Question

Cell Biology Tutorial: Membrane Proteins

Answer 1

Note: This is a reference for educational/studying purposes, not a question, please save all comments or questions for the end.

Answer 2

\({\bf{Three~Types}}\)
- integral/transmembrane proteins: span the membrane and contain hydrophilic cytosolic and exoplasmic domains + hydrophilic membrane-spanning domains
- lipid-anchored membrane proteins: bound to lipid molecules, has tail embedded into the leaflet which anchors the protein to the membrane, does not span the bilayer
- peripheral membrane proteins: bound indirectly through lipid anchor or integral proteins, can be associated with cytoskeletal elements or the ECM

\({\bf{Membrane~Spanning~Structures}}\) typically alpha-helices w/ hydrophobic side chains pointing away from the heli and interacting with the bilayer while the carbonyl groups form hydrogen bonds with the amino acids on the C-terminus
- can be single pass (Glycophorin A) or multipass (bacteriorhodopsin)
- aquaporins: transport water, glycerol, and other hydrophilic molecules, may have alpha-helices that cross membrane halfway and/or at oblique angles
- annular phospholipids: ring of lipids around protein
- electrostatic interactions between positive and negatively charged amino acid side chains -> guide assembly of membrane proteins
- porins: transmembrane proteins characterized by beta barrels where outer surface is hydrophobic and the core is hydrophilic. nonpolar b-strands form ring/band around the barrel

Answer 3

\({\bf{Anchoring~Proteins}}\)

- lipids may anchor water-soluble proteins to one leaflet, protein itself does not enter bilayer
- three groups:
1. fatty acyl group on N-terminal glycine residue (acylation)
2. hydrocarbon chain on cysteine residue on C-terminus (prenylation)
3. GPI anchors, contain phosphatidylinosol and phosphoethanolamine which anchors the membrane to the C-terminus of the protein
- glycoproteins are oriented so that their carb. chains are in the exoplasmic domains

\({\bf{Lipid~Binding~Motifs}}\)
- peripheral proteins bind to the head groups of phospholipids
- ring of positively charged arginine and lysine around catalytic channel
- enzymatic calcium+ active sites become buried in the hydrophobic amino acid channel

\({\bf{Detergents}}\) amphiphatic molecules that disrupt phospholipid bilayers
- detergent concentration increases -> micelle formation
- critical micelle formation: concentration at which micelles form, unique to each detergent
- ionic detergents: bind to hydrophobic regions of membrane proteins/water-soluble proteins
- non-ionic detergents: form mixed micelles that are not soluble
- peripheral proteins can be removed with highly ionic solutions which disrupt bonds but most peripheral proteins are already soluble in water

Answer 4

This is the end of my tutorial; I hope you found it helpful. If you have any ∗relevant∗ comments or questions I will attempt to address them to the best of my ability.

Thank you for reading!