chapter 9

Lipids: Structure and Function

• Lipids are an essential group of molecules found in all cells.
• While not polymeric like other macromolecules, lipids aggregate and perform crucial biological functions.
• These functions include:
  • Biological membrane components: Lipids with hydrocarbon chains are essential for forming biological membranes.
  • Energy storage: Lipids act as energy reserves in cells.
  • Cellular signaling: Lipids play key roles in various signaling pathways.
• Mycobacterium tuberculosis, the causative agent of tuberculosis, possesses a thick cell wall enriched with a unique lipid called mycolic acid, aiding its avoidance of the human immune system and antibiotics.
• Lipids are extracted using organic solvents like chloroform and methanol.
• They are separated using chromatography and identified through mass spectrometry.

Classes of Lipids: Fatty Acids

• Fatty acids are long-chain carboxylic acids with hydrocarbon side groups.
• They typically exist in esterified forms as major components of various lipids.
• Common biological fatty acids include:
  • Palmitic acid (C16)
  • Oleic acid (C18)
  • Linoleic acid (C18)
  • Stearic acid (C18)
• Fatty acids with an even number of carbon atoms are prevalent due to their biosynthesis by the catenation of C2 units.
• Unsaturated fatty acids contain double bonds; many are polyunsaturated (with two or more double bonds).
• ω-3 and ω-6 fatty acids are crucial polyunsaturated fatty acids.
• Saturated fatty acids are fully reduced, highly flexible, and can adopt numerous conformations.

Triacylglycerols: Energy Reservoirs

• Triacylglycerols, also known as triglycerides, are predominantly found in plants and animals.
• These nonpolar, water-insoluble substances are fatty acid triesters of glycerol.
• They function as energy reservoirs in animals and are the most abundant class of lipids in this group.
• The three fatty acid residues in triacylglycerols can vary, creating diverse structures like 1-Palmitoleoyl-2-linoleoyl-3-stearoylglycerol.

Steroids: Cholesterol and Hormones

• Steroids are lipids derived from cyclopentanoperhydrophenanthrene, a compound with four fused nonplanar rings.
• Cholesterol, the most abundant steroid in animals, is classified as a sterol due to its C3-OH group.
• It is a major component of animal plasma membranes, contributing 30-40% to its composition.
• Its polar OH group imparts weak amphiphilic character.
• Cholesterol can be esterified to long-chain fatty acids, forming cholesterol esters like cholesterol stearate.
• While plants contain minimal cholesterol, they synthesize other sterols.

Steroid Hormones: Regulating Function

• Cholesterol is the metabolic precursor for steroid hormones in mammals.
• These hormones are classified based on their physiological effects.
• Glucocorticoids like cortisol regulate carbohydrate, lipid, and protein metabolism, influence inflammatory reactions, and aid in stress responses.
• Aldosterone and other mineralocorticoids control salt and water excretion by the kidneys.
• Androgens and estrogens regulate sexual development and functions.
• Testosterone is an androgen (male sex hormone), and β-estradiol is an estrogen (female sex hormone).

Vitamin D: Sunlight and Calcium Absorption

• Vitamin D2 (ergocalciferol) is formed nonenzymatically in animal skin through the photolytic action of UV light on ergosterol, a plant sterol commonly added to milk.
• Vitamin D3 (cholecalciferol) is similarly formed from 7-dehydrocholesterol.
• Both vitamin D2 and D3 are inactive; their active forms are produced by adding an OH group by liver and kidney enzymes, yielding 1α,25-Dihydroxycholecalciferol.
• Active vitamin D elevates serum Ca2+ by promoting intestinal absorption of dietary Ca2+.
• It also enhances the absorption of Ca2+ in bones and teeth.

Vitamin D Deficiency: Rickets

• Vitamin D deficiency in children leads to rickets, a disease characterized by stunted growth and deformed bones.

Membrane Fluidity and Cholesterol

• Cholesterol does not form a bilayer independently.
• It decreases membrane fluidity because its rigid steroid ring system hinders the movement of fatty acid side chains in other membrane lipids.

Membrane Proteins: Aquaporin-O and Glycophorin A

• The integral membrane protein aquaporin-O (AQPO) is associated with lipids.
• It forms channels facilitating water transport through the membrane.
• Glycophorin A is a transmembrane (TM) protein completely spanning the erythrocyte membrane.
• It bears 15 O-linked oligosaccharides and one N-linked oligosaccharide in its extracellular domain.
• Its hydrophobic transmembrane region consists of 19 sequential amino acid residues.
• The terminal portion, located on the cytoplasmic face, is rich in anionic, cationic, and polar residues.

Glycophorin A Polymorphisms

• Two common genetic variants of glycophorin A exist.
• Glycophorin A^M^ has Ser and Gly at positions 1 and 5, whereas glycophorin A^N^ has Leu and Glu at these positions.

Bacteriorhodopsin: Generating Proton Gradient

• Bacteriorhodopsin, an integral membrane protein produced by the archaebacterium Halobacterium salinarum, uses electron crystallography for structure determination.
• Its structure consists of seven α-helical rods spanning the lipid bilayer perpendicularly.
• It generates a proton concentration gradient across the cell membrane, driving ATP synthesis.
• Retinal, a covalently bound molecule, acts as the light-absorbing group in the protein.

Identifying Transmembrane Helices: Hydropathy Plots

• Hydropathy plots help identify transmembrane helices in proteins.
• They plot the average hydropathy indices of amino acid residues within a sliding window against the position of the first amino acid.
• A positive value indicates a hydrophobic peptide segment, suggesting a transmembrane helix.
• Segments longer than 20 hydrophobic residues are likely to form TM helices.
• Glycophorin A has only one TM helix, while bacteriorhodopsin possesses seven.

β-Barrels: Porins

• Porins, transmembrane proteins, can form β-barrels.
• These structures facilitate the transport of specific molecules through the membrane.
• The X-ray structure of E. coli OmpF porin, a 16-stranded monomer, reveals its β-barrel architecture.

Prenylation: Anchoring Proteins to Membranes

• Some membrane-associated proteins have covalently attached lipids that anchor them to the membrane.
• Prenylated proteins have lipids built from isoprene units.
• The most common isoprenoid groups are the C15 farnesyl and C20 geranylgeranyl residues.
• Prenylation is a post-translational modification where lipids are added to proteins.
• The most frequent prenylation site is a C-terminal tetrapeptide C-X-X-Y, where C is cysteine, X is often an aliphatic amino acid, and Y influences the type of prenylation.
• The prenyl group is enzymatically linked to the Cys sulfur atom via a thioether linkage.

Membrane Fluidity: Lateral Diffusion

• The rate at which a bleached area recovers its fluorescence, as measured by fluorescence microscopy, indicates the rate of lateral diffusion of fluorophore-labeled molecules in the membrane.

Membrane Skeleton: Erythrocyte Ghost

• The membrane skeleton helps define cell shape.
• Erythrocyte ghosts, obtained by osmotic lysis, represent the membranous particles lacking cytoplasm.
• Spectrin, a major protein in erythrocyte ghosts, constitutes 75% of the erythrocyte membrane skeleton.
• Spectrin has two polypeptide chains: a 280-kDa α-subunit and a 246-kDa β-subunit.
• Each polypeptide contains multiple 106-residue repeats, forming flexibly connected triple-helical bundles.
• Two heterodimers join head-to-head to create an (αβ)2 heterotetramer.

Spectrin and Erythrocyte Membrane Skeleton

• The X-ray structure of three consecutive repeats of chicken brain α-spectrin reveals an up-down-up triple-helical bundle.
• Electron micrographs illustrate the stretched erythrocyte membrane skeleton, revealing its structure.
• A model of the erythrocyte membrane skeleton shows its relationship to the intact cell.
• Junctions between spectrin tetramers include actin, tropomyosin, and band 4.1 protein.

Ankyrin: Connecting Spectrin and Ion Channels

• Spectrin is associated with the 1880-residue protein ankyrin, which binds to integral membrane ion channel proteins.
• Ankyrin's N-terminal 798-residue segment consists primarily of 24 tandem ankyrin repeats.
• The X-ray structure of human ankyrin repeats 13-24 shows its structure.

Membrane Protein Mobility

• The interaction between membrane components and the underlying cytoskeleton explains the various mobilities of membrane proteins within the membrane.
• A model illustrates how the cytoskeleton influences membrane protein mobility.

Test your knowledge on the membrane skeleton of erythrocytes and its structural components. This quiz covers key proteins, microscopy techniques, and the influence of cholesterol on membrane fluidity. Dive deep into the specifics of membrane structure and function.