Protein comes from the Greek word “proteios”, which means first or primary. Protein, the building block of life, is synthesized in all forms of living cells. Humans have tens of thousands of types of proteins, which are all constructed from a set of 20 amino acids. A polymer of amino acid is a polypeptide.

All amino acids share common structures. Amino acids consist of an amino group and a carboxyl group connected by an alpha carbon. The R group, also known as side chain, differs with each amino acid. The side chain’s physical and chemical properties determine the functional role of the amino acid in the polypeptide. Because of the unique characteristic of the side group, an amino acid can be hydrophobic, hydrophilic, acidic or basic.

Amino acids are connected by a dehydration reaction, marked by the removal of water. The resulting covalent bond is called a peptide bond. A polypeptide, regardless of length, has a single amino acid end (N-terminus) and a single carboxyl end (C-terminus).

It is important to note that the term polypeptide is not synonymous with protein. A functional protein is not just an individual polypeptide chain, but actually one or more polypeptides precisely folded into a uniquely shaped molecule. A specific protein determines its function.

Amino Acid

Amino Acids Abbrev. (3 Letters) Abbrev. (1 Letter)
Alanine Ala A
Arginine Arg R
Asparagine Asn N
Aspartic Acid Asp D
Cysteine Cys C
Glutamic Acid Glu E
Glutamine Gln Q
Glycine Gly G
Histidine His H
Isoleucine Ile I
Leucine Leu L
Lysine Lys K
Methionine Met M
Phenylalanine Phe F
Proline Pro P
Serine Ser S
Threonine Thr T
Tryptophan Trp W
Tyrosine Tyr Y
Valine Val V

Protein Structure

Proteins have three structures: primary, secondary, and tertiary structure.

The primary structure of a protein is its polypeptide sequence. The secondary structure consists of the coil (alpha-helix) and folds (beta-sheet) that result from hydrogen bonds between repeating constituents of the polypeptide chains. The tertiary structure is the overall shape of the polypeptide resulting from interaction between the side chains of various amino acids. A quaternary structure also arises when a protein consists of two or more polypeptide chains.