4-3 THE PEPTIDE GROUP
The Peptide Group

Contains the two atoms involved in the peptide __________ along with their four
substituents (the carbonyl __________ atom, the amide __________ atom, and the two
adjacent __-__________ atoms).

The peptide group can be represented by multiple Lewis structures, making them
__________ structures. Figure A represents the common structure, Figure B represents
the other contributing resonance structure, and the last is a hybrid representation, which
best represents the resonance within the peptide group.
Figure A

Figure B
Figure C
This _______________ of electrons makes the C to O bond and the C to N bond behave
like __________ bonds, restricting any _______________ around the atoms in the
peptide group. The bonds between the carbonyl C and the -carbon, and between the N
and the -carbon can still rotate __________. Restrictions on free rotation within the
peptide group ultimately determine the __-__ _______________ of a protein.

The double-bond nature of the peptide bond restricts the peptide group to either the
_____ conformation or the _____ conformation.

These conformations arise during protein _______________, and they cannot be
interconverted around the peptide bond once it has formed.

The _____ conformation is favored because there is less _______________ between the
side chains in this conformation.

Specific enzymes called peptidyl prolyl cis/trans isomerases can catalyze the
interconversion of cis and trans conformations at _______________ residues. Human
peptidyl prolyl cis/trans isomerase is the target of the immunosuppressive drug
cyclosporin A, which is used with transplant patients to prevent the _______________ of
the donated organ.

Although nearly all peptide groups are in the trans conformation, interference can still
occur with the rotation of the N–C and the C–C bonds, as shown in the following
diagram:

The letters (_____) and  (_____) represent the angle of rotation allowed, which is
restricted by the _______________ on the carbonyl group and by the __________
__________.

Read pages 91 (3rd full paragraph) through page 92 (end of section) to learn how
Ramachandran plots are used to determine which and  angles are permissible in a
polypeptide chain.