Amino acids are the building blocks of proteins. They are essential for the running and maintenance of normal body systems. Proteins are polymers of amino acids, with each amino acid residue joined to its neighbor by a specific type of covalent bond.
General Properties of Amino Acids
Each of the amino acids used in protein synthesis has the same structure. They contain a carboxylate acid group, an amino group attached to the Alpha-carbon in an L-configuration, a hydrogen atom and a chemical group called the side chain. The side chain is different in each amino acid compound.
In a solution and at physiological pH, the free amino acids exist as Zwitterions: that is ions in which the amino group is positively charged and the carboxylate group is negatively charged.
Classification of Amino Acids
Amino acids are classified based on the properties of their side chains. The side chains determine the types of bonds and interactions each biomolecule in a polypeptide chain can make with other molecules. Thus, amino acids can be grouped into five main classes based on the properties of their R-group, where R-group means the side chains.
Nonpolar, Aliphatic R-Group
In this group, the R-groups are nonpolar and hydrophobic. Examples of amino acids in this group are Alanine, Valine, Leucine, and Isoleucine. They cluster together within proteins, stabilizing protein structure by means of hydrophobic interactions. Glycine is the smallest member of this group with its R-group being a hydrogen atom.
Nonpolar Aromatic R-Group
Members of this group are hydrophobic. Phenylalanine, Tyrosine, and Tryptophan are members of this group. The hydroxyl group of tyrosine can form hydrogen bonds, which makes it an important functional group in some enzymes. Tryptophan and phenylalanine absorb ultraviolet light. This accounts for the strong absorbance of light by most proteins at a wavelength of 280nm, and this properties are exploited by scientists in protein characterization.
Polar, Uncharged R-Group
The R-group of members of this family are hydrophilic, because they contain functional group that form hydrogen bonds with water. Members of this group include Serine, threonine, Cysteine, Asparagine and Glutamine.
Positively Charged (Basic) R-Group
Members of this group include Lysine, Arginine, and Histidine. At the physiologic pH, the R-group of Lysine and Arginine are fully ionized and positively charged. Histidine is however weakly basic and the free amino acid is largely uncharged at physiologic pH. When Histidine is incorporated into a protein, its R-group can be either positively charged or neutral, depending on the ionic environment provided by the protein.
Negatively charged (Acidic) R-Group
At physiologic pH, members of this group are fully ionized, containing a negatively charged carboxylate group (-Coo–). Aspartic and Glutamic acids are the two members of this group, and therefore, are called Aspartate and Glutamate respectively, to emphasize that the members of this are negatively charged at physiologic pH.
Optical Properties of Amino Acids
The α-carbon of Amino acids is said to be a chiral or optically active carbon atom. This is because the α-carbon atom is attached to four different chemical groups. An exception however, is glycine, which is attached to two hydrogen atoms.
Amino acid with an asymmetric center at the α-carbon can exist in two forms, which are designated D and L, and are mirror images of each other. The two forms in each pair are called stereoisomers, optical isomers, or enantiomers.
Amino acids found in proteins are of L configuration. D-amino acids are however found in some antibiotics and in bacteria cell walls.
Inborn errors of metabolism
Inborn errors of metabolism are caused by specific changes in certain enzymatic reactions, in which the synthesis of certain amino acids are hampered by the changes in the enzyme’s structural-functional activity. Inborn errors in metabolism attack about 1 in every 5000 newborns. Some examples of inborn errors of metabolism include Alkaptonuria, Phenylketonuria, maple syrup urinary disease, etc.
Importance of Amino acids
- Gamma amino butyric acid (GABA) is a derivative of glutamic acid and dopamine. It is a neurotransmitter. Gabapentin, which is an analog of GABA can pass through blood brain barrier and form GABA in brain. It is clinically used to relieve pain.
- Histidine is the mediator of allergic reactions.
- Thyroxine is an important thyroid hormone