1.4 Primary and Secondary Protein Structure [HY]

Question 1

Proteins

Answer 1

polypeptides that range from just a few amino acids in length
up to thousands. They serve many functions in biological systems, functioning as enzymes, hormones, membrane pores and receptors, and elements of cell structure.

Question 2

Primary structure of a protein

Answer 2

• the linear arrangement of amino acids coded in an organism’s DNA
• listed from the N-terminus, or amino end, to the C terminus, or carboxyl end
• stabilized by the formation
  of covalent peptide bonds between adjacent amino acids
• encodes all the information needed for folding at all of the higher structural levels; the secondary, tertiary, and quaternary

Question 3

Secondary Structure

Answer 3

• primarily the result of hydrogen bonding between nearby amino acids
• most common secondary structures are α-helices and β-pleated sheets
• key to the stability of both
  structures is the formation of intramolecular hydrogen bonds between different residues.

Question 4

α-Helices

Answer 4

• rodlike structure in which the peptide chain coils clockwise around a central axis.
• The helix is stabilized by intramolecular hydrogen bonds between a carbonyl oxygen atom and an amide hydrogen atom four residues down the chain
• side chains of the amino acids point away from the helix core
• important component in the structure of keratin

Question 5

β-Pleated Sheets

Answer 5

• can be parallel or antiparallel
• the peptide chains
  lie alongside one another, forming rows or strands held together by intramolecular hydrogen bonds between carbonyl oxygen atoms on one chain and amide hydrogen atoms in an adjacent chain
• To accommodate as many hydrogen bonds as possible, the β-pleated sheets assume a pleated, or rippled, shape.
• The R groups of amino residues point above and below the plane of the β-pleated sheet.

Question 6

Why are prolines rarely found in α-helices?

Answer 6

• Because of its rigid cyclic structure, proline will introduce a kink in the
  peptide chain when it is found in the middle of an α-helix.
• Found in helices that cross the cell membrane
• proline is often found in the turns between the chains of a β-pleated sheet, and it is often found as the residue at the start of an α-helix