Transcript Chapter 24 Amino Acids, Peptides, and Proteins

CHE 242
Unit VIII
The Structure, Properties,
Reactions and Mechanisms of
Carboxylic Acids and Their
Derivatives
CHAPTER TWENTY-FOUR
Terrence P. Sherlock
Burlington County College
2004
Structure of Proteins
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Stereochemistry of
-Amino Acids
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Essential Amino Acids
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Arginine (Arg)
Threonine (Thr)
Lysine (Lys)
Valine (Val)
Phenylalanine (Phe)
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Tryptophan (Trp)
Methionine (Met)
Histidine (His)
Leucine (Leu)
Isoleucine (Ile)
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Complete Proteins
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Provide all the essential amino acids.
Examples: those in meat, fish, milk, eggs.
Plant proteins are generally incomplete.
Vegetarians should eat many different
kinds of plants, or supplement diet with
milk or eggs.
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Rare Amino Acids
• 4-Hydroxyproline, 5-hydroxylysine found
in collagen.
• D-Glutamic acid in cell walls of bacteria
• D-Serine in earthworms
• -Aminobutyric acid, a neurotransmitter
• -Alanine, constituent of the vitamin
pantothenic acid.
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Zwitterion
• Amino acid exists as a dipolar ion.
• -COOH loses H+, -NH2 gains H+.
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Actual structure depends on pH.
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Properties of Amino Acids
• High melting points, over 200C
• More soluble in water than in ether.
• Larger dipole moments than simple
acids or simple amines.
• Less acidic than most carboxylic acids,
less basic than most amines.
pKa = 10
+
H 3N
O
CH
C
_
O
pKb = 12
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Structure and pH
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Isoelectric Point
• pH at which amino acids exist as the
zwitterion (neutral).
• Depends on structure of the side chain.
• Acidic amino acids, isoelectric pH ~3.
• Basic amino acids, isoelectric pH ~9.
• Neutral amino acids, isoelectric pH is
slightly acidic, 5-6.
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Electrophoresis
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Reaction with Ninhydrin
• Used to visualize spots or bands of amino
acids separated by chromatography or
electrophoresis.
• Deep purple color formed with traces of
any amino acid.
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Structure of Peptide
• The peptide bond is an amide bond.
• Amides are very stable and neutral.
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Peptide Bond Formation
• The amino group of one molecule condenses
with the acid group of another.
• Polypeptides usually have molecular weight
less than 5000.
• Protein molecular weight 6000-40,000,000.
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Classification
of Proteins
• Simple: hydrolyze to amino acids only.
• Conjugated: bonded to a nonprotein
group, such as sugar, nucleic acid, or
lipid.
• Fibrous: long, stringy filaments, insoluble
in water, function as structure.
• Globular: folded into spherical shape,
function as enzymes, hormones, or
transport proteins.
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Levels of
Protein Structure
• Primary: the sequence of the amino
acids in the chain and the disulfide links.
• Secondary: structure formed by
hydrogen bonding. Examples are helix and pleated sheet.
• Tertiary: complete 3-D conformation.
• Quaternary: association of two or more
peptide chains to form protein.
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Alpha Helix
Each carbonyl oxygen can hydrogen
bond with an N-H hydrogen on the next
turn of the coil.
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Pleated Sheet
Each carbonyl oxygen hydrogen
bonds with an N-H hydrogen on an
adjacent peptide chain.
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Summary of Structure
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Denaturation
• Disruption of the normal structure of a
protein, such that it loses biological
activity.
• Usually caused by heat or changes in pH.
• Usually irreversible. A cooked egg cannot
be “uncooked.”
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POWER POINT IMAGES FROM
“ORGANIC CHEMISTRY, 5TH EDITION”
L.G. WADE
ALL MATERIALS USED WITH PERMISSION OF AUTHOR
PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE
ORGANIC CHEMISTRY COURSE
BY:
ANNALICIA POEHLER STEFANIE LAYMAN
CALY MARTIN
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