Essential Cell Biology 5th edition

126 CHAPTER 4 Protein Structure and Function
(A) backbone model
Figure 4−11 Protein conformation can be represented in a variety
of ways. Shown here is the structure of the small bacterial transport
protein HPr. The images are colored to make it easier to trace the path
of the polypeptide chain. In these models, the region of polypeptide
chain carrying the protein’s N-terminus is purple and that near its
C-terminus is red.
conformation of each protein is unique, some regular folding patterns
can be detected, as we discuss next.
(B) ribbon model
(C) wire model
The α Helix and the β Sheet Are Common Folding
Patterns
More than 60 years ago, scientists studying hair and silk discovered two
regular folding patterns that are present in many different proteins. The
first to be discovered, called the α helix, was found in the protein α-keratin,
which is abundant in skin and its derivatives—such as hair, nails, and
horns. Within a year of that discovery, a second folded structure, called
a β sheet, was found in the protein fibroin, the major constituent of silk.
(Biologists often use Greek letters to name their discoveries, with the first
example receiving the designation α, the second β, and so on.)
These two folding patterns are particularly common because they result
from hydrogen bonds that form between the N–H and C=O groups in
the polypeptide backbone (see Figure 4−6). Because the amino acid side
chains are not involved in forming these hydrogen bonds, α helices and β
sheets can be generated by many different amino acid sequences. In each
case, the protein chain adopts a regular, repeating form. These structural
features, and the shorthand cartoon symbols that are often used to represent
them in models of protein structures, are presented in Figures 4−12
and 4−13.
α helix
R
amino acid
side chain
(D) space-filling model
R
R
R
oxygen
hydrogen bond
0.54 nm
carbon
R
R
hydrogen
R
R
nitrogen
R
carbon
nitrogen
(A)
(B)
(C)
Figure 4−12 Some polypeptide chains fold into an orderly repeating form
known as an α helix. (A) In an α helix, the N–H of every peptide bond is hydrogenbonded
to the C=O of a neighboring peptide bond located four amino acids away
in the same chain. All of the atoms in the polypeptide backbone are shown; the
amino acid side chains are denoted by R. (B) The same polypeptide, showing only
the carbon (black and gray) and nitrogen (blue) atoms. (C) Cartoon symbol used to
represent an α helix in ribbon models of proteins (see Figure 4−11B).
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