Download lecture CH21 chem131pikul

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CHAPTER 21: Amino Acids, Proteins, Enzymes
Learning Objectives:
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CH 21 Homework:
End of Chapter problems: 32, 36, 38, 42, 48, 50, 52, 62, 64, 68, 70, 74,
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Amino Acids
Definition
All amino acids contain two functional groups—an
amino group (NH2) and a carboxyl group (COOH).
Amino acids differ in the R group bonded to the ! carbon
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Amino Acids
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At isoelectric pH
•  The isoelectric point for amino acids is about 6, this is the
pH at which the amino acid exists as a neutral molecule
•  The pKa of the amine group is usually about 9-11
•  The pKa of the carboxylic acid group is usually about 2-3
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Amino Acids
Acid/Base Chemistry
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Amino Acids
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Stereochemistry
•  Carbohydrates: naturally occurring isomer is the Disomer (OH group on right in a Fischer projection)
•  Amino Acids: most naturally occurring isomers are the Lisomer (NH3 group on the left in a Fischer projection)
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The 20 common &
naturally
occurring amino
acids in humans.
Essential Amino Acids:
Isoleucine (Ile)
Leucine (Leu)
Methionine (Met)
Phenylalanine (Phe)
Threonine (Thr)
Tryptophan (Trp)
Valine (Val)
Arginine (Arg)
Histidine (His)
Lysine (Lys)
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Peptides
Definition
• Peptides and proteins are formed when amino
acids are joined together by amide bonds.
• A dipeptide has two amino acids joined together
by one amide bond.
• The amide bond is called
a peptide bond.
• Polypeptides have many amino acids, while
proteins have more than 40 amino acids.
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Peptides
Amide Bond Formation
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Peptides
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Amide Bond Formation
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Peptides
N & C Terminal Amino Acids
• The amino acid with the free –NH3+ group is the
N-terminal amino acid and is written on the left.
• The amino acid with the free –COO" group is the
C-terminal amino acid and is written on the right.
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Peptides/
Protein
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Insulin
Gly-Ile-Val-Glu-Gln-Cys-Cys-Thr-Ser-Ile-Cys-Ser-Leu-Tyr-Gln-Leu-Glu-Asn-Tyr-Cys-R
Phe-Val-Asn-Gln-His-Leu-Cys-Gly-Ser-His-Leu-Val-Glu-Ala-Leu-Tyr-Leu-Val-Cys-GlyGlu-Arg-Gly-Phe-Phe-Tyr-Thr-Pro-Lys-Thr-R’
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Proteins
Primary Structure
The primary structure of a protein is the sequence
of amino acids joined together by peptide bonds.
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Proteins
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Secondary Structure
• The secondary structure is the 3D arrangement of localized regions of a
protein.
• These regions arise due to hydrogen bonding between the N—H group
of one amide with the C#O group of another.
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Proteins
Secondary Structure
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Proteins
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Tertiary Structure
The tertiary structure is the 3D shape adopted by the entire
peptide chain:
•  Maximize Hydrogen bonding with water (hydrophilic)
•  Stabilize non-polar sidechains by london dispersion forces
(hydrophobic)
•  Polar functional groups H-bond with each other
•  Charged sidechains attracted through electrostatic interactions
•  Disulfide bonds form
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Proteins
Tertiary Structure
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Proteins
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Quaternary Structure
The quaternary structure of the protein is the shape
adopted when two or more folded poly-peptide chains come
together into one complex. Ex: Potassium Channel:
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Proteins
Quaternary Structure
Hemoglobin
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Proteins
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Quaternary Structure
Hemoglobin
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Proteins
1°, 2°, 3°, 4° Structure
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Proteins
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Peptide Hydrolysis
Protein hydrolysis involves breaking the peptide bonds by
treatment with aqueous acid, base, or certain enzymes: Pepsin
(gastric juices), Trypsin and Chymotrypsin (intestines)
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Proteins
Peptide Denaturation
Denaturation is the process of altering the shape of a protein
without breaking the amide bonds that form the primary
structure: heat, acid, base, or agitation
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Enzymes
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Definition
Enzymes are proteins that serve as biological catalysts
for reactions in all living organisms.
•  They increase the rate of a reaction (106 to 1012 times faster),
but are unchanged themselves.
•  Enzymes are very specific; each enzyme catalyzes a certain
reaction or type of reaction only.
•  The names of most enzymes end with the suffix -ase like
peptidase, lipase, and hydrolase
•  A cofactor is a metal ion or an organic molecule needed for an
enzyme-catalyzed reaction to occur.
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Enzymes
Function
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Enzymes
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Conformational Changes Upon Binding
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Enzymes
Inhibition
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Enzymes
Zymogens
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Superoxide Dismutase
Enzymes
H
N
O
OH2
OH
OAsp FeIII
O2-
OH
OAsp
NHis
Asp 156 O
NHis
O2-
H+
NHis
OAsp FeII
NHis
NHis
OH2
HOOH
H+
2H+
OAsp FeII
NHis
NHis
His 160
N
His 73
NH
NH
OH2
NHis
FeIII
N
His 26 N
O2
NHis
Fe
NHis
NHis
O2-
O2NHis
* O2- hydrogen bonds to residues in secondary
coordination sphere, positioning it near Fe(II),
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