21.6 : Amino acids

Amino acids are organic compounds that are predominantly made up of carbon, hydrogen, oxygen, and nitrogen. Although there are hundreds of amino acids, there are twenty that function as building blocks of proteins. These twenty amino acids are often referred to by their three or one-letter abbreviations.

All amino acids have a central carbon called the alpha carbon, which functions as a chiral center for most amino acids. It is attached to an amino group, a carboxyl group, a hydrogen atom, and a variable R group.

A chiral molecule has four distinct groups arranged around a particular atom, known as the chiral center. As glycine has a hydrogen atom as its R group, it does not have four distinct groups and therefore does not have a chiral center.

These four groups can form two distinct configurations that are stereoisomers, called L and D isomers. These isomers are mirror images that are not superimposable on each other. Amino acids used in protein synthesis are always L-isomers.

The amine group on an amino acid can act as a base and accept a proton resulting in a positive charge, whereas the carboxyl group can act as an acid and donate a proton resulting in a negative charge. At physiological pH, amino acids usually exist as dual-charged and are called zwitterions, molecules with an equal number of positive and negative charges.

Every amino acid has a unique R group that is responsible for their varied features that include their size, solubility, and charge. Amino acids are broadly classified into two categories based on their R-groups: non-polar and polar. 

Non-polar amino acids can be divided into aliphatic and aromatic. Polar amino acids can be divided into acidic, basic, and uncharged at a neutral pH.

Aliphatic groups are single bonded hydrocarbons that exist as straight, branched, or cyclic chains. The amino acids with aliphatic R groups are glycine, the simplest amino acid; alanine, valine, leucine, and isoleucine, which are extremely hydrophobic and associate with each other to form stable core structures in proteins; methionine, a sulfur-containing amino acid; and proline, which has an aliphatic ring.

Aromatic groups are hydrocarbons that exist as unsaturated rings with alternating single and double bonds. The aromatic amino acids are phenylalanine, tyrosine, and tryptophan, all of which are large and capable of participating in hydrophobic interactions.  

The acidic amino acids are aspartate and glutamate. They have carboxyl groups that donate a proton to become negatively charged. The basic amino acids are lysine, arginine, and histidine. They have amino groups on their side chains that accept a proton to become positively charged.

The polar uncharged amino acids are serine, threonine, cysteine, asparagine, and glutamine. They are hydrophilic and contain various polar functional groups: hydroxyl groups in serine and threonine, a sulfhydryl group in cysteine, and amide groups in asparagine and glutamine.