Proteins and Amino Acids

Introduction to Proteins

• Proteins are the most abundant macromolecules in living cells and are responsible for almost every function that occurs in the body.
• Proteins are linear chains of amino acids linked together by covalent, peptide bonds.

Amino Acids

• There are approximately 300 amino acids present in various animal, plant, and microbial systems, but only 20 amino acids are involved in the formation of proteins.
• Amino acids differ from each other in their side chains or R-groups, attached to the α-carbon.
• The 20 amino acids of proteins are often referred to as the standard or primary or normal amino acids.
• All the amino acids found in proteins are exclusively of the L-configuration.

Classification of Amino Acids

• Amino acids can be classified in five ways:
  • Chemical nature of the amino acid in the solution
  • Structure of the side chain of the amino acids
  • Nutritional requirement of amino acids
  • Metabolic product of amino acids
  • Nature or polarity of the side chain of the amino acids
• Classification based on chemical nature:
  • Neutral amino acids
  • Acidic amino acids
  • Basic amino acids
• Metabolic classification:
  • Glucogenic amino acids (14 out of 20 standard amino acids)
  • Ketogenic amino acids (2 amino acids: leucine and lysine)
  • Both glucogenic and ketogenic (4 amino acids: isoleucine, phenylalanine, tryptophan, and tyrosine)

Properties of Amino Acids

Physical Properties

• Amino acids are colorless, crystalline substances soluble in water.
• They exhibit acid/base behavior due to the amino and carboxyl groups attached to the α-carbon.

Optical Properties

• All naturally occurring amino acids are optically active except glycine, which is optically inactive.
• At pH 7.0, all amino acids have the same L-configuration.

Zwitter Ion Formation

• Amino acids exist in aqueous solution as dipolar molecules or zwitterions, having both positive and negative charges on the same amino acid.
• At acidic pH, amino acids are positively charged due to the protonated COO group.
• At alkaline pH, amino acids are negatively charged due to the deprotonated NH3+ group.
• The net charge of an amino acid depends on the pH of the medium.

Isoelectric pH (pI)

• The pH at which an amino acid bears no net charge (zwitter ion) and therefore does not move in an electric field is called the isoelectric pH.