Biochemistry Lecture 3: Calculating Isoelectric Point of Amino Acids

Amino Acids Properties

• Amino acids generally have high melting points, often above 200°C.
• Amino acids can have different tastes, such as sweet (Gly, Ala, Val), tasteless (Leu), or bitter (Arg, Ile).
• All amino acids except glycine possess optical isomers due to the presence of an asymmetric carbon atom.
• Some amino acids have a second asymmetric carbon, e.g., isoleucine and threonine.

Amino Acids as Ampholytes

• Amino acids contain both acidic (COOH) and basic (-NH2) groups.
• Amino acids can donate or accept a proton, making them ampholytes.
• Amino acids rarely exist in a neutral form with free carboxylic and free amino groups.
• In strongly acidic pH, amino acids are positively charged (cation), while in strongly alkaline pH, they are negatively charged (anion).
• Each amino acid has a characteristic pH at which it carries both positive and negative charges and exists as a zwitterion (dipolar ion).
• The isoelectric point (pI) of an amino acid is the pH at which it has no net charge.

Peptides

• Peptides are short chains of 2-50 amino acids linked by peptide bonds.
• Chains of less than 10-15 amino acids are called oligopeptides (e.g., dipeptides, tripeptides, tetrapeptides).
• When a polypeptide contains more than approximately 50 amino acids, it is known as a protein.
• Peptides are formed by covalently linking amino acids through peptide bonds, which are amide linkages between the α-carboxyl group of one amino acid and the α-amino group of another.
• A water molecule is released during peptide bond formation.

Peptide Bond Characteristics

• Peptide bonds have partial double-bond character.
• Peptide bonds are rigid and planar.
• Peptide bonds have a trans configuration.
• Peptide bonds are uncharged but polar.

Interesting Peptides

• Enkephalins are pentapeptides that control pain by binding to receptors in certain brain cells.
• Bradykinin, vasopressin, and oxytocin are peptide hormones that are nonapeptides.
• Bradykinin inhibits tissue inflammation, vasopressin regulates blood pressure, and oxytocin induces labor and stimulates milk production.
• Gramicidin S is a cyclic decapeptide antibiotic produced by a strain of bacteria.
• Aspartame is a synthetic sweetener and a methyl ester of a dipeptide of L-aspartate and L-phenylalanine.
• Glutathione is a tripeptide that destroys harmful oxidizing agents in the body by reducing them.