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CHAPTER 11: Proteins: Structure and Function
OUTLINE
•11.1 Amino Acids
•11.2 Chirality and Amino Acids
•11.3 Peptides
•11.4 Protein Architecture
•11.5 Enzymes
CHAPTER 11: Proteins: Structure and Function
BIOMOLECULES
• Biomolecules are large, complex organic molecules.
• They are present in all living things.
• They include:
• Proteins
• Carbohydrates
• Lipids
• Nucleic acids (DNA and RNA)
• The first of these is presented below, the remainder
in subsequent sets of slides.
• All obey general principles of chemical reactions
discussed previously.
CHAPTER 11: Proteins: Structure and Function
PROTEINS
• Proteins have some of the most diverse functions of
all biological molecules, ranging from the hemoglobin
that transports oxygen to tissues, to collagen and
elastin that provide structure to ligaments, tendons,
and blood vessels, to the enzymes that catalyze all
biochemical reactions.
• They are composed of amino acids linked together
in chains, folded into complex structures with specific
biological functions.
CHAPTER 11: Proteins: Structure and Function
PROTEIN FUNCTION IS DEPENDENT ON
STRUCTURE!
The change in a single component
of the protein hemoglobin results in
altered folding patterns for the
protein, which affects cellular
structure resulting in a serious
disease, sickle cell anemia.
CHAPTER 11: Proteins: Structure and Function
DID YOU KNOW?
1. What is the chemical basis for sickle cell anemia?
2. How does molecular polarity play a role in
hemoglobin function?
CHAPTER 11: Proteins: Structure and Function
DID YOU ANSWER?
Hemoglobin, like all globular proteins, has surface
polar amino acids and internal nonpolar amino acids.
In sickle cell anemia, one of its surface amino acids,
glutamic acid, is genetically exchanged for the
nonpolar amino acid, valine. This simple shift results
in the incorrect conformation, or folding, of the
protein, causing the disease.
CHAPTER 11: Proteins: Structure and Function
11.1 AMINO ACIDS
CHAPTER 11: Proteins: Structure and Function
RECALL
• Chemical properties of organic functional groups
• Effect of pH on acidic and basic groups
CHAPTER 11: Proteins: Structure and Function
PARTS OF AMINO ACIDS
• An amino acids consists of
• an amine
• a carboxylic acid
• a hydrogen atom
• one of about 20 side chains
(R groups or residues)
• All bonded to a central atom, the a-carbon
• The identity of an amino acid is determined by its
side chain.
CHAPTER 11: Proteins: Structure and Function
AMINO ACID STRUCTURE AND pH
• The amino group is basic; the carboxylic acid group
is acidic.
• At neutral pH, each is ionized.
• Compounds such as amino acids containing both a
negative and a positive charge are called zwiterions.
• As the pH of an amino acid solution changes, its
charges change.
CHAPTER 11: Proteins: Structure and Function
AMINO ACID SIDE CHAINS
• The 20 amino acids are differentiated by their side
chains.
• The side chains may be either polar or nonpolar.
• Nonpolar side chains are generally alkanes or have
aromatic ring structures.
• Polar side chains may be divided into acidic, basic,
or neutral side chains.
• At physiological pH,
• the 3 basic side chains have positive charge
• the 2 acidic side chains have negative charge
CHAPTER 11: Proteins: Structure and Function
EXAMPLES OF AMINO ACIDS
• Aspartic acid has an acidic side chain, lysine has a basic
side chain, and cysteine has a neutral side chain.
• Each amino acid may be designated by a 3-letter
abbreviation, as shown.
CHAPTER 11: Proteins: Structure and Function
NONPOLAR AMINO ACIDS
*Essential Amino Acid
CHAPTER 11: Proteins: Structure and Function
POLAR AMINO ACIDS
CHAPTER 11: Proteins: Structure and Function
ESSENTIAL AMINO ACIDS
• Essential amino
acids are amino acids
required in our diets.
• About half may be
synthesized by our
bodies, the rest must
be consumed in the
foods we eat.
• Essential amino
acids include the
following:
CHAPTER 11: Proteins: Structure and Function
PRACTICE PROBLEM
For each of the following amino acids check the
boxes that apply.
CHAPTER 11: Proteins: Structure and Function
11.2 CHIRALITY AND AMINO ACIDS
CHAPTER 11: Proteins: Structure and Function
AMINO ACID CHIRALITY
• Chirality is a property associated with many
molecules, particularly those with biomedical
applications such as most amino acids.
• Objects, such as a glove or certain molecules, are
chiral if their mirror images are non-superposable.
• Your right and left hands have chirality because they are
mirror images, and yet are not the same as each other.
• The term “chiral” means “handed.”
• Objects, such as a sock or mitten, are achiral
because their mirror images are superposable.
• Chiral molecules have at least one tetrahedral atom
with four different atoms or groups attached.
CHAPTER 11: Proteins: Structure and Function
ENANTIOMERS
• Enantiomers are pairs of chiral objects, one being
the mirror image of the other.
• Amino acids may consist of enantiomers.
• But only one of the pair is found in nature.
• It is customary to refer to each member of the
enantiomeric pair with the prefixes D- or L-.
• Only the L-form of amino acids is common. Other
common classes of materials are in the D-form.
• Other naming rules may use R- and S, (+) and (-),
or d-, and l-.
CHAPTER 11: Proteins: Structure and Function
EXAMPLE: A PAIR OF ENANTIOMERS
• The common form of alanine is the L-form. Each
has the same set of groups bonded to the a-carbon,
but in a different 3-dimensional relationship
• Note the mirror-image relationship between them:
CHAPTER 11: Proteins: Structure and Function
FISCHER PROJECTIONS
• A simplified way to indicate the structure of
enantiomers is to use Fischer projections.
• These indicate chiral atoms as crosshairs (+) with
the main carbon chain written vertically.
• For L-amino acids, the alpha carbon is shown at the
crosshair with the carboxylate to the top, the amino
group to the left and the side chain down.
• D-amino acids would be the mirror image of this.
CHAPTER 11: Proteins: Structure and Function
ALANINE AS A FISCHER PROJECTION
CHAPTER 11: Proteins: Structure and Function
PROPERTIES OF ENANTIONMERS
• When enantionmers are in an achiral environment,
their physical and chemical properties are identical.
• In a chiral environment, such as within a cell, their
properties may be quite different.
• Because proteins are composed of L-amino acids,
they are chiral.
• For example two common drugs, Darvon and
Novrad are enantiomers, but one is an analgesic and
the other is a cough suppressant.
CHAPTER 11: Proteins: Structure and Function
A PAIR OF ENANTIONMERS
• These pharmaceuticals are mirror images of each
other, yet they show very different effects in our
bodies.
CHAPTER 11: Proteins: Structure and Function
RACEMIC MIXTURES
• A racemic mixture is a mixture containing both
enantiomers, the L- form and the D-form.
• Since one of the pair usually does not have
biological activity, the racemic mixture is likely to have
reduced potency.
• These are usually designated by indicating both
forms, such as D/L.
• In many cases, the enantiomer without biological
activity displays adverse effects if consumed.
• Most pharmaceuticals contain only one of the two
enantiomers.
CHAPTER 11: Proteins: Structure and Function
PRACTICE PROBLEMS
1. Indicate whether the following statements are TRUE or
FALSE.
A chiral molecule:
a. is superposable on its mirror image
b. has an enantiomer
c. may exhibit different chemical properties from its
enantiomer in the body
d. typically exhibits different chemical properties from its
enantiomer in an achiral environment
2. Ibuprofen, the active ingredient in Motrin and other
OTC analgesics, is a chiral drug sold as a racemic
mixture. What does this mean?
CHAPTER 11: Proteins: Structure and Function
11.3 PEPTIDES
CHAPTER 11: Proteins: Structure and Function
PEPTIDE BONDS
• Peptides are molecules formed by joining two or
more amino acids.
• Peptides may be
• small oligopeptides (2 amino acids to about a dozen)
• larger polypeptides (up to about 50 amino acids)
• very large molecules called proteins (>50 amino acids)
• To form these, amino acids are joined by peptide
bonds, essentially an amide group.
• Peptide bonds form between the carboxylate group
of one amino acid and the ammonium on another.
CHAPTER 11: Proteins: Structure and Function
PEPTIDE FORMATION
•The formation of
a peptide bond is
an example of an
acyl group
transfer reaction.
•The reverse is a
hydrolysis.
•Note the
involvement of a
water molecule in
these reactions.
CHAPTER 11: Proteins: Structure and Function
OLIGOPEPTIDES
• Two amino acids in a peptide are a dipeptide.
• A third joined to these constitute a tripeptide.
• Structures of any length may be formed.
• One end of the chain will always have an
ammonium ion at neutral pH, and is called the Nterminus; the opposite end is a carboxylate ion and
is called the C-terminus.
CHAPTER 11: Proteins: Structure and Function
A SPECIFIC TRIPEPTIDE
• The tripeptide shown consists of the amino acids
alanine, glycine, and valine. Alanine is the N-terminal
amino acid and valine is the C-terminal amino acid.
Peptides are conventionally written with the Nterminus to the left.
CHAPTER 11: Proteins: Structure and Function
COMMON OLIGONUCLEOTIDES
• Aspartame, commercially the artificial sweetener
Nutrasweet™, is the dipeptide asp-phe.
• Oxytocin, a hormone involved in contractions during
labor and in lactation, consists of the 9 amino acids:
cys - tyr - ile - gln - asn - cys - pro - leu – gly
• The endorphin, met-enkephalin, mentioned in
chapter 7, has 5 amino acids. This helps reduce pain
following injury.
CHAPTER 11: Proteins: Structure and Function
PRACTICE PROBLEM
Are the dipeptides gly-phe and phe-gly different
compounds? Are they structural isomers or
stereoisomers?
CHAPTER 11: Proteins: Structure and Function
11.4 PROTEIN ARCHITECTURE
CHAPTER 11: Proteins: Structure and Function
PROTEIN STRUCTURE
• Proteins are composed of the 20 L-amino acids.
• A protein may have from about 50 to many
thousands of amino acids, joined linearly by way of
peptide bonds.
• The information for determining the sequence of
amino acids resides in the DNA of most cells.
• A gene is the region of DNA responsible for the
coding of a protein.
• There are thousands of different proteins in the
body, each has a specific function dependent on its
structure.
CHAPTER 11: Proteins: Structure and Function
THREE-DIMENSIONAL SHAPE OF PROTEINS
• Proteins are not strictly linear structures.
• The chain of amino acids folds into a threedimensional structure termed its native
conformation.
• Electrostatic interactions between atoms within the
protein and between protein atoms and external
atoms, such as solvent, determine folding patterns.
• Amino acid side chains contribute interacting groups
to this folding.
CHAPTER 11: Proteins: Structure and Function
FOUR LEVELS OF PROTEIN STRUCTURE
• Primary structure
• Sequence of amino acids from Nterminus to C-terminus
• Secondary structure
• Localized regular folding stabilized
by hydrogen bonds
• Tertiary structure
• Complex irregular folding of entire
protein
• Quaternary structure
• Association of two or more subunits
CHAPTER 11: Proteins: Structure and Function
PRIMARY STRUCTURE OF A PROTEIN
• The sequence of amino acids determines all other
aspects of a protein’s structure and function.
• If this sequence is altered, the protein may not
function properly.
• Genetic disease is usually a disruption of the
primary structure, often by replacing a single amino
acid.
• In sickle cell anemia, a single amino acid change
from glutamic acid to valine results in improper
function of hemoglobin.
• An amino acid is located in sequence by the number
of its position from the N-terminus.
CHAPTER 11: Proteins: Structure and Function
SECONDARY STRUCTURE
• Secondary protein structure refers to the regular
folding patterns in localized regions of a protein.
• Most interactions stabilizing secondary structure
occur between carbonyl oxygens and amide
hydrogens by way of hydrogen bonds:
CHAPTER 11: Proteins: Structure and Function
PATTERNS IN SECONDARY STRUCTURE
• There are many examples of extended patterns of
secondary structure.
• The two most common types of secondary structure
are
•a-helix
•b-pleated sheet
• Typical proteins may contain some of each of these
or be mostly one or the other; some proteins contain
neither.
CHAPTER 11: Proteins: Structure and Function
THE a-HELIX
• An a-helix is a coiled segment of a
polypeptide held in place by hydrogen
bonds between carbonyl and amide
groups along the protein backbone.
CHAPTER 11: Proteins: Structure and Function
RIBBON DRAWING OF a-HELIX
The diagrams show a portion of the oxygen-binding
protein, myoglobin. The ribbons on the left indicate
the seven regions of a-helix. The two ribbons in red
are expanded on the right to show hydrogen bonds.
CHAPTER 11: Proteins: Structure and Function
THE β-PLEATED SHEET
• b-pleated sheets occur when two or more
polypeptide chains stack in a pattern similar to that of
a pleated skirt.
• These are stabilized by hydrogen bonding, and the
chains may be either parallel to each other (each
chain in the same orientation N-terminal to Cterminal) or antiparallel (chains in opposite
orientation).
• The strength of silk is due to the presence of bpleated sheets in the protein fibroin.
CHAPTER 11: Proteins: Structure and Function
THE β-PLEATED SHEET
Ribbon drawings of b-pleated sheets usually indicate
these structures with broad flat arrows, pointing in the
direction of the C-terminal amino acid.
CHAPTER 11: Proteins: Structure and Function
PRACTICE PROBLEMS
Orient the peptide structures below to show how they
would form hydrogen bonds, and add dashes to show
the corresponding hydrogen bonds.
Label the partial positive and negative charges in the
atoms involved in hydrogen bonding.
Why are there partial charges?
CHAPTER 11: Proteins: Structure and Function
TERTIARY STRUCTURE
• The tertiary structure of proteins describes the
entire folding pattern of the protein and consists of
complex and irregular folding, not patterned as found
in secondary structure.
• Tertiary structure is determined by interactions that
include distant amino acid residues as well as the
surrounding environment.
• For some proteins, tertiary structure also includes
prosthetic groups—non-peptide organic molecules
or metal ions that are strongly bound to the protein.
CHAPTER 11: Proteins: Structure and Function
EXAMPLES OF TERTIARY STRUCTURE
CHAPTER 11: Proteins: Structure and Function
FOUR LEVELS OF PROTEIN STRUCTURE
• Primary structure
• Sequence of amino acids from Nterminus to C-terminus
• Secondary structure
• Localized regular folding stabilized
by hydrogen bonds
• Tertiary structure
• Complex irregular folding of entire
protein
• Quaternary structure
• Association of two or more subunits
CHAPTER 11: Proteins: Structure and Function
WHAT CAUSES A PROTEIN TO FOLD?
• The fundamental driving force behind protein folding
is energy.
• A folded protein and its environment are lower in
potential energy than the unfolded protein.
• Proteins fold spontaneously into their native
conformation under physiological conditions.
• Protein folding is not random, but is defined by the
sum of all the electrostatic interactions present in the
protein and its surroundings.
CHAPTER 11: Proteins: Structure and Function
TYPES OF ELECTROSTATIC INTERACTIONS
IN PROTEINS
• There are 4 main types of electrostatic interactions
resulting in the tertiary structure of a protein:
• Disulfide bridges (covalent bond, hydrophobic)
• Salt bridges (ionic bond, hydrophilic)
• Hydrogen bonding (hydrophilic)
• Dispersion forces (hydrophobic)
CHAPTER 11: Proteins: Structure and Function
DISULFIDE BRIDGES
Disulfide bridges are covalent bonds between two
cysteines. Their thiols (-SH) react to form a disulfide
(-S-S-). This is an example of an oxidation-reduction
reaction.
CHAPTER 11: Proteins: Structure and Function
DISULFIDE BRIDGES
Perming hair involves a reduction of the disulfide
bonds in keratin — a hair protein, followed by an
oxidation that forms new disulfide bonds, giving the
hair the shape of the rollers.
CHAPTER 11: Proteins: Structure and Function
SALT BRIDGES
• Salt bridges are ionic
bonds between
oppositely charged
amino acid residues.
• These are stronger
than hydrogen bonds
because of the fully
charged groups
attracted to each other.
CHAPTER 11: Proteins: Structure and Function
HYDROGEN BONDS
• Hydrogen bonding occurs between polar residues
when one residue contains either an N-H or an O-H
bond.
• While hydrogen bonding is weaker than disulfide
bridges or salt bridges, they are so numerous as to
contribute significantly to protein conformation.
CHAPTER 11: Proteins: Structure and Function
DISPERSION FORCES IN PROTEINS
• Dispersion forces are the weakest of the
interactions stabilizing protein conformation.
• These cause attractions between nonpolar amino
acid residues.
• Because nonpolar groups only interact with other
nonpolar groups, proteins fold so that nonpolar amino
acids are only in the interior.
CHAPTER 11: Proteins: Structure and Function
PROSTHETIC GROUPS
• Prosthetic groups are non-peptide organic
molecules or metal ions, or a combination of both,
that are essential to a protein’s function.
• The diagram shows heme, an organic molecule
containing Fe2+, a prosthetic group in the proteins
hemoglobin and myoglobin.
CHAPTER 11: Proteins: Structure and Function
PRACTICE PROBLEMS
Cystic Fibrosis (CF) is a disease caused by a missing
phenylalanine-508 in a protein containing 1480 amino acids.
The protein is necessary for chloride ions (Cl-) to pass through
the cell membrane. The abnormal CF protein does not fold
correctly as a result of the missing amino acid. As a result,
mucus accumulates in the lungs causing respiratory infections
and difficulty breathing. Many individuals with CF die by age
30, and require a lifetime of specialized care.
a. Write the structure of the missing amino acid.
b. Where in the primary structure is a phe missing?
c. What is a gene? Would you expect cystic fibrosis to be a
hereditary disease or an infectious disease? Explain.
d. What causes the protein that causes CF not to fold
correctly?
CHAPTER 11: Proteins: Structure and Function
PRACTICE PROBLEMS
1. Write the products formed in the following reduction.
2. To remove the odor from a dog sprayed by
a skunk, the dog can be bathed in water
containing a small amount of bleach (NaOCl)
or hydrogen peroxide (H2O2). Both bleach and
hydrogen peroxide are strong oxidizing
agents. The chemical structure of one of the
compounds that give skunks their offensive
odor is shown to the right. Write the oxidation
reaction that occurs when the thiol functional
group in this compound reacts with an
oxidizing agent.
CHAPTER 11: Proteins: Structure and Function
QUATERNARY STRUCTURE
• Some proteins consist of more than one polypeptide
chain; each is termed a subunit.
• The quaternary structure of a protein is the
relative arrangement and position of the subunits
within the protein.
• The subunits are stabilized by the same interactions
responsible for tertiary structure: disulfide bridges,
salt bridges, hydrogen bonds, and dispersion forces.
CHAPTER 11: Proteins: Structure and Function
HEMOGLOBIN HAS QUATERNARY
STRUCTURE
This diagram shows each of the levels of structure in
hemoglobin.
CHAPTER 11: Proteins: Structure and Function
PROTEIN DENATURATION
• The denaturation of a protein is any process
disrupting its folding.
• Denaturation does not alter the primary structure.
• Denaturation is only rarely reversible.
• Denaturants are agents capable of denaturing
proteins and include the following:
• Heat
• pH changes
• Detergents
• Some metal ions such as Pb+2, Hg+2
• Cooking an egg is an example of irreversible protein
denaturation.
CHAPTER 11: Proteins: Structure and Function
PRACTICE PROBLEMS
For the following pairs of amino acids, indicate how
they might interact to contribute to the tertiary or
quaternary structure of a protein. Choose from the
following: disulfide bond, salt bridge, hydrogen
bonding, or dispersion forces.
aspartic acid and histidine
serine and lysine
leucine and valine
two cysteines
tryptophan and isoleucine
CHAPTER 11: Proteins: Structure and Function
TYPES OF PROTEINS
• There are several ways to categorize proteins. One
way is by solubility properties:
• Fibrous proteins
• Insoluble, consists of long fibers or sheets
• Examples include collagen, keratin, elastins
• Globular proteins
• Compact, soluble
• Examples include enzymes, antibodies, and hormones
• Membrane proteins
• Contained within cell membrane; nonpolar exterior
• Examples include receptors, channels
CHAPTER 11: Proteins: Structure and Function
PRACTICE PROBLEMS
1. List three ways to denature a protein.
2. State two differences between globular and fibrous
proteins.
CHAPTER 11: Proteins: Structure and Function
11.5 ENZYMES
CHAPTER 11: Proteins: Structure and Function
HOW DO ENZYMES WORK?
• Enzymes are globular proteins able to catalyze
many chemical reactions in living things.
• Enzymes work by lowering the activation energy,
EA, for a reaction.
• Enzymes do not alter the overall change in energy
of a reaction, ∆E.
• The reactant in an enzyme-catalyzed reaction is
called the substrate.
CHAPTER 11: Proteins: Structure and Function
ENERGY DIAGRAM FOR ENZYMES
Reaction diagram showing energy pathway for a reaction with an enzyme
(blue line) and without an enzyme (red line). The highest point on the
energy diagram (EA) is lower in the enzyme catalyzed reaction
CHAPTER 11: Proteins: Structure and Function
ENZYME CLASSIFICATIONS
• Generally, enzymes are named after the reaction
they catalyze, with the suffix –ase added.
• Examples are shown:
CHAPTER 11: Proteins: Structure and Function
THE ENZYME-SUBSTRATE COMPLEX
• An enzyme is able to act on its substrate by binding
to it at the enzyme’s active site.
• The shape of the substrate is complementary to the
shape of the active site.
• Binding occurs through weak intermolecular forces
such as hydrogen bonds, dipole-dipole interactions,
and dispersion forces.
• The relationship between the active site and the
bound substrate is termed the enzyme-substrate
complex.
• This relationship is described through the lock-andkey model: they fit the same way a key fits in a lock.
CHAPTER 11: Proteins: Structure and Function
ENZYME RATES
• An enzyme speeds up a reaction by a considerable
factor.
• Reactions mediated by enzymes may proceed as
slowly as two substrate molecules acted on per
second to as fast as 10 million molecules per second!
• These reactions tend to be reversible.
CHAPTER 11: Proteins: Structure and Function
COFACTORS AND COENZYMES
• Cofactors are generally metal ions required by the
enzyme for its catalytic function.
• Examples include Mg2+, Zn2+, and Fe2+.
• Coenzymes are organic molecules with a similar
function.
• Examples include NAD+/NADH, FAD/FADH2, and
coenzyme A, all derived from B vitamins.
• While enzymes are not changed through the reaction,
coenzymes are changed and must be recycled to be used
again.
CHAPTER 11: Proteins: Structure and Function
pH AND TEMPERATURE DEPENDENCE
• Both pH and temperature affect enzyme performance.
• Most enzymes are optimal under physiological conditions of
pH 7 and 37°C.
• Changes in these conditions denature the enzymes, hence
the need to keep these values constant.
CHAPTER 11: Proteins: Structure and Function
PRACTICE PROBLEMS
1. What is the active site of an enzyme?
2. Which of the following changes when a catalyst is
added to a reaction:
• EA or ∆E?
• The heat released from the reaction or the rate of the
reaction?
3. How is the rate of an enzyme catalyzed reaction
affected by the following changes?
a. an increase in pH
b. a decrease in pH
c. a decrease in temperature
d. an increase in temperature
CHAPTER 11: Proteins: Structure and Function
ENZYME INHIBITORS
• Enzyme inhibitors prevent an enzyme from
performing its function.
• Non-specific inhibitors act on all enzymes.
• Examples include pH and temperature.
• Specific inhibitors act on a selected few
enzymes. Two categories of these:
• Competitive inhibitors
• Noncompetitive inhibitors
CHAPTER 11: Proteins: Structure and Function
COMPETITIVE INHIBITORS
• Competitive inhibitors act by competing with
substrate for the active site of the enzyme, preventing
their binding, and thus formation of product.
• One example is the use of alcohol to treat ethylene
glycol poisoning:
• Many prescription drugs are competitive inhibitors.
CHAPTER 11: Proteins: Structure and Function
COMPETITIVE INHIBITORS
• Noncompetitive inhibitors bind at a site other than
the active site to inactivate an enzyme.
• They change the enzyme’s active site so it can no
longer bind substrate.
• In enzyme pathways, the final product may inhibit
the first step in a process termed feedback.
• The synthesis of cholesterol in our bodies is
controlled by feedback:
CHAPTER 11: Proteins: Structure and Function
PRACTICE PROBLEM
Which type of enzyme inhibitor binds to the active site
of an enzyme: a competitive inhibitor or a
noncompetitive inhibitor?
CHAPTER 11: Proteins: Structure and Function
SUMMARY OF MAIN CHAPTER POINTS, P1
• Amino Acids
• Proteins are constructed from a set of 20-22 amino acids
• An amino acid contains an amine and a carboxylic acid covalently bonded to a carbon
atom—the a-carbon. The a-carbon also contains a side chain, R, which determines the
identity of the amino acid.
• The side chains of the 20 natural amino acids can be classified as either nonpolar or
polar.
• The polar side chains can be classified as acidic, basic, and neutral.
• Chirality and Amino Acids
• A chiral object has a non-superposable mirror image.
• Nineteen of the amino acids are chiral.
• Peptides
• Peptides are molecules composed of two or more amino acids held together by peptide
bonds.
• A peptide bond is an amide formed between the carboxylic acid of one amino acid and
the amine of another amino acid.
CHAPTER 11: Proteins: Structure and Function
SUMMARY OF MAIN CHAPTER POINTS, P2
• Protein Architecture
• Proteins are composed of anywhere from 50 to thousands of amino acids.
• The four levels of protein architecture are defined as primary structure, secondary
structure, tertiary structure, and quaternary structure.
• The primary structure of a protein is its amino acid sequence.
• Secondary protein structure refers to regular folding patterns in local regions of the
polypeptide backbone.
• The tertiary structure of a protein is the complex and irregular folding of the entire
polypeptide.
• The quaternary structure of a protein is the relative arrangement and position of
each of the subunits within the overall structure of the protein.
• Denaturation of a protein is the disruption of the secondary, tertiary, or quaternary
structure of a protein so that it can no longer perform its function.
CHAPTER 11: Proteins: Structure and Function
SUMMARY OF MAIN CHAPTER POINTS, P3
• Enzymes
• Enzymes act as catalysts to speed up chemical reactions.
• Enzymes work by lowering the energy of activation, EA,
for a reaction.
• Cofactors and coenzymes sometimes help an enzyme
achieve its catalytic effect.
• Enzyme inhibitors are compounds that prevent an
enzyme from performing its function.
• A competitive inhibitor competes with the substrate for the
active site of the enzyme.
• Noncompetitive inhibitors bind at a location on the
enzyme other than the active site and prevent the substrate
from binding to the substrate.