Amino Acids & Peptides. Chapter Outline. Amino Acids 5.1 Amino acid classes (G1) Stereoisomers (G2) Bioactive AA Titration of AA (G3) Modified AA AA reactions Peptides 5.2. General Properties.
Note that at physiological pH of 7, amino acids are charged as shown to right. They can act as either acid or base.
The side chains of aspartic acid and glutamic acid are negatively charged above pH 3.
Lysine – has butylammonium side chain
Arginine – bears a guanidino group
Histidine – carries a imidazoliummoeity
The D-isomer of aspartic acid is found in some proteins as the result of a spontaneous post-translational modification associated with protein aging or as the by-product of enzymatic modification catalyzed by protein L-isoaspartylmethyltransferase.
All amino acids (except glycine) recovered from polypeptides are optically active and direction of angle of rotation is measured by polarimeter.
These asymmetric centers give rise to enantiomers (nonsuperimposable mirror images of one another) or are stereoisomers (chiral molecules with different configurations about at least one of their asymmetric centers but which are otherwise identical).
Large number of proteins can exist. A small protein of 100 amino acids can have 20100 or 1.27x10130 possible unique polypeptide chains.
Glutathione: the reduced form
reduces oxidizing agents by dimerizing to
form the disulfide bond with release of 2 H.