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Proteins
Proteins are the third major group of macromolecules that make up
the bodies of organisms. Proteins play perse roles in living things.
Perhaps the most important proteins are enzymes, proteins capable
of speeding up specific chemical reactions. Other short proteins called
peptides are used as chemical messengers within your brain and throughout
your body. Collagen, a structural protein, is an important part of
bones, cartilage, and tendons. Despite their varied functions, all
proteins have the same basic structure: a long chain of amino acids
linked end to end.
Amino acids are small molecules containing
an amino group (-NH2), a carboxyl group (-COOH),
a hydrogen atom, a carbon atom, and a side chain that differs among
amino acids. In a generalized formula for an
R
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H2N--C--COOH
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H
amino acid, the side chain is shown as R. The identity and unique
chemical properties of each amino acid are determined by the nature
of the R group.
Only 20 different amino acids make
up the perse array of proteins found in living things. Each protein
differs according to the amount, type, and arrangement of amino acids
that make up its structure. Notice that each has the same chemical
backbone (shown by the box) as in the generalized formula. The R groups
are outside these boxes. Those amino acids with R groups that form
ring structures are termed aromatic compounds and those without ring
structures are nonaromatic. (This term was coined as chemists discovered
that many fragrant compounds had this distinctive ring structure.)
Those amino acids that are ionizable have R groups that become charged
when in solution.
Figure 15 How a polypeptide chain is formed and broken.
(a) During dehydration synthesis, peptide bonds are formed between
adjacent amino acids, forming a polypeptide. (b) During hydrolysis,
a molecule of water is added to each peptide bond that links adjacent
amino acids, breaking the bond between them. This separates the molecules
into inpidual amino acids.
Those with special chemical (structural)
properties play important roles in forming links between protein chains
or forming kinks in their shapes.
Each amino acid has a free amino group
(-NH2) at one end and a free carboxyl group
(-COOH) at the other end. During dehydration synthesis,
each of these groups on separate amino acids loses a molecule of water
between them, forming a covalent bond that links the two amino acids
(Figure 15a). This bond is called a peptide bond. A long chain of
amino acids linked by peptide bonds is a polypeptide. Proteins are
long, complex polypeptides. The great variability possible in the
sequence of amino acids in polypeptides is perhaps the most important
property of proteins, permitting tremendous persity in their structures
and functions.
The sequence of amino acids that makes
up a particular polypeptide chain is termed the primary structure
of a protein (Figure 16 ). This sequence determines the further levels
of structure of the protein molecule resulting from bonds that form
between these groups. Having the proper sequence of amino acids, then,
is crucial to the functioning of a protein. If the protein does not
assume its correct shape, it will not work properly or at all. Because
different amino acid functional groups have different chemical properties,
the shape of a protein may be altered by a single amino acid change.
Figure 16 Primary structure determines a protein's
shape due to bonding along the chain.
The functional groups of the amino
acids in a polypeptide chain interact with their neighbors, forming
hydrogen bonds. In addition, portions of a protein chain with many
nonpolar side chains tend to be shoved into the interior of the protein
because of their hydrophobic properties. Because of these interactions,
polypeptide chains tend to fold spontaneously into sheets or wrap
into coils. This folded or coiled shape is called its secondary structure
2. Proteins made up largely of sheets often form fibers such as keratin
fibers in hair, fibrin in blood clots, and silk in spiders' webs.
Proteins that have regions forming coils frequently fold into globular
shapes such as the globin subunits of hemoglobin in blood.
Hydrogen bonding can also result in
proteins with more complex shapes than the secondary structure. The
next level of structure is called tertiary structure 3. For proteins
that consist of subunits (separate polypeptide chains), the way these
subunits are assembled into a whole is called the quaternary structure
4.
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