#transmembrane proteins

Plasma Membrane

Which type of membrane transport is facilitated by transmembrane proteins?

Passive entry of polar solutes across 4 nm hydrophobic core is energetically prohibited, necessitating membrane proteins that provide facilitated diffusion pathway without ATP input. Transmembrane proteins create two mechanisms: channel proteins like aquaporin-1, potassium channels with selectivity filter TVGYG, and porins with beta-barrels that form continuous aqueous pores allowing diffusion at rates approaching 10^8 ions per second down electrochemical gradient, gating regulated by voltage, ligand, or mechanical force. Carrier proteins like GLUT1 glucose transporter and AE1 anion exchanger bind solute specifically, undergo conformational inversion from outward-open to inward-open, increasing permeability and specificity while still moving down gradient. Simple diffusion of O2, CO2, and steroid hormones occurs directly through lipid matrix independent of proteins due to high partition coefficient. Passive osmosis follows water activity gradient but accelerated by aquaporins. Lipid bilayer flipping of polar lipids requires flippases and is not spontaneous transport. Hence transmembrane proteins convert impermeable barrier into selective gateway enabling facilitated diffusion essential for nutrient uptake and excitability.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 11: Facilitated Diffusion and Transporters.

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Plasma Membrane Proteins

Which of the following is a common feature of transmembrane β-barrel proteins?

Beta-barrel membrane proteins display characteristic functional and structural attributes correlating with aqueous pore formation. Architecture comprises even number amphipathic beta-strands eight to twenty four organized antiparallel hydrogen bonded sheet closed cylindrically first strand bonded last strand strands tilted thirty to sixty degrees relative to barrel axis. Even residues hydrophobic facing lipid core, odd hydrophilic facing lumen generating water-filled channel diameter about seven to fifteen angstroms variable. Extracellular loops often long folding into pore constriction loop three in OmpF forming eyelet governing size exclusion approximately six hundred daltons cutoff and charge selectivity via acidic glutamate aspartate and basic arginine lysine lining. These porins facilitate passive diffusion of small polar nutrients including sugars, amino acids, phosphate, nucleosides, and antibiotics down concentration gradient rates up to million per second without energy, essential for Gram-negative bacterial survival. Trimeric assembly stabilizes. Eukaryotic mitochondria VDAC transports ATP ADP metabolites. Alpha-helical transporters active secondary carriers elsewhere, beta-barrels exclusively allow diffusion of small polar molecules through relatively nonselective aqueous pore.

Ref: Nikaido, Molecular Basis of Bacterial Outer Membrane Permeability, Microbiol Mol Biol Rev 2003.

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Plasma Membrane Proteins

Which feature distinguishes β-barrel transmembrane proteins from α-helical transmembrane proteins?

Two fundamental integral protein folds crossing lipid bilayers differ in secondary structure forming membrane-spanning segments. Alpha-helical type uses hydrophobic alpha-helices of nineteen to twenty seven residues where backbone carbonyl amide hydrogen bonds internal satisfying polarity, side chains aliphatic contacting lipid tails, found predominantly in plasma membrane and inner membranes of Gram-negative bacteria, mitochondria inner membrane, endoplasmic reticulum; multispan examples Band 3 fourteen passes, GPCR seven passes, glycophorin one pass. Beta-barrel type folds as antiparallel amphipathic beta-strands seven to twenty two residues where alternating residues face lipid hydrophobic and lumen hydrophilic, strands connected by short turns and long loops, hydrogen bond network closes cylinder first to last strand. Barrels exclusively discovered in outer membranes of Gram-negative bacteria as porins OmpF OmpC PhoE, mitochondria VDAC, chloroplast outer envelope Toc75, inserted by BAM and SAM machinery. Functionally alpha-helical receptors transporters, beta-barrel nonspecific channels for small polar solutes. Thus formation of membrane-spanning beta-strands distinguishes beta-barrel architecture from amphipathic alpha-helices.

Ref: Schulz, Beta-Barrel Membrane Proteins – Structure and Assembly, Biochim Biophys Acta 2002.

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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 anti-M anti-N antibodies. Transmembrane segment residues seventy three to ninety five forms nineteen residue alpha helix enriched leucine isoleucine valine with dimerization motif L75 IxxG79 V80 xxG83 V84 xxT87 allowing tight van der Waals packing and strong association measured minus twelve kilocalories per dimer widely studied for helix-helix energetics. Cytosolic tail thirty six residues acidic interacts with protein 4.1R linking to spectrin actin junction regulating lateral mobility. Unlike seven-pass GPCR and fourteen-pass Band 3, single-pass architecture simplifies biophysical and invasion studies as receptor for Plasmodium falciparum EBA-175.

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

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A Cell Signaling

G protein-linked receptors are transmembrane proteins of:

Seven-pass, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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