Amino acids are molecules that, from a chemical point of view, are characterized by the presence of two different functional groups. The name itself refers to them: an amine group and a carboxylic group (that is the most relevant acid group in biochemistry). Although normally when we hear the word “amino acid” we think in proteins, actually any molecule that presents these two functional groups is an amino acid.
From a functional standpoint, the amino acids play many important functions in our body. All our proteins are formed from different combinations of a set of 20 different amino acids, the so-called "standard amino acids".
This is actually the best known function of the amino acids, they are the building blocks of the thousands of proteins and peptides that exist in nature. However, to summarize all the functions in this one is a very narrow perspective. The amino acids play many other important physiological functions, such as: they are the donors of nitrogen atoms for the synthesis of several molecules, such as nitrogen bases, other amino acids, polyamines, heme group, ...; they are present in the composition of certain lipids, such as phosphatidylserine; they perform the function of neurotransmitters (glutamate and glycine, for example) or are used as precursors in the synthesis of other neurotransmitters (GABA, for example); they function as carriers of nitrogen in the blood stream, with glutamine and alanine with a particular relevant role in this function.
In their chemical composition, the standard amino acids have a common structure among the majority of their constituents. All have a central carbon, designated a-carbon, to which there are attached the amine group and the carboxylic group, to define them as amino acids. As these groups are bonded to a-carbon are often referred as a-amine and a-carboxylic acid, and amino acids are referred to as a-amino acids. In addition to these groups, there is also a hydrogen atom bonded to the central carbon. The 4th substituent is designated by R group or side chain and it is this group that defines the identity of the amino acid, since all other components are common to the 20 standard amino acids. Soon I will talk in more detail about the properties of this R group...