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Amino Acids and Proteins
Biology 12
E. McIntyre
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• 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?
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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
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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
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• 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
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• 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
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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.
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Solution AA1
Identify each as polar or nonpolar
A
B
polar
Non-polar
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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
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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
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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.
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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.
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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.
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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
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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
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Protein Structure
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