This document provides information about amino acids, peptides, and proteins. It discusses how proteins are the most abundant and functionally diverse molecules in living systems, performing essential roles like enzymatic functions and muscle movement. It describes the common components of amino acids, including carboxyl groups, amino groups, and variable side chains. Twenty amino acids are most commonly found in mammalian proteins. The document classifies these 20 amino acids and discusses their properties, including how nonpolar amino acids cluster inside proteins. It also covers cysteine and cystine, disulfide bonds, and properties of amino acids like solubility and acid-base characteristics. The polymerization of amino acids via peptide bonds to form polypeptides and proteins is also summarized.

1. AMINO ACIDS, PEPTIDES
AND PROTEIN
LECTURE # 5
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2. Proteins
Proteins are the most abundant and functionally diverse molecules in living systems.
Virtually every life process depends on this class of macromolecules.
FUNCTIONS
enzymes and polypeptide hormones direct and regulate metabolism
contractile proteins in muscle permit movement
In bone, the protein collagen forms a framework
hemoglobin and plasma albumin, shuttle molecules essential to life, whereas immunoglobulins
fight infectious bacteria and viruses.
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3. Amino Acids
More than 300 different amino acids have been described in nature.
Only 20 are commonly found as constituents of mammalian proteins.
Each amino acid has a
1. carboxyl group,
2. a primary amino group (except for proline, which has a
secondary amino group), and
3. a distinctive side chain (“R group”) bonded to the α-carbon atom
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Classification of the 20
amino acids commonly found
in proteins, according to
the charge and polarity of
their side chains at acidic pH

6. TYPES
A. Amino acids with nonpolar side chains
B. Amino acids with uncharged polar side chains
C. Amino acids with acidic side chains
D. Amino acids with basic side chains
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7. Amino acids with nonpolar side chains
 Each of these amino acids has a nonpolar side chain that does not gain or lose
protons or participate in hydrogen or ionic bonds (see Figure 1.2).
The side chains of these amino acids can be thought of as “oily” or lipid-like, a
property that promotes hydrophobic inter-actions
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8. Location of nonpolar amino acids in proteins:
In proteins found in aqueous solutions (a polar environment) the
side chains of the nonpolar amino acids tend to cluster together in
the interior of the protein
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10. Proline
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11. Proline
Proline differs from other amino acids in that
1. side chain and α-amino N form a rigid, five-membered ring structure (Figure
1.5).
2. Proline, then, has a secondary (rather than a primary) amino group.
3. It is frequently referred to as an“imino acid.”
4. The unique geometry of proline contributes to the formation of the fibrous
structure of collagen
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13. 1. Disulfide bond:
1. The side chain of cysteine contains a sulfhydryl (thiol) group (–SH), which is an important
component of the active site of many enzymes.
2. In proteins, the –SH groups of two cysteines can be oxidized to form a covalent crosslink
called a disulfide bond (–S–S–).
3. Two disulfide-linked cysteines are referred to as “cystine.”
Many extracellular proteins are stabilized by disulfide bonds. Albumin, a blood
protein that functions as a transporter for a variety of molecules, is an
example.
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14. 2. Side chains as sites of attachment for
other compounds:
The polar hydroxyl group of
serine; threonine; and, rarely,
tyrosine, can serve as a site of
attachment for structures such
as a phosphate group.
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BREAK

18. Cystine
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19. A derived amino acid found in many proteins is
cystine.
It is formed by the oxidation of two cysteine thiol side
chains, joined to form a disulfide covalent bond
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Cystine

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Properties

21. General properties of Amino acids
They have a very high melting and boiling point.
Amino acids are white crystalline solid substances.
In taste, few Amino acids are sweet, tasteless, and bitter.
Most of the amino acids are soluble in water and are insoluble in organic solvents.
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22. ACIDIC AND BASIC PROPERTIES OF
AMINO ACIDS
1. Amino acids in aqueous solution contain weakly acidic α-carboxyl
groups
2. weakly basic α-amino groups.
3. ionizable group in its side chain.
Thus, both free amino acids and some amino acids combined in
peptide linkages can act as buffers.
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23. The Solubility of Amino Acids Reflects
Their Ionic Character
The charged functional groups of amino acids ensure that they are readily solvated by—and thus
soluble in—polar solvents
such as water and ethanol but insoluble in nonpolar solvents such as benzene, hexane, or ether.
Amino acids do not absorb visible light and thus are colorless.
However, tyrosine, phenylalanine, and especially tryptophan absorb high-wavelength (250–290
nm) ultraviolet light.
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24. FUNCTIONAL GROUPS DICTATE THE CHEMICAL
REACTIONS OF AMINO ACIDS
1. Each functional group of an amino acid exhibits all of its
characteristic chemical reactions.
2. For carboxylic acid groups, these reactions include the formation of esters, amides, and
acid anhydrides;
3. For amino groups, esterification; and for —OH and —SH groups, oxidation and
esterification.
4. The most important reaction of amino acids is the formation of a peptide bond.
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25. Amino Acids Are Polymerized into Peptides
and Proteins
Polymerization of the 20 common amino acids into polypeptide
chains in cells is catalyzed by enzymes and is associated with the
ribosomes .
Chemically, this polymerization is a dehydration reaction
1. The a carboxyl group of an amino acid with side chain R1 forms a
covalent peptide bond with the a amino group of the amino acid
with side chain R2 by elimination of a molecule of water.
2. The dipeptide (two amino acid residues joined by a single
peptide bond) can then
form a second peptide bond through its terminal carboxylic acid
group and the amino gp of a third amino acid (R3), to generate a
tripeptide
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Thyrotropin releasing hormone Secreted by hypothalamus;
causes anterior pituitary gland to release thyrotropic hormone
Vasopressin (antidiuretic hormone) Secreted by posterior pituitary gland;
causes kidney to retain water from urine
Methionine enkephalin Opiate like peptide found in brain that
inhibits sense of pain
Little gastrin (human) Hormone secreted by mucosal cells in stomach;
Causes parietal cells of stomach to secrete acid
Glucagon (bovine) Pancreatic hormone involved in regulating glucose metabolism
Angiotensin II (horse) Pressor or hypertensive peptide; also stimulates release of
aldosterone from adrenal cortex
Plasma bradykinin (bovine) Vasodilator peptide Substance P Neurotransmitter

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