It is the specific sequence of amino acids that determines the structures and properties of a protein. Each protein has a unique amino acid sequence. The 20-odd amino acids are arranged in a definite order in a protein, which may have hundreds of amino acid residues, just as the 26 letters of the alphabet can form many words and sentences. Proteins with similar structures may have entirely different physiological action if they differ in just one or few amino acid residues. Thus the peptide hormones oxytocin and vasopressin, which are secreted by the posterior lobe of the pituitary, have nearly identical structure, except for two amino acid residues.
There exists 20 required amino acids and they are known such that: essential, nonessential and conditional.
There exists 8 essential aminoacids that need to be obtained from the food. They are called "essential" because of the fact that the body cannot produce them and they need to be aquired from the daily diet. These 8 essential aminoacids are: leucine, isoleucine, valine, histidine, tryptophan, phenylalanine, methionine, threonine.
The nonessential aminoacids are produced by the body and not from daily diet. There exists 4 nonessential aminoacids such that: glutamic acid, aspartic acid, asparagine, alanine.
The conditional aminoacids are useful only in case of certain diseases or an increased level of stress. There exists 8 known conditional aminoacids such that: tyrosine, arginine, glycine, cysteine, glutamine, praline, serine and ornithine.