Last revised by Daniel J Bell on 27 Jul 2022

Proteins comprise long chains of amino acids joined together by peptide bonds.

A peptide bond is formed by the carboxyl group of one amino acid linking to the amino group of the next one. 

Arbitrarily proteins are categorized according to length of the amino acid chain: if less than one hundred amino acids, then they are peptides, subdivided into oligopeptides (2-10 amino acids) and polypeptides (11-99 amino acids). Conversely, chains of approximately 100 amino acids or more are termed proteins 1.

Glycoproteins are proteins with a carbohydrate component and lipoproteins are proteins with a lipid component.

The sequence of amino acids in the protein chain is called the primary structure. The basic chains are then folded and twisted in complex ways to form the secondary structure. A typical example is the α-helix, the simplest secondary structure of proteins (helical form). As these folded twisted chains are further organized into larger crystals (etc.) the arrangement is called the tertiary structure. Finally some protein macromolecules are comprised of several protein subunits and the overall arrangement is known as the quaternary structure, e.g. hemoglobin.

The three-dimensional structures of proteins (and nucleic acids) are stored in a database, the Protein Data Bank, which is accessible free of charge to users 7.

It was H. Emil Fischer, a German chemist, who first hypothesized the chemical bond that holds two amino acids together in a single structure (dipeptide) 8.

Peptide receptor radionuclide therapy (PRRT) is the technique by which some peptides, capable of binding specific transmembrane receptors, are labeled with ß- emitting radionuclides, such as Y-90 (90Y-DOTATOC, 90Y-DOTALAN) and Lu-177 (177Lu-DOTATATE) and used for the medical nuclear therapy of numerous tumors 2-6

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