BCHM3050 Chapter 8: Lipids and Membranes Study Notes

BCHM3050 Chapter 8: Lipids and Membranes

Introduction to Lipids

  • Definition and Functions:
    • Lipids serve multiple critical functions, including:
    • Energy storage
    • Membrane structure
    • Cell signaling
  • Classes of Lipids:
    • Lipids can be categorized into the following classes:
    • Fatty acids
    • Triacylglycerols
    • Glycerophospholipids
    • Sphingolipids
    • Isoprenoids

Fatty Acids

  • Structure:
    • Composed of a carboxylic acid group and a long hydrocarbon tail, generally consisting of 12 to 20 carbon atoms.
    • Most naturally occurring fatty acids feature an even number of carbons.
    • Fatty acids are numbered starting from the carboxylate end, with the terminal methyl carbon referred to as the omega (ω) carbon.
    • Fatty acids play a crucial role in forming triacylglycerols and phospholipids.
The Structure of Fatty Acids Affects Lipid Properties
  • Types of Fatty Acids:
    • (a) Saturated fatty acid
    • (b) Monounsaturated fatty acid (MUFA)
    • (c) Polyunsaturated fatty acid (PUFA)
  • Saturated fatty acids are straighter in structure and pack more tightly together, leading to higher melting points compared to MUFAs and PUFAs.
Saturated Fatty Acids
  • Defined as fatty acids that are fully “saturated” with hydrogen atoms, meaning there are no carbon-carbon double bonds.
  • Characteristics:
    • Tightly packed structure
    • Higher melting temperatures
    • Examples: Palmitate, Stearate
Unsaturated and Polyunsaturated Fatty Acids
  • Unsaturated fatty acids contain one or more double bonds.
  • They can exist in two isomeric forms:
    • Cis: Similar groups on the same side.
    • Trans: Similar groups on opposite sides.
  • Natural unsaturated fatty acids predominantly feature cis bonds.
  • Unsaturated fatty acids do not pack tightly, resulting in lower melting temperatures.
Comparison of Saturated vs Unsaturated Fatty Acids
  • Stearate ion: Represents a saturated fatty acid (deprotonated form of stearic acid); hydrophilic head and hydrophobic tail.
  • Oleate ion: Represents an unsaturated fatty acid with one cis double bond.
Common Fatty Acids
  • Table 8.1 Some Common Fatty Acids:
    • Saturated fatty acids:
    • Lauric acid (12 carbons), Myristic acid (14 carbons), Palmitic acid (16 carbons), Stearic acid (18 carbons), Arachidic acid (20 carbons)
    • Unsaturated fatty acids:
    • Palmitoleic acid (16 carbons), Oleic acid (18 carbons, 1 double bond), Linoleic and α-Linolenic acids (18 carbons, 2 and 3 double bonds respectively)
    • Arachidonic acid (20 carbons, 4 double bonds), Eicosapentaenoic acid (EPA, 20 carbons, 5 double bonds), Docosahexaenoic acid (DHA, 22 carbons, 6 double bonds).
Delta and Omega Naming Systems
  • Represents how double bonds are specified within fatty acids.

Triacylglycerols

  • Triacylglycerols consist of glycerol linked to three fatty acids.
  • Characteristics:
    • Lacks a significant polar portion.
    • Fatty acids may differ in structure.
  • Functions:
    • Primarily for energy storage and insulation.

Glycerophospholipids

  • Structure consists of:
    • Glycerol backbone
    • A polar head group containing phosphate and potentially additional polar or charged groups
    • Two fatty acid tails
  • Amphipathic Molecule:
    • Contains both hydrophobic (fatty acids) and hydrophilic (polar head) regions.
  • Function: Critical component of cell membrane structure.

Sphingolipids

  • Built on a sphingosine backbone that typically includes:
    • One fatty acid
    • One polar group (phosphate or sugars)
  • Found as vital components in both animal and plant membranes.
Types of Sphingolipids
  • Sphingomyelin:
    • Composed of a sphingosine backbone + fatty acid + phosphate.
    • Predominantly located in cell membranes with significant abundance in the myelin sheath of nerve cells.
  • Glycolipids:
    • Comprised of lipids with attached sugar residues.
    • Different subclasses such as cerebrosides and gangliosides, used for cellular recognition and signaling.

Sphingolipid Storage Diseases

  • Examples of lysosomal storage diseases linked to sphingolipid metabolism:
    • GM1 gangliosidosis
    • Tay-Sachs disease
    • Gaucher disease
    • Niemann-Pick disease A and B
    • Metachromatic leukodystrophy
    • Krabbe disease

Isoprenoids

  • Diverse biomolecules characterized by repeating five-carbon isoprene units.
  • Examples: citronella, carotenoids, pinene.

Steroids

  • Steroid structure includes four fused rings, derivatives of isoprenoids.
  • Found in eukaryotes and some bacteria; function is membrane stability.
  • Variants based on substitutions and double-bond placements include:
    • Cholesterol
    • Estrogen
    • Testosterone

Membrane Structure

Lipid Bilayer
  • Forms spontaneously due to the hydrophobic effect.
  • Composed of:
    • Glycerophospholipids
    • Sphingolipids
    • Cholesterol
    • Membrane proteins
Melting Points of Fatty Acids
  • Saturated fatty acids yield a waxy consistency at room temperature with higher melting points.
  • Unsaturated fatty acids manifest as oily liquids with lower melting points.
  • The degree of saturation directly correlates to packing efficiency and melting temperatures.
Fluidity in Membranes
  • Unsaturated Fatty Acids:
    • Kinks in tails decrease packing density, increasing fluidity.
  • Saturated Fatty Acids:
    • Straight tails allow for better packing, decreasing fluidity.
  • Ranking:
    • Saturated fatty acids: Longer carbon chains result in higher melting points.
    • Unsaturated fatty acids: More double bonds lead to lower melting points.
Cholesterol's Role in Membrane Fluidity
  • Cholesterol acts as a buffer to maintain membrane integrity:
    • Prevents excessive packing of lipids at low temperatures.
    • Holds lipids together at higher temperatures.
Lipid Asymmetry and Membrane Proteins
  • Lipid asymmetry is maintained via enzymes such as:
    • Scramblase
    • Flippase
    • Floppase

Membrane Proteins

Classification of Membrane Proteins
  • Integral Membrane Proteins:
    • Span the membrane, contain hydrophobic amino acids flanked by charged regions, facilitating transport.
  • Peripheral Membrane Proteins:
    • Interact electrostatically with lipid head groups or integral proteins.
  • Lipid-anchored Proteins:
    • Covalently attached hydrophobic anchors embedded in the membrane.
    • Include myristoylation, palmitoylation, prenylation.

The Fluid Mosaic Model

  • Describes how proteins are embedded in the lipid bilayer, allowing lateral movement but with a fixed orientation.
  • Factors limiting protein mobility include interactions with cytoskeletal elements and other membrane components.

Exam Review Topics

  • Recognizing structures and functions of lipids discussed, including:
    • Saturated and unsaturated fatty acids
    • Triacylglycerols
    • Glycerophospholipids
    • Sphingolipids including types such as sphingomyelin, cerebrosides, and gangliosides
  • Understanding how hydrophobic characteristics influence lipid locations and functions, and effects on membrane dynamics.
  • The roles of cholesterol and lipid translocases in maintaining membrane integrity and asymmetry.