Transcript Amino Acids & Peptides

Amino Acids & Peptides
Introduction:
Both D-amino acids and non-α-amino acids occur in
nature, but only L-α-amino acids are present in
proteins. All amino acids possess at least two weakly
acidic functional groups, R-NH3+ & R-COOH.
Many
also
possess
additional
weakly
acidic
functional groups such as -OH, -SH, guanidino, or
imidazole groups.
While proteins contain only L- α -amino
acids, microorganisms elaborate peptides
that contain both D- and L-α-amino acids.
Several of these peptides are of therapeutic
value, including the antibiotics bacitracin
and gramicidin A and the antitumor agent
bleomycin. Certain other microbial peptides
are toxic.
Neither humans nor any other higher
animals can synthesize 10 of the 20
common L- α -amino acids in amounts
adequate to support infant growth or to
maintain health in adults.
The Genetic Code Specifies20 L-Amino
Acids
Of the over 300 naturally occurring amino
acids, 20 constitute the monomer units of
proteins
Amino Acids May Have Positive,
Negative,or Zero Net Charge
Charged and uncharged forms of the ionizable
-COOH and -NH3 + weak acid groups
pKa Values Express the Strengths of
Weak Acids.At Its Isoelectric pH (pI), an
Amino Acid Bears No Net Charge.
The isoelectric, species is the form of a molecule that has an
equal number of positive and negative charges and thus is
electrically neutral.
The isoelectric pH, also called the pI, is the pH midway
between pKa values on either side of the isoelectric species.
For an amino acid such as alanine that has only two
dissociating groups,there is no ambiguity.
The first pKa (R-COOH) is 2.35 and the second pKa (RNH3 +) is 9.69. The isoelectric pH (pI) of alanine thus is:
For
polyfunctional acids, pI is also the pH midway
between the pKa values on either side of the isoionic
species. For example, the pI for aspartic acid is:
For lysine, pI is calculated from:
The Solubility and Melting Points of Amino
Acids Reflect Their Ionic Character