1 Chapter 16 Amino Acids, Proteins, and Enzymes 16.1 Functions of Proteins 16.2 Amino Acids 16.3 Amino Acids as Acids and Bases
Feb 13, 2016
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Chapter 16 Amino Acids, Proteins, and Enzymes
16.1 Functions of Proteins16.2 Amino Acids
16.3 Amino Acids as Acids and Bases
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Functions of Proteins
Proteins perform many different functions in the body.
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Amino Acids
Amino acids • are the building blocks of proteins.• contain a carboxylic acid group and an amino group on
the alpha () carbon.• are ionized in solution.• each contain a different side group (R). R side chain R
│ + │H2N—C —COOH H3N—C —COO−
│ │ H H ionized form
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Examples of Amino Acids
H + │H3N—C—COO−
│ H glycine
CH3
+ │H3N—C—COO−
│ H alanine
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Types of Amino Acids
Amino acids are classified as• nonpolar (hydrophobic)
with hydrocarbon side chains.
• polar (hydrophilic) with polar or ionic side chains.
• acidic (hydrophilic) with acidic side chains.
• basic (hydrophilic) with –NH2 side chains.
Nonpolar Polar
AcidicBasic
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Nonpolar Amino Acids
An amino acid is nonpolar when the R group is H, alkyl, or aromatic.
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Polar Amino Acids
An amino acid is polar when the R group is an alcohol, thiol, or amide.
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Acidic and Basic Amino Acids
An amino acid is • acidic when the R group is a carboxylic acid.• basic when the R group is an amine.
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Learning Check
Identify each as (1) polar or (2) nonpolar. +
A. H3N–CH2–COO− (Glycine)
CH3 | CH–OH + │
B. H3N–CH–COO − (Threonine)
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Solution
Identify each as (1) polar or (2) nonpolar. +
A. H3N–CH2–COO− (Glycine) (2) nonpolar
CH3
| CH–OH + │
B. H3N–CH–COO − (Threonine) (1) polar
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Fischer Projections of Amino Acids
Amino acids • are chiral except glycine.• have Fischer projections that are stereoisomers.• that are L are the only amino acids used in proteins.
L-Alanine D-Alanine L-Cysteine D-Cysteine
CH2SH
H2N H
COOH
CH2SH
H NH2 COOH
CH3
H NH2
COOH
CH3
H2N H
COOH
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A zwitterion • has charged −NH3
+ and COO– groups.• forms when both the –NH2 and the –COOH groups in an
amino acid ionize in water.• has equal + and – charges at the isoelectric point (pI).
O O ║ + ║
NH2—CH2—C—OH H3N—CH2—C—O–
glycine zwitterion of glycine
Zwitterions
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In solutions more basic than the pI,• the —NH3
+ in the amino acid donates a proton.
+ OH–
H3N—CH2—COO– H2N—CH2—COO– zwitterion Negative ion at pI pH > pI
Charge: 0 Charge: 1-
Amino Acids as Acids
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In solution more acidic than the pI,• the COO- in the amino acid accepts a proton.
+ H+ +
H3N—CH2—COO– H3N—CH2—COOH zwitterion Positive ionat pI pH< pI
Charge: 0 Charge: 1+
Amino Acids as Bases
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pH and ionization
H+ OH–
+ +H3N–CH2–COOH H3N–CH2–COO– H2N–CH2–COO– positive ion zwitterion negative ionlow pH pI high pH
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Separation of Amino Acids
When an electric current is used to separate a mixture ofamino acids• the positively charged amino acids move towards the
negative electrode.• the negatively charged amino acids move toward the
positive electrode. • an amino acid at its pI does not migrate.• the amino acids are identified as separate bands on
the filter paper or thin layer plate.
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Separation of Amino Acids
With an electric current, a mixture of lysine, aspartate,and valine are separated.
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CH3 CH3
+ | |H3N—CH—COOH H2N—CH—COO–
(1) (2)Which structure represents:
A. Alanine at a pH above its pI?
B. Alanine at a pH below its pI?
Learning Check
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CH3 CH3
+ | |H3N—CH—COOH H2N—CH—COO–
(1) (2)Which structure represents:
A. Alanine at a pH above its pI? (2)B. Alanine at a pH below its pI? (1)
Solution