chapter 16 amino acids, proteins, and enzymes
DESCRIPTION
Chapter 16 Amino Acids, Proteins, and Enzymes. 16.1 Functions of Proteins 16.2 Amino Acids 16.3 Amino Acids as Acids and Bases. Functions of Proteins. Proteins perform many different functions in the body. Amino Acids. Amino acids are the building blocks of proteins. - PowerPoint PPT PresentationTRANSCRIPT
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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