Amino Acid Functions
Amino acids are organic molecules comprised mainly of carbon, hydrogen, oxygen, and nitrogen. They contain a side chain (R group), which is specific for each amino acid, in addition to an amine (-NH2) and a carboxyl (-COOH) functional group. Around 500 amino acids are known, whereas only 20 are coded by the DNA. For humans nine amino acids like Histidine or Lysine are essential, meaning they cannot be produced by the body itself and have to be taken up with food.
In vivo, amino acids are the building blocks of peptides and proteins. Through translation of RNA amino acids are attached step-by-step with each other. The sequence for these polymer chains is encoded by the DNA which is then transcribed into RNA followed by the translation into proteins.
Peptide Bond Formation
The peptide bond is formed by the reaction of the carboxyl group with the amino group of the following amino acid resulting in an amide bond. In most cases the in vivo polymerization is catalyzed by ribosomes together with different other molecules like transfer RNA (tRNA) – which carry the next amino acid –, initiation factors, elongation factors and termination factors. tRNAs are linked with only one specific amino acid through an ester bond by enzymes called aminoacyl tRNA synthetases. The right sequence of the proteins is achieved by complementary base pairing of the loaded tRNAs with the mRNA situated in the ribosomes. The genetic code is defined by codons, a sequence of three DNA bases which code for one amino acid. Since the genetic code has redundancy but no ambiguity, there are codons translate into the same amino acid but no codon that can translate into two different amino acids.
All proteinogenic amino acids belong to the group of α-amino acids – the amine and carboxyl group are attached to the first (alpha-) carbon atom – and occur in two chiral forms, called L- or D-isomer. Most proteins are composed of L-amino acids, but rarely enzymatic posttranslational modifications can produce proteins with D-amino acids.
In terms of proteins, the amino acid sequence is called primary structure. Along with posttranslational modifications, the primary structure determines the secondary and the tertiary structure of a protein.
Beside the use in protein synthesis, amino acids can exert non-protein functions. Through oxidation to urea and carbon dioxide, they may be converted into energy. The synthesis of neurotransmitters like gamma-amino-butyric acid (GABA) and other molecules require amino acids as precursors.
Labeling of amino acids is widely used in the field of proteomics, e.g. in SILAC (stable isotope labeling by amino acids in cell culture) experiments. You have the choice between various labeling techniques like radioactive isotope, non-radioactive isotope or dye labeling.
In the industry animal feed is complemented with amino acids to compensate the absence of essential amino acids in the bulk components.
Glutamic acid is often used as a flavor enhancer, and aspartame as a low-calorie sugar replacement.