Membranes 2

Question 1

Lipid bilayer impermeable to:

Answer 1

small hydrophobic molecules: O2, CO2, N2, benzene
small uncharged polar molecules: H2O, glycerol and ethanol
Larger uncharged polar molecules: AAs, glucose and nucleotides
Ions: H+, Na+ etc

Question 2

Types of membrane protein

Answer 2

integral, peripheral, lipid anchored

Question 3

Lipid anchored proteins

Answer 3

Covalently attached lipids can anchor some typically water soluble proteins to one or other leaflet

Question 4

Integral

Answer 4

Spans bilayer

have TM proteins

Question 5

Peripheral

Answer 5

often lipid associated/protein associated (via integral)
Many ORFs (protein coding regions) in genomes encode membrane proteins

Question 6

which AAs most suited to spanning hydrophobic part of bilayer

Answer 6

AAs with non polar R groups

When looking at integral proteins TM domain ie part imbedded in bilayer- made up of non polar AAs

Question 7

Peptide bonds

Answer 7

Polar because electrons drawn towards carbonyl oxygen and away from amide hydrogen, creating a partial negative/positive charge
So a lot of polar groups exposed to interior of bilayer- unfavourable

Question 8

How is the problem of peptide bond polarity overcome?

Answer 8

Forming some secondary structure- hydrogen bonds between partial negative charge in carbonyl and partial positive of amide hydrogen in a regular pattern (peptide backbone)

Question 9

Types of secondary structure in integral membrane proteins

Answer 9

alpha helixes, beta pleated sheets

Question 10

Alpha helix

Answer 10

Formed when backbone hydrogens and oxygens form regular hydrogen bonding pattern
Carbonyl oxygen in peptide bond 1 hydrogen bonded to amide in 4, 2 to 5 stc
Each AA contribute 1.5 A to full length vs in fully extended where each AA contributes 3.5 A
Membrane sapnning domains of integral proteins ahave 20 AAs in a helix conformation (vs only 8-9 in fully enxtended)
R groups all pointing to outside of helix

Question 11

Where are most proteins made

Answer 11

ribosome

Question 12

what is energetically unfavourable to move within proteins

Answer 12

To move large hydrophilic portions of PP chain through bilayer

Question 13

How can this be solved

Answer 13

Use protein conducting channel- allows hydrophilic PP chain to pass through, but as soon as hydrophobic part reached, channel opens and allows hydrophobic stretch to move out into interior of membrane

Question 14

Extracytoplasmic tend to be

Answer 14

hydrophilic

Question 15

Transmembrane tend to be

Answer 15

hydrophobic

Question 16

Cytoplasmic tend to be

Answer 16

hydrophilic

Question 17

Beta pleated sheets

Answer 17

antiparallel B pleated sheet is formed with adjacent Beta strands running in opposite directions
H bonds perpendicular to the chains