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#membrane biology

13 public questions tagged with this topic.

Plasma Membrane

Which membrane component is responsible for forming a hydrophilic barrier?

Aqueous compartments surround cells, so membrane lipids must present hydrophilic surface to water while hiding hydrophobic interior away from solvent. Phospholipid headgroups such as phosphocholine, phosphoethanolamine, phosphoserine, and phosphoinositol are zwitterionic or anionic, heavily hydrated, forming hydrogen bonds with water and creating interfacial region of high polarity and dielectric constant. This polar curtain, two molecules thick around bilayer perimeter, repels hydrophobic compounds and prevents uncontrolled leakage of cellular metabolites, establishing hydrophilic barrier ess

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Membrane Barrier and Headgroup Hydration.

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Glycolipid and Phospholipid

Which statement about lipid droplets is correct?

Lipid droplets defy classic bilayer vesicle definition possessing neutral core surrounded by monolayer explaining unique buoyancy density microscopy appearance. Emergence begins when DGAT1 DGAT2 synthesize triacylglycerol DAG acyl CoA ACAT1 ACAT2 forms cholesteryl ester accumulating between ER leaflets forming oil lens twenty to sixty nanometers growing via Ostwald ripening coalescence regulated Seipin LDAF1 perilipin recruitment. Lens buds toward cytosol coated phospholipid monolayer enriched PC PE lyso PC preventing coalescence decorated perilipin family PLIN1 five CIDE proteins Seipin Rab18

Ref: Walther TC & Farese RV, Annu Rev Biochem 2012, Lipid droplets neutral lipid storage monolayer organization and functions.

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Glycolipid and Phospholipid

Which of the following is NOT a glycolipid?

Lipid classification relies headgroup chemistry: glycolipid defined direct glycosidic linkage sugar ceramide without phosphate intermediate distinct from phospholipid. Glucosylceramide galactosylceramide lactosylceramide GM1 ganglioside exemplify category synthesized UGCG glucosyltransferase B4GALT5 galactosyltransferases producing beta linkage to ceramide hydroxyl. Sphingomyelin instead ceramide one phosphate linked choline ethanolamine via phosphodiester synthesized sphingomyelin synthases SGMS1 Golgi SGMS2 plasma membrane transferring phosphocholine phosphatidylcholine ceramide releasing di

Ref: Shayman JA, J Lipid Res, Sphingomyelin is sphingophospholipid not glycolipid distinction and metabolism.

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Glycolipid and Phospholipid

Which of the following best describes cholesterol’s role in lipid rafts?

Liquid ordered raft stability arises favorable packing between sphingolipid and sterol driven enthalpy entropy compensation. Cholesterol small polar hydroxyl rigid planar tetracycle inserts into voids between long saturated amide linked acyl chains sphingosine backbone maximizing van der Waals contacts permitting hydrogen bond between sphingosine C3 hydroxyl cholesterol hydroxyl and amide carbonyl to amide donor. This promotes condensed complex formation increasing bilayer thickness four angstroms decreasing permeability solutes and creating diffusion distinct from liquid disordered with FRAP

Ref: Simons & Sampaio, Cold Spring Harb Perspect Biol 2011, Cholesterol sphingolipid stabilization rafts formation.

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Properties of Plasma Membrane

Which of the following is NOT involved in maintaining plasma membrane asymmetry?

Maintenance of nonrandom phospholipid distribution across leaflets relies on specialized lipid translocators rather than hydrolytic enzymes. Flippases are P4 ATPases such as ATP11C and ATP11A complexed with CDC50A chaperone that consume ATP to move phosphatidylserine and phosphatidylethanolamine inward against gradient maintaining inner negativity. Floppases are ABC transporters ABCA1 ABCB1 ABCB4 that export phosphatidylcholine and cholesterol outward crucial for HDL biogenesis and bile secretion. Scramblases like TMEM16F ANO6 and Xkr8 family are calcium activated or caspase activated providin

Ref: Pomorski & Menon, Annual Review Cell Biology, Lipid translocators flippase floppase scramblase functions.

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Properties of Plasma Membrane

Which of the following is NOT a function of lipid bilayers?

Biological membranes excel at compartmentalization, selective permeability, scaffolding receptors, and regulating fluidity, but they do not directly catalyze polymerization of deoxyribonucleotides into DNA strands. Lipid bilayers form continuous hydrophobic barriers five nanometers thick preventing free diffusion of ions and polar metabolites, enabling maintenance of sodium potassium gradients by Na K ATPase and creation of organelle specific lumens for oxidative folding. They anchor integral proteins via hydrophobic matching, recruit peripheral proteins through PIP2 and phosphatidylserine ele

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Membrane functions and Chapter 5: DNA replication machinery.

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Plasma Membrane Proteins

Which of the following proteins spans the membrane 14 times?

Band 3 anion exchanger family SLC4A1 represents prototypical high abundance multi-pass transporter in human erythrocytes comprising about twenty five percent integral membrane protein. Full length nine hundred eleven amino acids organized into two domains: N-terminal cytoplasmic domain approximately four hundred three residues forming elongated dimeric structure binding ankyrin, protein 4.2, deoxyhemoglobin, and glycolytic enzymes including aldolase regulating metabolism; C-terminal membrane domain approximately five hundred residues comprising fourteen transmembrane helices arranged into two

Ref: Arakawa et al., Crystal Structure of AE1 Band 3 Anion Exchanger, Science 2015.

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Plasma Membrane Proteins

Which of the following is an example of a single-pass transmembrane protein?

Single-pass type I membrane protein topology illustrated by glycophorin A major sialoglycoprotein CD235a abundant about million copies per red cell. Gene GYPA encodes one hundred fifty amino acids including signal peptide nineteen residues removed cotranslationally. Extracellular domain seventy residues heavily O-glycosylated with fifteen O-linked tetrasaccharides NeuAc-alpha2-3Gal-beta1-3 GalNAc plus one N-linked complex glycan contributing sixty percent mass, terminal sialic acid provides negative zeta potential preventing aggregation and defines M and N blood group antigens recognized by an

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 10, Single-Pass Proteins – Glycophorin.

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Plasma Membrane Proteins

Which of the following interactions primarily stabilizes transmembrane proteins within the lipid bilayer?

Stabilization of transmembrane segments inside bilayer originates from hydrophobic effect plus van der Waals packing. Sec61 translocon provides aqueous pore laterally gated allowing nascent hydrophobic sequence to partition directly into lipid phase assessed by apparent free energy scale of Hessa and von Heijne measuring contribution per residue leucine minus zero point five six kilocalories, arginine positive two point five kilocalories. Burying non-polar side chains of leucine isoleucine valine phenylalanine methionine avoids forcing water to form ordered clathrate cages around them, releasi

Ref: White and von Heijne, How Translocons Select Transmembrane Helices, Annu Rev Biophys 2008.

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Lipid Link Protein

Which of the following lipids forms a thioester bond with cysteine?

Distinction between cysteine lipidations clarifies regulation of membrane affinity. S-palmitoylation involves nucleophilic attack of cysteine thiolate on palmitoyl-CoA thioester producing thioester bond between sixteen-carbon saturated palmitate and side chain catalyzed by membrane-integrated DHHC zinc finger enzymes containing conserved Asp-His-His-Cys motif utilizing palmitoyl-CoA donor. This thioester is reversible rapidly cleaved by serine hydrolases APT1, APT2, PPT1 and ABHD17 allowing depalmitoylation cycles controlling localization of HRas trafficking between Golgi and plasma membrane,

Ref: Resh, Palmitoylation of Ligands, Receptors and Intracellular Signaling Molecules, Sci STKE 2006.

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Lipid Link Protein

What happens when detergent concentration is below the CMC?

Understanding CMC highlights phase behavior of amphiphiles. At subsaturation detergent monomers exist dissolved in aqueous phase hydration shell disordered, partial insertion into outer leaflet may increase permeability but leaves bilayer structure largely intact. When monomer concentration reaches thermodynamic threshold where entropic cost of ordering water around hydrophobic tails outweighs micellization free energy, spontaneous aggregation into micelles occurs with hydrocarbon tails sequestered interior and polar heads facing water forming spherical or ellipsoidal particles thirty to fifty

Ref: le Maire et al., Interaction of Membrane Proteins and Detergents, Biochim Biophys Acta 2000.

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Lipid Link Protein

Which of the following is a membrane-permeable detergent?

Detergent physicochemical properties determine permeabilization without denaturation. Triton X-100 tert-octylphenyl polyethoxyethanol consists of hydrophobic tert-octylphenol moiety and hydrophilic polyoxyethylene chain averaging nine to ten units, conferring CMC near zero point two four millimolar, aggregation number about one hundred to one fifty and micellar weight around ninety kilodalton. It partitions efficiently into phospholipid bilayers increasing permeability to macromolecules like antibodies and substrates, forming mixed micelles that solubilize lipids while protein-protein electros

Ref: Seddon et al., Methods Mol Biol, Membrane Protein Solubilization and Detergent Properties.

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