Question

1)

a)draw the structure of histidine at pH 4 and at pH 8. Make sure you clearly indicate all charges.

b)Briefly describe the nature of the H-bonds that stabilize parallel and antiparallel -sheets. How do the H-bonds in these two types of secondary structure differ? You can draw simple diagrams if you wish, but there is no need for detailed structures.

Answer 1

1. a)

b) β-sheets are made up of β-strands joined with each other by backbone of two or three hydrogen bonds and form pleated sheet. This structure is formed when overlapping of two segments of a polypeptide chain occur. There are two types of β-sheets i.e., parallel and antiparllel. In parallel condition C and N terminus of both segments are on same side whereas in antiparallel both c and N terminus of both segments are on opposite to each other. Hydrogen bons in parallel β-sheets formed at an angle and longer. Longer hydrogen bonds are weaker due to weaker hydrogen bond parallel β-sheets are less stable. whereas, in antiparallel β-sheets hydrogen bonds are formed directly between opposite segments. Hydrogen bonds in antiparallel β-sheets are stronger and make it stable. (hydrogen bonds are shown by dotted line).