lecture 17 - protein structure: alpha helix

Question 1

what composes the 2° structure of proteins?

Answer 1

• α-helix
• β-sheets
• turns & loops

Question 2

explain the α-helix

Answer 2

“slinky”, spiral spring

Question 3

how does the α-helix occur?

Answer 3

• by H bonding between residue “i” to residue “i+4”

- Hbonding is maximal within the α-helix

Question 4

secondary structure is contained within:

except:

Answer 4

a continuous sequence

• *except 1st 4 residues don’t bond bc their N grp is bonded to residues in another helix
• *except last 4 residues don’t bond bc their Oh grp is bonded to residues in another helix

Question 5

the repetitive geometry of an α-helix corresponds to:

Answer 5

repetitive phi/psi angles

Question 6

in which direction do R groups project from the α-helix?

Answer 6

perpendicular to the helical axis

Question 7

the helical wheel is comprised of approximately how many residues per turn?

Answer 7

36 residues/ 360° turn

Question 8

approximately how many degrees per residue in a helical wheel?

Answer 8

100°/residue

Question 9

why are degrees/residue important if residues 2, 3, 4, 5 and 6 are hydrophobic and 1, 4, 7 and 8 are hydrophilic?
what does this create?

Answer 9

• one face of the α-helix is hydrophobic and the other is hydrophilic
• AMPHIPATHIC helix
• hydrophobic face will tend to be burried while hydrophilic face will be exposed to aq solution

Question 10

which amino acid is most preffered for an α-helix?

Answer 10

Ala

Question 11

which amino acid is least common in α-helices?

why?

Answer 11

Gly

• bulky side chain
• lack an NH hydrogen and therefore cannot H-bond to residue “i+4”

Question 12

which amino acid is never found in α-helices?

Answer 12

Pro

• it’s an α-helix breaker
• can be 1st res of α-helix but nothing other