Amino Acids Proteins, and Enzymes - Hartnell：氨基酸的蛋白质,与酶- Hartnell-精品文档99页

When an electric current is used to separate a mixture of amino acids • the positively charged amino acids move towards the
negative electrode. • the negatively charged amino acids move toward the
An amino acid is nonpolar when the R group is H, alkyl, or aromatic.
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by Pearson Education, Inc.
4
Polar Amino Acids
An amino acid is polar when the R group is an alcohol, thiol, or amide.
• acidic (hydrophilic) with acidic side chains.
• basic (hydrophilic) with –NH2 side chains.
Copyright © 2009 by Pearson Education, Inc.
Polar Basic
3
Nonpolar Amino Acids
+
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.
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.
Solution
CH3 +| H3N—CH—COOH
(1)
CH3 | H2N—CH—COO–
(2)
Which structure represents
A. alanine at a pH above its pI? (2)
B. alanine at a pH below its pI? (1)
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Learning Check
CH3 +| H3N—CH—COOH
(1)
CH3 | H2N—ຫໍສະໝຸດ H—COO–(2)Which structure represents
A. alanine at a pH above its pI?
B. alanine at a pH below its pI?
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• the backbone of a peptide chain or protein.
CH3
CH3
S
CH CH3 SH
CH2
CH3 O
CH O
CH2 O
CH2 O
H3N CH C N CH C N CH C N CH C O-
H
H
H
Ala─Leu─Cys─Met
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Primary Structures
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5
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.
and the amino group of the next amino acid.
O
CH3 O
+
||
+ | ||
H3N—CH2—C—O– + H3N—CH—C—O–
O H CH3 O
+
|| | | ||
H3N—CH2—C—N—CH—C—O–
peptide bond
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Formation of a Dipeptide
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20
Naming Dipeptides
A dipeptide • is named from the free amine (NH3+) using a -yl
ending for the name. • names the last amino acid with the free carboxyl group
amino acid ionize in water.
• has equal + and – charges at the isoelectric point (pI).
O ║ NH2—CH2—C—OH
Glycine
O
+
║
H3N—CH2—C—O–
Zwitterion of glycine
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Amino Acids as Acids
Positive ion
pH< pI
Charge: 1+
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pH and ionization
H+ + H3N–CH2–COOH
Positive ion low pH
OH–
+
H3N–CH2–COO– H2N–CH2–COO–
Zwitterion
Negative ion
pI
high pH
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Separation of Amino Acids
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Separation of Amino Acids
With an electric current, a mixture of lysine, aspartate, and valine are separated.
Copyright © 2009 by Pearson Education, Inc.
Chapter 16 Amino Acids, Proteins, and Enzymes
16.4 Formation of Peptides
Copyright © 2009 by Pearson Education, Inc.
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The Peptide Bond
A peptide bond • is an amide bond. • forms between the carboxyl group of one amino acid
In solutions more basic than the pI, • the —NH3+ in the amino acid donates a proton.
+
H3N—CH2—COO–
Zwitterion
at pI Charge: 0
OH–
H2N—CH2—COO–
Negative ion pH > pI Charge: 1-
CH3 │
CH3
S
│
│
CH–CH3 SH
CH2
│
│
│
CH3 O H CH O H CH2 O H CH2 O +│ ║ │ │ ║ │ │ ║ │ │ ║
H3N–CH–C–N–CH–C–N–CH–C–N–CH–C–O–
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Solution
Ala-Leu-Cys-Met
Alanylleucylcysteinylmethionine
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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
7
Solution
Identify each as (1) polar or (2) nonpolar.
the alpha () carbon.
• are ionized in solution. • each contain a different side group (R).
R side chain │ H2N—C —COOH │ H
R +│ H3N—C —COO−
│ H Zwitterion (ionized form)
(COO–) by its amino acid name.
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Learning Check
Write the three-letter abbreviations and names of the tripeptides that could form from two glycine and one alanine.
2
Types of Amino Acids
Amino acids are classified as Nonpolar
• nonpolar (hydrophobic) with hydrocarbon side chains.
• polar (hydrophilic) with
Acidic
polar or ionic side chains.
• The nonapeptides oxytocin and vasopressin have similar primary structures.
• Only the amino acids at positions 3 and 8 differ.
Glycylalanylglycine
Gly-Ala-Gly
Alanylglycylglycine
Ala-Gly-Gly
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Learning Check
Write the three-letter abbreviation and name for the
following tetrapeptide.
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
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Solution
Write the names and three-letter abbreviations of the tripeptides that could form from two glycine and one alanine.
Glycylglycylalanine
Gly-Gly-Ala
16.5 Levels of Protein Structure
Copyright © 2009 by Pearson Education, Inc.
26
Primary Structure of Proteins
The primary structure of a protein is
• the particular sequence of amino acids.
COOH
H2N
H
CH3
L-Alanine
COOH
H
NH2
CH3
D-Alanine
COOH
COOH
H2N
H
CH2SH
L-Cysteine
H
NH2
CH2SH D-Cysteine
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Zwitterions
A zwitterion
• has charged −NH3+ and COO– groups. • forms when both the –NH2 and the –COOH groups in an
11
Amino Acids as Bases
In solution more acidic than the pI, • the COO– in the amino acid accepts a proton.
+
H3N—CH2—COO–
Zwitterion
at pI
Charge: 0
H+
+
H3N—CH2—COOH
CH3 │
CH3
S
│
│
CH–CH3 SH
CH2
│
│
│
CH3 O H CH O H CH2 O H CH2 O +│ ║ │ │ ║ │ │ ║ │ │ ║ H3N–CH–C–N–CH–C–N–CH–C–N–CH–C–O–
Ala
Leu
Cys
Met
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Chapter 16 Amino Acids, Proteins, and Enzymes