lecture 18 - protein structure: beta sheets

Question 1

explain beta-sheet structure

Answer 1

• repetitive phi/psi angles
• zig-zag comfirmations of backbone
• not formed by a contiguous region of amino acid sequence
• minimum of 2 beta strands come together to form a beta sheet

Question 2

what is a beta sheet stabilized by?

Answer 2

H-bonds between strands (can be parallel or anti-parallel)

Question 3

explain anti-parallel H-bonds between beta sheets

Answer 3

• R groups on adjacent strands are oriented in the same direction (e.g. all R groups are above the plane)
• H-bonds are perpendicular to strands and in the plane of the sheet

Question 4

explain parallel H-bonds between beta sheets

Answer 4

-some R groups are above the plane, some R groups are below the plane

Question 5

explain the relationship between parallel H bonds and an amphipathic B-sheet

Answer 5

• can create an amphipathic B-sheet by having hydrophobic R groups on face and hydrophilic groups on another face
• implies mechanism for folding B-sheets to bury hydrophobic face

Question 6

beta sheets often have a twist (not strictly planar), what does this mean? what does this require?

Answer 6

• have a mix of anti-parallel and parallel H-bonds

- need more than 2 strands

Question 7

explain how secondary structures link together

Answer 7

• flexible regions (have primary structure) allow the secondary structure to fold into 3D fold
• have alternate folding pattern: parallel beta sheet with alpha helix on top, etc.

Question 8

define motif

Answer 8

collection of secondary structural elements