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1
Amino Acids and Proteins
Biology 12
E. McIntyre
2
• What do gelatin desserts, hair, antibodies, spider webs, blood clots, egg whites, tofu, and fingernails all have in common? They are all made of protein.
Where do we find proteins?
3
Proteins
• genetic information in DNA codes specifically for the production of proteins
• 50% of the dry mass of most cells
• As with carbohydrates and lipids, their three-dimensional structure is directly related to their function
• enzymes are proteins
4
Proteins: Basic Characteristics
• Proteins are amino acid polymers• central carbon atom surrounded by
an amino group, a carboxyl group, a hydrogen atom, and a variable group of atoms called a side chain, usually symbolized by the letter ‘R’. Remember that carboxyl and amino groups are functional groups.
• There are 20 different R groups commonly found in living organisms and, so there are 20 different amino acids.
General structure of an amino acid
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• There are 20 amino acids.
• Note all are the same except for the side chains
6
• Amino acids are amphiprotic (which means that they possess both acidic (carboxyl) and basic (amino) functional groups.)
• When dissolved in water, the carboxyl group donates an H ion to the amino group, causing the carboxyl group to become negatively charged and the amino group to possess an extra hydrogen and a net positive charge.
Amino acids: General Properties
7
• Amino acids may be polar (hydrophilic), nonpolar (hydrophobic), or charged (acidic or basic)
• Acidic amino acids possess a carboxyl group on their side chains, and basic amino acids contain amino groups on their side chains.
• These characteristics are determined by an amino acid’s side chain.
…Amino acids: General Properties
8
Examples of Amino Acids
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Types of Amino Acids
Nonpolar R = H, CH3
OPolar ll
R = –CH2OH, –CH2SH, –CH2C–NH2, (polar groups with –O-, -SH, -N-)
Acidic
R = –CH2COOH, or -COOHBasic
R = –CH2CH2NH2
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Learning Check AA1
Identify each as polar or nonpolar
A.
B.
11
Solution AA1
Identify each as polar or nonpolar
A
B
polar
Non-polar
12
Essential Amino Acids
• 10 amino acids not synthesized by the body• Arginine (arg) histadine (his) isoleucine (ile)
leucine (leu) threonine (thr) lysine (lys), methionine (met) phenylalanine (phe) trytophan (trp)valine (val)
• Must obtain from the diet• All in diary products • 1 or more missing in grains and vegetablesWant to remember the essential amino acids?
Any Help In Learning These Little Molecules Proves Truly Valuable
13
Amino Acids as Acids and Bases
• Ionization of the –NH2 (amino) and the –COOH (carboxyl) group
• An amphiprotic molecule has both a + and – charge
• An amphiprotic molecule is neutral overall
Zwitterion of alanine
In water, an amino acid loses a H+ ion from the carboxyl group and
donates it to the amino group.
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pH and ionization
H+ OH–
+ +
H3N–CH2–COOH H3N–CH2–COO– H2N–CH2–
COO–
Positive ion zwitterion Negative ion
Low pH neutral pH High pH
15
Learning Check AA2
CH3 CH3
+
H3N–CH–COOH H2N–CH2–COO–
(1) (2)
Select from the above structures
A. Alanine in base.
B. Alanine in acid.
16
Solution AA2
CH3 CH3
+
H3N–CH–COOH H2N–CH2–COO–
(1) (2)
Select from the above structures
A. (2) Alanine in base.
B. (1) Alanine in acid.
17
Stop & Think
• Remember that carboxyl and amino groups are functional groups.
• Think: What are functional groups?
• How are polymers of amino acids made?
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Solution to Stop & Think
• Carboxyl groups and amino groups react together to link amino acids.
19
The Peptide Bond
Amide bond formed by the –COOH of an amino acid and the –NH2 of the next amino acid
O CH3
+ | | + |
NH3–CH2–COH + H3N–CH–COO–
O CH3
+ | | |
NH3–CH2–C – N–CH–COO–
| peptide bond H
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Peptides
• Amino acids linked by amide (peptide) bonds
Gly Lys Phe Arg Ser
H2N- -COOH
end Peptide bonds end
Glycyllysylphenylalanylarginylserine
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Learning Check AA3
What are the possible tripeptides formed from one each of leucine, glycine, and alanine?
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Solution AA3
Tripeptides possible from one each of leucine, glycine, and alanine
Leu-Gly-AlaLeu-Ala-GlyAla-Leu-GlyAla-Gly-LeuGly-Ala-LeuGly-Leu-Ala
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Learning Check AA4
Write the three-letter abbreviations for the following tetrapeptide:
H3N CH
CH3
C
O
N
H
CH C
O
N
H
CH C
O
N
H
CH C O-
OCH
CH CH3
CH3
CH2
SH
CH2
CH2
S
CH3
25
Solution AA4
Ala-Leu-Cys-Met
H3N CH
CH3
C
O
N
H
CH C
O
N
H
CH C
O
N
H
CH C O-
OCH
CH CH3
CH3
CH2
SH
CH2
CH2
S
CH3
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Types of Proteins
Type Examples• Structuraltendons, cartilage, hair, nails• Contractile muscles• Transporthemoglobin• Storage milk• Hormonal insulin, growth hormone• Enzyme catalyzes reactions in cells• Protectionimmune response
27
Protein Structure