ionized form (oleic acid); common unsaturated fatty acid with a similar pK.

Amphipathic Characteristics of Fatty Acids

  • Molecular Structure: Composition includes a polar head group (COO-) and a non-polar hydrocarbon tail.

  • In Aqueous Solutions: Forms micelles to eliminate unfavorable contacts between water and hydrophobic tails, while allowing solvation of polar headgroups.

  • Drives Micelle Formation: Governed by the hydrophobic effect and favorable van der Waals interactions.

Nomenclature of Fatty Acids

  • Descriptive Naming:

    • Systematic names often include structural information.

    • Example: Cis-9-octadecenoic acid (Oleic acid).

    • D numbering: e.g., 18:1Δ9 indicates the location of the first carbon of the alkene in relation to the carboxyl carbon.

    • W numbering: e.g., 18:1ω9 indicates the terminal alkene relationship.

Naturally Occurring Fatty Acids (Tables 10-1 and 10-2)

  • Table 10-1:

    • Lists various naturally occurring fatty acids, their structures, properties, and nomenclatures, along with solubility and melting points.

  • Key Fatty Acids:

    • Lauric Acid (12:0), Myristic Acid (14:0), Palmitic Acid (16:0), Stearic Acid (18:0), Arachidic Acid (20:0), Lignoceric Acid (24:0).

Conformation and Melting Points of Fatty Acids

  • Chain Conformations: Saturated chains adopt extended conformations, while double bonds usually present in cis configuration create kinks.

  • Melting Point Relationships:

    • Shorter chains mean fewer van der Waals interactions, leading to lower melting points.

    • Compare saturated fatty acids: 16:0 and 18:0; unsaturated fatty acids will have lower melting points due to kinks allowing less tight packing.

Properties of Biological Membranes and Storage Fats

  • Reflect Fatty Acid Composition: Different types of fats (e.g., olive oil vs. butter vs. beef fat) have varying percentages of fatty acid types, impacting physical states (liquid or solid).

Categories of Biological Lipids

  • Types include:

    • Glycerolipids

    • Glycerophospholipids

    • Sphingolipids

    • Sterols

Triacylglycerols

  • Definition: Fats and oils serve as the primary storage form of lipids.

  • Solubility: They are less soluble in water than free fatty acids due to esterification of the carboxylate.

Storage Fats

  • Energy Storage Efficiency:

    • More energy per carbon and more reduced, carrying less water compared to glycogen and starch, suitable for long-term storage.

Polar Membrane Lipids

  • Structure: Composed of polar head groups and nonpolar tails, diversifying through modifications in backbones, fatty acids, and head groups.

  • Properties: Properties of head groups influence surface characteristics of membranes, with variation across different organisms, organs, and cellular membranes.

Composition of Different Membranes

  • Membrane Lipid Composition Variability: Varies across cell types and organs, reflecting differing functional requirements.

  • Example - Rat Hepatocyte Membranes: Show variations in percentage composition of cholesterol, cardiolipin, minor lipids, phosphatidylcholine, phosphatidylethanolamine, and sphingolipids.

Phosphoglycerides

  • Structure: Composed of glycerol backbone with two OH groups esterified to fatty acids R1 and R2, and the third OH group is esterified to a phosphorylated alcohol.

Sphingolipids

  • Comparison with Phospholipids:

    • Comprised of sphingosine and fatty acids connected by amide linkages.

    • Sphingomyelin as a major type, often present in myelin sheath of neurons, whereas phospholipids like phosphatidylcholine have a glycerol backbone.

Cholesterol

  • Structure: Contains a steroid nucleus made of four fused rings, contributing rigidity to membrane structures.

  • Polarity: Contains a weakly polar hydroxyl group contributing to some hydrophilicity, while the rest is predominantly nonpolar.

Function of Cholesterol

  • Membrane Role: Modulates fluidity and permeability of biological membranes, critical for maintaining membrane integrity.

  • Dietary Sources: Obtained through diet or synthesized in the liver; cholesterol is transported via blood bound to proteins.

Major Components of Membranes

  • Table of Major Components and Their Percentages:

    • Varies across different organisms such as human myelin sheath, mouse liver, maize leaf, yeast, Paramecium, and E. coli reflecting protein, phospholipid, and sterol components.

Membranes and Membrane Proteins

  • Functions of Membranes:

    • Define cell boundaries, control intracellular environment, and act as selective permeability barriers.

  • Membrane-Associated Activities:

    • Functions of membrane proteins include acting as receptors, transporters, and sites for energy conversion.

Membrane Bilayer

  • Structure: Comprises two leaflets of monolayers with hydrophilic head groups interacting with water and hydrophobic tails oriented inward.

  • Stabilizing Forces: Include hydrophobic effects, van der Waals interactions, and interactions between head groups and water molecules.

Membrane Properties

  • Determined by Composition: The fluidity and permeability are influenced by factors such as saturated vs. unsaturated fatty acid tails and lipid composition, which varies by cell types.

Membrane Protein Types

  • Categories Include:

    • Peripheral

    • Integral

    • Monotopic, bitopic, polytopic proteins

Peripheral Membrane Proteins

  • Characteristics:

    • Loosely associated, held together through ionic interactions with head groups.

    • Easily released by altering ionic conditions.

Integral Membrane Proteins

  • Characteristics:

    • Embedded within the membrane, typically traverse the bilayer, requiring detergents for extraction.

  • Functions: Often involved in biochemical activities related to transport, signaling, or energy conversion.

Detergents and Membrane Proteins

  • Function of Detergents: Amphipathic molecules that can solubilize membrane proteins by disrupting lipid membranes, allowing proteins to associate and disperse.

  • Examples of Detergents:

    • Octyl-D-glucoside

    • Sodium dodecyl sulfate

Summary of Learning Goals

  • The content covered throughout the review of fatty acids, lipids, and membranes enhances the student's understanding of biochemical structure and function.

Note: This set of study notes compiles comprehensive information based on the provided transcript, adjusted and organized for a thorough learning experience about lipids and membranes. Ensure to reference relevant chapters and textbook problems to reinforce understanding of key concepts as outlined.