Chapter 19:
Proteins
1
AMINO ACIDS
• The building blocks of proteins
Only 20 amino acids are common in proteins
Called -amino acids because amino group is attached
to the -carbon, next to the carboxylate group
2
AMINO ACID “R” GROUPS
• Amino acids are categorized into four groups based on the
“R” group characteristics. The “R” group can be:
1. neutral and nonpolar (e.g. -CH3)
2. neutral and polar (e.g. -CH2-OH)
3. basic (e.g. -CH2CH2CH2CH2-NH3+)
4. acidic (e.g.-CH2-COO-)
3
AMINO ACID STEREOCHEMISTRY
• In 19 of the 20 amino acids, the -carbon is chiral.
• With few exceptions, the amino acids in living systems are
in the L form.
• Glycine is the achiral amino acid.
4
AMINO ACID ZWITTERIONS
• Amino acids exist as zwitterions, a dipolar ion that results
from an internal acid-base reaction.
• Note that the net charge of the zwitterion is zero.
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ZWITTERIONS, cont.
• The isoelectric point is the pH at which an amino acid has
a net charge of zero.
“R” Group
Neutral
Acidic
Basic
Isoelectric Point
About pH 6
Less than pH 6
More than pH 6
• At pH values above the isoelectric point, the amino acid
has a net negative value.
• At pH values below the isoelectric point, the amino acid
has a net positive value.
• Amino acid solutions can act as buffers because they react
with both H3O+ and OH-.
6
AMINO ACID REACTIONS
• Oxidation of cysteine, the only –SH containing amino acid,
to form a disulfide (-S-S-) bridge:
7
REACTIONS, cont.
• Peptide bond formation – amino acids can make polymers
by forming amide (peptide) linkages
8
REACTIONS, cont.
• Peptides – an amino acid polymer of short chain length
• Polypeptide – intermediate chain length polymer, less than
50 amino acids
• Protein – polymer with more than 50 amino acids
• Amino acid residue – an amino acid that is part of a
polymer
• By convention, peptides are written with the N-terminal
residue on the left and the C-terminal residue on the right
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IMPORTANT PEPTIDES
• Vasopressin and oxytocin – hormones released by
pituitary gland
• Both have disulfide bridges
• Vasopressin (antidiuretic hormone) decreases urine
formation
• Oxytocin causes uterine contractions.
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IMPORTANT PEPTIDES, cont.
• Adrenocorticotropic hormone – released by pituitary gland
• Has no disulfide bridges
• Regulates the production of steroids by the adrenal gland
11
PROTEIN SIZE
12
PROTEIN FUNCTIONS
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PROTEIN FUNCTIONS
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PROTEIN SHAPES
• Fibrous proteins – long rod-shaped or stringlike molecules
that intertwine to form fibers (collagen, elastin, keratin)
• Globular proteins – spherical-shaped proteins that form
stable suspensions in water, or is water soluble
(hemoglobin, enzymes)
15
PROTEIN COMPOSITION
• Simple proteins – contain only amino acid residues
• Conjugated proteins – contain amino acid residues and
other organic or inorganic components (prosthetic groups)
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PRIMARY PROTEIN STRUCTURE
• The linear sequence of amino acids in a protein chain
• Determines secondary and tertiary structures
17
SECONDARY PROTEIN STRUCTURE
• Determined by hydrogen bonding between amide groups
of amino acid residues in the chain.
• Two basic types of secondary structure:
-helix – keratin,
myosin,
epidermin,
Fibrin
-pleated sheets – found
extensively only
in silk protein
Random coil molecular
structure
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-HELIX
19
-PLEATED SHEET
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TERTIARY PROTEIN STRUCTURE
• The specific 3-D shape of a protein resulting from
interactions between “R” groups of amino acid residues
• “R” group interactions include:
Disulfide bridges – form between cysteine residues
Salt bridges – ionic bonds form between acidic and basic
residues
Hydrogen bonds – form between polar residues
Hydrophobic interactions – form between nonpolar
residues
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TERTIARY STRUCTURE, cont.
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TERTIARY STRUCTURE, cont.
• In an aqueous environment, the interaction of hydrophilic
and hydrophobic side chains with water also determines
shape.
23
QUATERNARY PROTEIN STRUCTURE
• All proteins have primary, secondary, and tertiary
structure, but not all proteins have quaternary structure.
• Quaternary structure is the arrangement of subunits that
form a larger protein.
• Subunits are polypeptides that have primary, secondary,
and tertiary structure.
• Conjugated proteins with quaternary structure contain
subunits as well as prosthetic groups, which may be
organic or inorganic components.
• Heme is an example of a
prosthetic group:
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QUATERNARY PROTEIN STRUCTURE
• Hemoglobin
• 4 chains (subunits): two identical alpha chains and two
identical beta chains
• 4 heme groups
• Hydrophobic forces hold subunits together
25
PROTEIN HYDROLYSIS
• Heat and acid or base can completely hydrolyze proteins.
• This is an important process in protein digestion.
26
PROTEIN DENATURATION
• The process by which a protein loses its characteristic
quaternary, tertiary, and secondary structure
• Leads to a loss of biological activity (function)
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PROTEIN DENATURATION
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