Protein Structure
Four Levels of Protein Structure
(1) Primary
– amino acid sequence
easy to determine by sequencing reactions
(2) Secondary
– primarily
a
-helices,
b
-sheets, and random coils; also includes disulfide bonds
determined by crystallography – can make inferences based upon primary structure
a
-helices
– stabilized by hydrogen bonds between keto oxygen and amino hydrogen
involves backbone – sidechains not involved (bulky side chains point away from each other)
keratin
– unique double
a
-helix – makes complete revolution every 3.6 residues
7 amino acid repeat in which 1
st
and 4
th
are hydrophobic – glue two
a
-helices together
coiled coil – every 35 amino acids, complete revolution of the
a
-helices around each other
also covalent disulfide bond
collagen
– triple
a
-helix – repeat (glycine-x-hydroxyproline) – hydroxyproline allows hydrogen bonding
scurvy is inability to make hydroxyproline (lack vitamin C)
also covalent bonds between allysines (made from lysine by lysyl oxidase; sweet peas inhibit this)
lathyrism
– no crosslink –
b
-aminopropionylnitrile (in sweet peas) inhibits lysyl oxidase
b
-sheets – also stabilized by hydrogen bonds – interacts with remote residues (requires a loop)
parallel – N
Þ
C in both directions; antiparallel – reverse directions
parallel must be remote – antiparallel can have small hairpin
silk
– unique
b
-sheet – repeat of (glycine-serine-glycine-alanine-glycine-alanine)
glycines zip together, as do serines and alanines, to make a strong sheet responsible to silk’s stability
(3) Tertiary
– three-dimensional structure – position of every atom (secondary structure glued by covalent bonds)
determined by crystallography – can only make vague preductions based upon homology to known protein
(4) Quaternary
(only for proteins with polypeptide subunits) – subunit structure