Essential Cell Biology 5th edition

122 CHAPTER 4 Protein Structure and Function
Figure 4–5 Hydrophobic forces help
proteins fold into compact conformations.
In a folded protein, polar amino acid side
chains tend to be displayed on the surface,
where they can interact with water; nonpolar
amino acid side chains are buried on the
inside to form a tightly packed hydrophobic
core of atoms that are hidden from water.
nonpolar
side chains
unfolded polypeptide
polar
side chains
polypeptide
backbone
polar side chains
can form hydrogen
bonds to water
nonpolar side chains
are packed into
hydrophobic core region
folded conformation in aqueous environment
Proteins Fold into a Conformation of Lowest Energy
Each type of protein has a particular three-dimensional structure, which
is determined by the order ECB5 of m3.05/4.05 the amino acids in its polypeptide chain.
The final folded structure, or conformation, adopted by any polypeptide
chain is determined by energetic considerations: a protein generally folds
into the shape in which its free energy (G) is minimized. The folding process
is thus energetically favorable, as it releases heat and increases the
disorder of the universe (see Panel 3−1, pp. 94–95).
Figure 4–6 Hydrogen bonds within a
protein molecule help stabilize its folded
shape. Large numbers of hydrogen bonds
form between adjacent regions of a folded
polypeptide chain. The structure shown is a
portion of the enzyme lysozyme, between
amino acids 42 and 63. Hydrogen bonds
between two atoms in the polypeptide
backbone are shown in red ; those between
the backbone and a side chain are shown
in yellow ; and those between atoms of two
side chains are shown in blue. Note that
the same amino acid side chain can make
multiple hydrogen bonds (red arrow). In this
diagram, nitrogen atoms are blue, oxygen
atoms are red, and carbon atoms are gray;
hydrogen atoms are not shown. (After
C.K. Mathews, K.E. van Holde, and K.G.
Ahern, Biochemistry, 3rd ed. San Francisco:
Benjamin Cummings, 2000.)
42
backbone to backbone
hydrogen bond between
atoms of two peptide
bonds
backbone to side chain
63
hydrogen bond between
atoms of a peptide bond
and an amino acid side chain
side chain to side chain
hydrogen bond between
atoms of two amino
acid side chains