Proteins and Amino Acids

Study Notes

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.

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.

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)

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.

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.

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.