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Encyclopedia of Physical Science and Technology EN013D-616 July 27, 2001 12:5

202 Protein Structure

residues in the helical conformation (∼80%) with com- Three general types of reverse turn have been described;
paratively few devoted to connecting regions. In proteins types I, II, and III, which all contain four amino acid
that are dominated by β-strands, typically <50% are in the residues and normally exhibit a hydrogen bond between
β-conformation. This occurs because for every three to six C O (i) and H N (i+3) (Fig. 8). Of these, the type III turn
residues in each strand there must be an equivalent number consists of a short section of residues in the 3 10 helical con-
of amino acids devoted to a turn to bring the polypeptide formation. Additional variants of the type I and II class are

chain back into a position where it can hydrogen bond to observed in the I and II turns. These exhibit conforma-
the same or neighboring β-strand. This emphasizes the tional angles for the central two residues of the turn that
importance of turns in protein structures. are the mirror image of types I and II. The observed con-
formation angles favor the presence of certain amino acids
at specific locations in the turns. For example, glycine pre-
D. Turns and Random Coil
dominates at position (i + 3) and proline predominates at
Many proteins contain secondary structure that cannot be position (i + 1) in both types I and II turns. In all turns
described as either helix or turn. This is typically clas- the central two amino acid residues do not form peptidyl
sified as turn, loop, or random coil. These sections of hydrogen bonds within the turn itself and thus must either
the polypeptide chain are characterized by nonrepetitive accommodate their hydrogen bonding potential via a side
conformational angles; however, this does not necessar- chain interaction with a neighboring residue or through in-
ily imply that these residues are less stable or less well teractions with the solvent. Thus polar or charged residues
ordered than the regular secondary structural elements. (Asp, Asn, Ser) are often located at the first residue of the
Many active site residues and components critical for lig- turn so that they can form a hydrogen bond to the amide
and recognition reside in loops or random coil and adopt hydrogen of residue (i +2). The need to satisfy the hydro-
an exquisitely well-defined conformation. genbondingpotentialofthemainchainatomsaccountsfor
On average, one third of all residues in proteins are the placement of most turns at the surface of the protein.
involved in turns that serve to reverse the direction of
the polypeptide chain. These turns are an essential fea-
ture of globular proteins and are almost always located at VI. PROTEIN STABILITY
the surface. In contrast to α-helices and β-strands which
have repetitive conformational angles, the conformational The term protein stability refers to the energy difference
angles observed in turns occur in sets that are character- between the folded and unfolded state of the protein in
istic of each type (Table II). Turns have been classified solution. Remarkably, the free energy difference between
according to the commonly observed groups of confor- these states is usually between 20 and 80 kJ/mol, which is
mational angles and the number of residues involved. Of of the magnitude of one to four hydrogen bonds. Although
these the β-hairpin or reverse turn is the most common. this suggests that proteins are only marginally stable, the
This type of turn is frequently used to connect antiparallel stability is sufficient to prevent spontaneous unfolding at
β-strands. normal temperatures.

TABLE II Conformational Angles of the Major Secondary Structural Elements

Secondary structure φ ψ
Helical conformations
α-Helix −57 −47
3 10 Helix −49 −26
Collagen Helix −78 +149
β-Strands
Antiparallel −139 +135
Parallel −119 +113
β-Turns φ(i + 1) ψ(i + 1) φ(i + 2) ψ(i + 2)
Type I −60 −30 −90 0
Type I +60 +30 +90 0
Type II −60 +120 +80 0
Type II −60 −120 −80 0
Type III −60 −30 −60 −30