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#lipid movement

3 public questions tagged with this topic.

Properties of Plasma Membrane

Which lipid movement occurs the fastest in a membrane?

Biological membranes support diverse motions separated by orders of magnitude based on activation energy required to traverse hydrocarbon core. Lateral diffusion within same leaflet requires only breaking weak van der Waals contacts and creating small free volume achieving diffusion coefficient about ten to minus eight centimeters squared per second meaning lipid circumnavigates Escherichia coli in microseconds and mammalian cell in seconds. Axial rotation and wobbling of acyl chains are even faster nanoseconds observed by ESR. In contrast flip flop forcing zwitterionic or anionic headgroup through low dielectric interior faces energy barrier near sixty to ninety kilojoules per mole yielding spontaneous half times hours to days for phosphatidylcholine without catalysis. Cells overcome slow step using flippase scramblase lowering barrier via hydrophilic groove. FRAP bleaching NBD PC spot shows rapid recovery via lateral exchange while spin label EPR assays of flip flop show negligible recovery. Cytoskeletal picket fence model by Kusumi explains macroscopic confinement despite local speed. Understanding rapid lateral versus sluggish transverse motions explains how compositional asymmetry persists despite fluid mosaic mobility fundamental for exams.

Ref: Alberts et al., MBOC 7th ed., Chapter 10: Rates of lipid movement lateral vs transverse.

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Properties of Plasma Membrane

Which experimental technique is used to study lipid movement in membranes?

Experimental investigation of lipid dynamics in plasma membranes of living cells requires fluorescence techniques capable of micrometer spatial resolution and millisecond temporal. FRAP fluorescence recovery after photobleaching involves incorporating fluorescent lipid analogs NBD-phosphatidylethanolamine BODIPY-phosphatidylcholine DiI dialkylcarbocyanine into bilayer at low mole fraction, or fluorescent protein fusions GFP-tagged for proteins. Focused laser beam bleaches circular spot one to four micrometer diameter irreversible photodestruction, subsequent recovery of fluorescence intensity monitored at low laser power as unbleached fluorophores diffuse from adjacent regions into bleached zone following Fick laws. Analysis yields diffusion coefficient D and mobile fraction Mf reflecting barriers cytoskeleton fences eighty nanometer corrals and lipid raft domains ordered phases slowing recovery. Complementary methods single particle tracking gold conjugated lipids high speed video, fluorescence correlation spectroscopy FCS confocal fluctuation. Western blotting separates denatured subunits by SDS-PAGE immunodetection does not report movement, X-ray crystallography determines static atomic coordinates crystalline lattice, PCR amplifies DNA. FRAP remains gold standard quantifying lateral movement of lipids and proteins supporting fluidity concept and membrane organization measurement.

Ref: Axelrod et al., Fluorescence Recovery After Photobleaching FRAP Method, Biophys J 1976.

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Properties of Plasma Membrane

Which type of lipid motion requires enzymatic activity?

Lipid mobility modes illustrate fluid dynamics of membranes. Four motions described: lateral diffusion random walk within leaflet plane diffusion coefficient ten to minus eight centimeters squared per second for phospholipids measured by FRAP single particle tracking; rotational diffusion around long axis nanosecond timescale; trans-gauche isomerization flexing of acyl tails picosecond; transverse flip-flop movement of entire lipid from one leaflet to opposite requiring polar headgroup passage through hydrocarbon interior large activation barrier twenty to fifty kilocalories per mol depending on head size charge hydration, spontaneous half-life hours days for phosphatidylcholine phosphatidylserine phosphatidylethanolamine, cholesterol faster minutes due to small hydroxyl head. Lateral rotation flexing thermally driven spontaneously because head remains at interface maintaining solvation, no covalent bond breakage. Flip-flop biologically relevant rates require protein catalysts: P4-ATPase flippases inward ATP driven, ABC floppases outward ATP dependent, calcium activated scramblases TMEM16F bidirectional nonselective. Therefore flip-flop uniquely requires enzymatic activity for physiologically relevant timescales, distinguishing it from spontaneous thermal motions that underlie fluid mosaic membrane liquid crystalline behavior.

Ref: Kol et al., Flip-Flop and Enzymatic Lipid Transport, Nature Reviews Mol Cell Biol 2004.

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