The amino acids of protein
General properties
AMINO ACID AND
PEPTIDE:
LECTURE
OUTLINE
Peptide bonds
Classifications
Acid-base properties
Optical Activity
Chirality
PROTEINS ARE
MADE FROM
AMINO ACIDS
 Amino acids are carboxylic
acids which also contain an
amino (- NH2) group.
 Those occurring in proteins
are almost exclusively amino acids ie the carboxyl
and amino groups are
attached to the same carbon
atom (the -carbon)
AA are classified according to the side chain, R: the most
useful way to do this is according to their hydrophobicity,
which determines their function in proteins.
CLASSIFICATION
The most hydrophobic are those with large apolar side
chains, the most hydrophilic those with charged side
chains.
The structures of the 20 aas commonly occurring in
proteins are given.
AA are often denoted by abbreviations of three letters (in
most cases the first 3 letters of the name)
20 MAJOR AMINO
ACIDS: TWO
MAJOR
CATEGORIES
BASED ON RGROUPS
Nonpolar (Hydrophobic)
Amino Acids
Polar (Hydrophilic)
Amino Acids
THREE LETTER
AND ONE
LETTER CODES
FOR THE AMINO
ACIDS
THE PEPTIDES
◼ Compounds of two amino acids linked by a peptide bond
are known as dipeptides
◼ 3 aa are called as tripeptides and so on
◼ Oligopeptides contain an unspecified but small number
of aa residues
◼ Polypeptides comprise larger numbers
◼ Natural peptides of 50 or more residues are called
proteins
Condensation of two -amino acids to form a dipeptide.

Linear polymers of aa have one -NH2 and one -COOH
which are not involved in the peptide bond formation

The aa bearing these are known as the N-terminus and Cterminus respectively

AA sequences are conventionally written from N-terminus
to C-terminus
Asp-Arg-Val-Tyr-Ile-His-Pro-Leu-Phe
N-terminus
C-terminus
WHAT IS CHIRALITY ?
A chiral object is not
superimposable on its
mirror image. Examples
include hands, socks and
keys…..
The two forms of a chiral
object are known as
Enantiomers, namely (R)and (S)-enantiomers.
Enantiomers are mirror
images of each other.
These molecules do differ
in the way that they rotate
plain polarized light and
the way that they react
with and interact with
biological molecules
NONSTANDARD AA
• The 20 common aa are not the only
aa that occur in the biological
systems.
• Nonstandard aa residues are often
important constituents of proteins.
• These residues result from the
specific chemical modifications of
aa residues in preexisting proteins.
•
Roles of these aa includes as
neurotransmitters, metabolic
intermediates and poisons.