Membrane proteins

Question 1

What are the functions of the plasma membrane in multicellular organisms

Answer 1

• regulate transport of nutrients into the cell
• regulate transport of waste out of the cell
• maintain proper chemical conditions in the cell
• provide a site for chemical reactions not likely to occur in an aqueous environment
• detect signals in/from extracellular environment
• interact with other cells or the extracellular matrix

Question 2

Membrane proteins have various functions. What is the function of transporters?

Answer 2

Actively pumps Na+ out of cells and K+ in
eg Na+ pump

Question 3

Membrane proteins have various functions. What is the function of Anchors

Answer 3

link intracellular actin filaments to extracellular matrix proteins
eg integrins

Question 4

Membrane proteins have various functions. What is the function of receptors

Answer 4

binds extracellular PDGF and, as a consequence, generates intracellular signals that cause the cell to grow and divide
eg platelet derived growth factor (PDGF) receptor

Question 5

Membrane proteins have various functions. What is the function of enzymes?

Answer 5

catalyses the production of intracellular signalling molecule cyclic AMP in response to extracellular signals
eg adenylyl cyclase

Question 6

Proteins can associate with lipid bilayer in various ways. What are these?

Answer 6

• Transmembrane
• Mono layer associated
• Lipid linked
• Protein attached

Question 7

Describe how a polypeptide chain crosses the bilayer as an alpha helix

Answer 7

• Hydrophobic side chain of amino acids forming a helix contact hydrophobic tails of phospholipids
• hydrophilic part of peptide backbone form H bonds in the interior of the helix

Question 8

What can transmembrane proteins be formed by?

Answer 8

multiple alpha helices

Question 9

How do porin protein channels work?

Answer 9

• forms water filled channels
• B-sheets curve into cylinder forming a B-barrel
• amino acids facing inside of barrel are hydrophilic
• amino acids facing outside are hydrophobic
• found in mitochondria outer membrane

Question 10

why does plasma membrane study use rbcs the most?

Answer 10

• no nucleus of internal membranes
• osmotically lysed to release contents
• virtually pure membrane preparation can be collected by centrifugation
• aka membrane ghosts

Question 11

What is a glycophorin

Answer 11

• first membrane protein sequenced (131 residues)
• C terminus external and heavily glycosylated (60 sugars per protein)
• single transmembrane helix
• short intracellular N terminus

Question 12

How is the transmembrane segment predicted?

Answer 12

• From a hydropathy plot
• The hydrophobicity of a segment of the sequence is calculated
• This is expressed as a hydropathy index and plotted against the residue number

Question 13

What is Band3 protein and how was it discovered?

Answer 13

• large intrinsic membrane protein
• functions as anion channel (exchanges HCO3- and Cl-)
• revealed by freeze fracture method

Question 14

What are the possible mechanisms of facilitated passive transport?

Answer 14

• carrier mediated
• pore mediated

Question 15

What are microbial ionophores (substance that conducts ions across a membrane)?

Answer 15

• Small, antibiotic peptides produced by a certain bacteria
• can dissolve in the membrane and catalyse transport of cations

Question 16

How do microbial ionophores provide models for the two types of transport mechanisms?

Answer 16

• Valinomycin: specific for K+ works by a carrier mechanism

• Gramicidin A: transports for a variety of monovalent cations and acts by a pore mechanism