Food Chemistry

1.4 Proteins 49

small cyclic peptides or in proteins before proline
residues).
Thus in ribonuclease A, two X-Pro bonds
have trans-conformation (Pro-42 and Pro-
117), and two have cis-conformation (Pro-93
and Pro-114). The equilibrium between the
two isomers is catalyzed by specific en-
zymes (peptidyl-prolyl-cis/trans-isomerases).
This accelerates the folding of a peptide
chain (cf. 1.4.2.3.2), which in terms of the
biosynthesis occurs initially in all-trans-
conformation.
Six atoms of the peptide bonds, Cαi,C′i,Oi,Ni+ 1 ,
Cαi+ 1 and Hi+ 1 , lie in one plane (cf. Fig. 1.16).
For a trans-peptide bond,ωiis 180◦. The position
of two neighboring planes is determined by the
numerical value of the anglesψi(rotational bond
between a carbonyl carbon and anα-carbon) and
φi(rotational bond between an amide-N and an
α-carbon). For an extended peptide chain,ψi=
180 ◦andφi= 180 ◦. The position of side chains

can also be described by a series of anglesχ^1 i−n.

Fig. 1.16.Definitions for torsion angles in a peptide
chain
ωi= 0 ◦for Cαi−C′i/Ni+ 1 −Cαi+ 1 →cis,
ψi= 0 ◦for Cαi−Ni/C′i−Oi→trans,
φi= 0 ◦for C′i−C′i/Ni−H→trans,

χi= 0 ◦for Cαi−NiCiβ−Ci→cis.
The angles are positive when the rotation is clock-
wise and viewed from the N-terminal side of a bond
or (for X) from the atom closer to the main chain re-
spectively. (according toSchulzandSchirmer, 1979)

1.4.2.2 Secondary Structure (Regular Structural Elements)...........

The primary structure gives the sequence of

amino acids in a protein chain while the sec-

ondary structure reveals the arrangement of the

chain in space. The peptide chains are not in

an extended or unfolded form (ψi,φi= 180 ◦).

It can be shown with models thatψi andφi,

at a permissible minimum distance between

non-bonding atoms (Table 1.20), can assume

only particular angles. Figure 1.17 presents the

Table 1.20.Minimal distances for nonbonded

atoms (Å)

CNOH

C3. 20 a 2. 90 2. 80 2. 40

(3.00)b (2.80) (2.70) (2.20)

N2. 70 2. 70 2. 40

(2.60) (2.60) (2.20)

O2. 70 2. 40

(2.60) (2.20)

H2. 00

(1.90)

aNormal values.

bExtreme values.

Fig. 1.17.φ,ψ-Diagram (Ramachandranplot). Al-

lowed conformations for amino acids with a Cβ-atom

obtained by using normal(−)and lower limit (- - -)

contact distances for non-bonded atoms, from Ta-

ble 1.20.β-Sheet structures: antiparallel (1); paral-

lel (2), twisted (3). Helices:α-, left-handed (4), 3 10 (5),

α, right-handed (6),π(7)