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.

Learn how to calculate the isoelectric point (pI) of amino acids by determining the average pKa values of ionizable groups. Explore an example with leucine, which forms a zwitterion at its pI. Understand how the pH of the medium influences the ionic nature of amino acids and peptides.