Proteins are chains of amino acids joined together by peptide bonds (also called amide bonds). Hence proteins are also called peptides or polypepetides depending on the number of amino acids in the chain. A chain of over 50 amino acids is usually a polypeptide.
The general form of amino acids (figure 1.1) consists of a carboxylic acid group (COOH); an amine group (NH2) a hydrocarbon group (G or R); and a hydrogen atom (H); all bonded to a carbon atom called the α carbon (alpha carbon). An amino acid is distinguished from another by the hydrocarbon group, G. Just 20 amino acids are commonly found in protein molecules. All except one of these amino acids have the general form of fig 1.1. The amino acid, proline is the exception. In the proline structure, a cyclic structure results because a hydrogen atom of the amine group has been substituted by the hydrocarbon group.
Figure 1.2 illustrates the linking of the amino acid Glycine with the amino acid Alanine to form Glycylalanine dipeptide. The peptide bond (or amide bond) is the bond that links a carboxylic acid group of an amino acid with the amine group of the other amino acid. In other words, the peptide bond (or amide bond) links the carbon atom and the nitrogen atom in the amide group of a protein molecule. The Glycylalanine dipeptide shown in figure 1.2 has an amine group at one end and a carboxylic acid group at the other end. This amine group left-end and carboxylic acid group right-end are characteristics of all protein molecules. Consequently, the identity of a protein is determined by its side chains and its amino-acid sequence. Therefore, Glycylalanine dipeptide is a different protein from Alanylglycine dipeptide eventhough both are constituted by the same two amino acids. In the latter, the amine group left-end belongs to Alanine.
Proteins are very important substances in living organisms. The expression of genetic information is realized ultimately through proteins, especially the enzymes that catalyse metabolic reactions.
The matter that bond to form amino acids; the chemical reactions that establish the bonds; the chemical bonds; the grouping of amino acids to form peptides and polypeptides and the work proteins do; are all expressions of Pj Problems. Consequently, proteins are expressions of Pj Problems.