1. AMINO ACIDS AND PEPTIDES
MR. R. PHIRI
CH 2011
March-June 2021 SEMESTER SESSION
2. •Amino acids are the monomer units of proteins.
•There are over 300 amino acids that occur in nature but of
these only 20 amino acids repeatedly occur in proteins.
The 20 amino acids are referred to as standard amino
acids.
•Amino acids contain 2 functional groups: amino and
carboxyl group bonded to an 𝛼 −carbon. There’s an
𝑅 −group attached to the 𝛼 −carbon. The 𝑅 −group
specifies the type of amino acid.
•The carboxyl end of amino acids is acidic whilst the amino
end is basic.
•Amino acids show stereoisomerism about the 𝛼 −carbon:
most amino acids are 𝛼 − L− amino acids. Proteins are
synthesized from these 20 L-α-amino acids
5. Biomedical importance of amino acids
1. Amino acids are the building blocks of proteins.
2. Amino acids and their derivatives are important in the
biosynthesis of porphyrins, purines, pyrimidines, and urea.
3. Amino acids and their derivatives participate in diverse
cellular functions such as nerve transmission, and as
chemical messengers.
6.
7. Classification of amino acids
Amino acids can be classified based on:
1. Structure
2. their metabolic fate
3. Chemical nature
4. Nutritional requirement.
8. 1.Classification based on structure and
chemical nature
The 20 amino acids found in proteins
can be classified into 6 major groups.
9. 1.Non polar Aliphatic side chains
• These are amino acids that contain non polar aliphatic side
chains.
• These are generally simple amino acids.
• They include glycine, alanine, leucine, isoleucine, valine
and proline
• (Leu, lle, Val) contain branched aliphatic side chains
• Proline can be classified as an imino acid due to
possessing a secondary amine group
10.
11. 2.Amino acids containing hydroxyl (𝑂𝐻−
) groups
•Serine, threonine and tyrosine
•Tyrosine is also aromatic and is therefore
considered under aromatic amino acids too.
12. 3. Sulfur containing amino acids
•Cysteine and Methionine
•Cystine, another important sulfur containing amino
acid formed by condensation of two molecules of
cysteine.
13. 4.Acidic amino acids and their amide Derivatives
• Glutamic, Aspartic, Asparagine,Glutamine
• asparagine and glutamine are their respective amide
derivative of aspartic and glutamic
14.
15. 5. Basic amino acids
•Lysine, Histidine, Arginine
•These are highly basic. Histidine forms an imidazole and
it may also be considered under aromatic amino acids.
16.
17. 6.Aromatic amino acids
• Phenylalanine, Tyrosine, Tryptophan
• Histidine may also be considered to be an aromatic
amino acid.
18.
19. 2. Classification based on their chemical
properties
1.Non-polar amino acids - these are also said to be
hydrophobic
2. Polar amino acids with no charge -These are
hydrophilic amino acids that do not contain any charges
on them
3.Polar with a positive charge - polar amino acids with a
net positive charge on them.
4.Polar amino acids with a negative charge - carboxylic
20. 3. Nutritional classification of amino acids
•Amino acids can be classified as essential or non –
essential.
•Some essential amino acids include: Arginine, Valine,
Histidine, lsoleucine, Leucine, Lysine, Methionine,
Phenylalanine, Threonine, and Tryptophan.
•But Arginine and Histidine can be synthesized by adults
though children are unable to synthesize it and so they are
considered semi - essential amino acids.
•There are about 10 amino acids that are non-essential.
They include: glycine, alanine,
serine,cysteine,aspartate,asparagnine, glutamate,
glutamine, tyrosine and proline.
21. 4.Amino acid classification based on their
metabolic fate
• The carbon skeleton of amino acids can serve as a
precursor for the synthesis of glucose( glycogenic) or
fat (ketogenic).
• Based on metabolic fate amino acids can be classified
into 3 groups.
1. Glycogenic
2. Ketogenic
3. Both ketogenic and glycogenic
22. Modified amino acids
•In addition to the 20 standard amino acids, there are other
amino acids found in proteins which are a result of
posttranslational modification of parent amino acid.
•These include: Hydroxyproline, hydroxylysine and thyroxine.
•Hydroxyproline and hydroxylysine differ from the parent
amino acids in that they have hydroxyl groups on their side
chains
•Thyroxine differs from tyrosine in that it contains an iodine-
containing aromatic group on the side chain. Thyroxine is a
hormone.
23. Amino Acids Can Act as Both Acids and Bases
• Amino acids have both acidic and basic properties
(amphoteric).
• They are generally weak acids and bases. They can
donate protons and are generally polyprotic acids.
• Most amino acids are diprotic.
• Carboxyl (negatively charged) and amino (positively
charged) are charged at neutral pH.
• Amino acids can dissociate and thus donate protons
to a solution
24. Titration of amino acids
• When an amino acid is titrated, its titration curve indicates the
reaction of each functional group with hydrogen ion.
• At very low pH, amino acids are protonated and positively charged.
• When we add a base to this solution, the amino acid starts to lose its
protons and there are changes in pH. As we add more of the base,
amino acids become negatively charged at very high pHs.
• Titratable groups of each of the amino acids have characteristic pKa
values. The pKa values of α-carboxyl groups are fairly low, around 2.
The pKa values of amino groups are much higher, with values ranging
from 9 to 10.5.
• The pKa values of side-chain groups, including side-chain carboxyl
and amino groups, depend on the groups’ chemical nature.
27. •Amino acids can act as buffers, the maximum buffering
capacity of an amino acid is at at it’s pka whilst maximum
buffering range is within ±1 pKa value.
•Amino acids have different pKa values and that means each
amino acid will have different charge at a given pH and this
property can be used to separate amino acids by
electrophoresis
28. •For example alanine and histidine, both have a -1 charge at
high pH but at lower pH of about 5, alanine is a zwitterion
whilst histidine is positive. Thus these two amino acids can
be separated from each other at this pH in an electric field.
•The pH at which a molecule has zero net charge is called the
isoelectric pH (pI). At its isoelectric pH, a molecule will not
migrate in an electric field. The net charge of an amino acid
determines its mobility in an electric field. This property can
also be used in separating amino acids.
29. Calculating pI
• The isoelectric point (pI) value can be calculated by
taking the average pKa values of corresponding
ionizable groups.
pI =
𝑝𝐾1 +𝑝𝐾2
2
30. Example
• Calculate the pI of leucine with pka COOH (2.4) and
pka 𝑁𝐻3
+
9.6 .
𝒑𝑰 =
𝒑𝑲𝒂𝟏 + 𝒑𝑲𝒂𝟐
𝟐
𝒑𝑰 =
𝟐. 𝟒 + 𝟗. 𝟔
𝟐
=6.0
31. Peptides
• Peptides are compounds in which an amide bond links the
amino group of one a-amino acid and the carboxyl group of
another.
• Peptides can be said to be compounds formed by linking
small numbers of amino acids, ranging from two to several
dozen.
• An amide bond of this type is often referred to as a peptide
bond.
• The carbon–nitrogen bond formed when two amino acids
are linked in a peptide bond is usually written as a single
bond
32. Importance of peptides
• In the neuroendocrine system peptides useful as hormones
(vasopressin & oxytocin), hormone-releasing factors,
neuromodulators, and neurotransmitters.
• Some peptides are useful as analgesics Leucine and
methionine enkephalin.
• Some Bacteria peptides contain both D-and L-α-amino acids
which possess therapeutic value such as the antibiotics
Bacitracin and the antitumor agent Bleomycin.
• Some peptides are useful as antioxidants in the body e.g.
Glutathione
• Some peptides such as aspartame are used as food
sweeteners.
33. Formation of a peptide
Glycine
Alanine
+
Alanylglycine
+ 𝐻2𝑂
→
The two α- amino acids are joined by a peptide bond in
alanylglycine by a condensation reaction. They form a
dipeptide.
Ala-Gly
AG
34.
35. • The peptide bond can be written as a resonance hybrid of two
structures, one with a single bond between the carbon and
nitrogen and the other with a double bond between the carbon
and nitrogen.
• The peptide bond is also stronger than an ordinary single bond
because of this resonance stabilization.
• The peptide bond is characterized by a planar geometry, partial
double bond characteristic and inability to rotate.
• Which bonds have the ability to rotate on a peptide ?
• There is free rotation around the bonds between the α-carbon
of a given amino acid residue and the amino nitrogen (phi) and
carbonyl carbon(Psi) of that residue but no rotation on the
peptide bond.
• This constraint plays a role in how the protein backbone folds.
36. Classification of peptides
•Peptides are classified according to the number of
amino acids linked together.
• dipeptides, tripeptides, tetrapeptides, etc
•Some examples of peptides include Glutathione and
Oxytocin
37. Oxytocin
• Oxytocin induces labor in pregnant women and controls contraction
of uterine muscle. Oxytocin also plays a role in stimulating the flow of
milk in a nursing mother
• Oxytocin is a cyclic Nona peptide. Instead of having its amino acids
linked in an extended chain, two cysteine residues are joined by an
S—S bond
38. Oligopeptide
•Peptides made of small numbers of amino acids are
called oligopeptides.
•Oligopeptides have about 2 to 20 amino acids residues
and some even up to 40 amino acid residues.
•Thus oligopeptides include dipeptides, tripeptides,
tetrapeptides, pentapeptides and octapeptides etc.
