560 MHR • Appendix 5
and other interactions between R-groups in
different parts of a protein can cause the
protein to remain fixed in a complicated three-
dimensional shape, called a tertiary structure.Note that this three-dimensional shape is in
addition tothe alpha-helices and beta-sheets
already formed. Part C of Figure A5.3 shows the
tertiary structure of a polypeptide.O C
N
R C
N
O C
R C
H
OC
NH
H N
OC
O C
N
R C
OC
NH
O C
N
R C
N
O C
R C
H
OC
NH
C R
H N
C R
OC
O C
N
R C
OC
NH
H
H
H
H
C R
C
C
C C
N
CH
R
O
O
O
O
H
H
H
H
H
O
O
O
C
CH
R
C N
C
CH
R
C
N
C
CH
R
N
C
CH
R
N
CH
R
N
C
N
CH
R
peptide bondCH
pleated sheetα (alpha) helix+H 3 N amino acidCOO−Figure A5.3 Levels of protein organizationPrimary Structure
This level of structure
is determined by the
sequence of amino
acids that join to form
a polypeptide.A
Secondary Structure
Bonding between amino
acids causes the
polypeptide to form
an alpha-helix or a
pleated sheet.B
Tertiary Structure
The helix folds into a globular
shape due (partly) to bonding
between the functional groups.C Quarternary Structure
This level of structure occurs
when two or more polypeptides
join to form a single protein.