l8-7 TertiaryStructureof Proteins
567
In addjtion, collagenis a glycoprotein (seesec. 18.11).sugar units (usually
disaccharidesofglucose and galactose)are covalentlyattachedto the peptide chains.
18.7Tertiorystructureof proteins
AIMS: To describethe forcesthat help determinethe tertiory
structureof proteins. Todefine o coniugotedprotein. To
describethe tertiary structureof myoglobin.
Focus
Folding of polypeptide chains
produces tertiary structure.
The polypeptide chains of proteins fold to form three-dimensionalstructures. Thefolding of a polypeptide chain of a protein morecureinto a relatiuely stable three-dimensionalshapeconstitutesthe protein'stertiarystructure. Like the formation of a secondary structure, the formation of the
tertiary structure of a protein is determined by its primary structure. The
tertiary structure may contain within it regions of secondary structure,
such asalphahelixandbeta-pleatedsheet.The overallfolding of apobpeptide into its tertiary structure results from interactions between the side
chains of the amino acid residues within the primary structure of the protein. For example, disulfide bridges can hold two regions of the same
polypeptide chain together (Fig. lB.9). weak attractions between amino
acid side chains are also important. A polypeptide chain folds in the way
that maximizes energetically favorable hydrogen bonds. Ionic bonds, also
called salt bridges, are usually formed between the negatiuelycharged carboxylate ion side-chain groups of aspartate or glutamate and the positiuety
charged amino side-chain groups of lysine or arginine. one major factor in
how a protein folds is the presence of aliphatic or aromatic amino acid
residues in the peptide chain. The hydrophobic side chains of these
residues tend to face the interior of protein molecules, much as the
hydrophobic tails of soap molecules exclude water by forming micelles.
Protein molecules that have different primary structures experience different side-chain interactions and therefore fold into differeni tertiary structures. conversely, protein molecules that have the same primary suucrure
experiencethe same side-chain interactions and therefore fold into the
same tertiary structure.
Hldrophobic
aggregation
Figure18.9
Manyforcesstabilize
the tertiary
structure
of proteins.
568
CHAPTER
l8 Amino Acids,Peptides,and Proteins
The unusually large amount of
myoglobin in whale muscle enables whales to remain submerged
in water for long periods of time
without rising to the surface for
oxygen.
Myoglobin- the oxygen-storageprotein of mammalian muscle-was
the first protein to have its tertiary structure determined. Myoglobin is a
globular protein.ln contrast to rod-shapedfibrous proteins,globular proteins haue lrlore or lessspherical shapes.Like many other globular proteins,
myoglobin is soluble in water and dilute salt solutions. Myoglobin consists
of a single peptide chain of 153 amino acids. This relatively small protein
contains 1260atoms, not counting hydrogens. Myoglobin is an example of
a conjugatedprotein. Conjugated proteins hauestructures that incorporate
nonprotein portions calledprosthetic groups. Prosthetic groups are necessary for the protein to perform its biological function. They are usually
organic molecules that are permanently attached to the protein by covalent
bonds. The myoglobin molecule contains a prosthetic group called a heme
group.The heme group contains an iron atom in the iron(Il) or ferrous
(Fe'*) state (Fig.18.10).Oxygenis bound to the heme iron to form oxymyoglobin.
In 1957,Iohn C. Kendrew of the Medical ResearchCouncil laboratories
in Cambridge, England, and his colleagues determined the three-dimensional structure of myoglobin by a method called X-ray crystallography.
(SeeA CloserLook X-ray Crystallography,for more about the X-ray method
for determining protein structures.) The X-ray crystallographic picture of
myoglobin in Figure 18.11 shows a globular structure in which almost
three-fourths of the peptide chain is alpha helix. The tertiary structure of
myoglobin's single chain consists of segmentsof alpha helix between turns
of the polypeptide chain. Because of the turns, the molecule takes on a
CH:CHz
Water molecule
cHs
cH2cHzco2H
F i g u t e1 8 . 1 0
Heme. Noticethat the structure
containsfour linked pyrrolerings
(color) with an iron(ll) ion (Fe2*)
at the center.
F i g u r el 8 . l !
The tertiarystructureof myoglobin,
a globularprotein.The heme group
and alpha-helical
segmentsare
clearlyvisible.A water moleculeis
adjacentto the heme iron in deoxymyoglobin,but it is replacedby
orygen in orymyoglobin.
18.7Tertiary
Structure
of Proteins 569
X-ray Crystallography
The determination of the three-dimensional
structure of a biologically interesting molecule is
fascinating and practical. The fascination comes
from learning how the atoms in the molecule are
arrangedin spaceand trying to figure out howthis
arrangement enables the molecule to perform its
biological function. The practical aspect is that
this knowledge,once obtained, can sometimesbe
used to design compounds that will either mimic
or block the action of the biological molecule.
X-ray crystallography is a very powerful tool
for determinationof the three-dimensionalstrucfures of molecules,including largemoleculessuch
as those of proteins and deoxyribonucleicacids
(DNA). Exposinga crystal of a substanceto a narrow beam of X rays causesthe X rays to be scattered, or diffracted, by their interactionswith the
electronsof the atoms in the crystal.Hundreds or
thousands of diffraction patterns are recorded
from different anglesall around the crystal.
The spacingand intensity of the spots on diffraction patterns contain information about the
electron densitiesin molecules.Computers are
used to translate the diffraction information into
three-dimensionalmaps of the electrondensities
all over the molecule (seefigure).The electrondensity maps give very accurate,complete molecular structures that pinpoint the positions of the
atoms in space.For example,a region in the molecule that has a high electron density must be a
NZC\
lN-
-Hzc ,/c\r-/
i
H
F i g u r e1 8 . 1 2
Hemeis attachedto myoglobin
througha bond from the iron of the
heme groupto the nitrogenof a
histidylresidue.
Electron-density
mapof myoglobin.
Thehemegroupis
seenedge-onwith its two associated
histidine(His)
residues
anda watermolecule
(W).
covalent bond, because electrons are concentrated in covalentbonds.Conversely,a region that
contains essentiallyno electron densiw must be a
nucleus of an atom, becauseelectrons are not
found in the nucleus.The identity of a chemical
bond, say,a carbon-carbonbond, is conflrmed if
the distance between two nuclei connected bv a
region of high electron density is 0.15 r,rn, th"
length of an ordinary C-C bond. Hydrogenaroms
aretoo small to be directlydetectedbyX-ray crystallography.However,the positions of hydrogens
can he inferredonce the positionsof the heavier
atoms in a molecule are knornm.
globular shapewith the heme group resting in a "basket" formed by segments of alpha helix. There are other interestingfeatures.The entire molecule is very compact-no more than tvvo water molecules will fit on the
inside. All the hydrophobic amino acid residues,those with aliohatic and
aromaticsidechains,are turnedtowardthe interior of the moleCuleso that
they are not exposedto water. All the charged side chains are exposedto
water on the exterior of the molecule. Finally, as Figure l8.l2 shows, the
heme group is attachedto the protein by a bond betweenthe iron(Il) ion of
the heme group and a nitrogen of a histidyl residue of the polypeptide
chain.