What is the Difference Between L and D Amino Acids

The main difference between L and D amino acids is that the amine group of L-amino acids occurs in the left-hand side when drawn in the Fischer projection, keeping the carboxylic acid group on top and the carbon chain in the bottom, whereas the amine group of the D-amino acids occurs in the right. 

L- and D-amino acids are the two isomeric forms of amino acids which occur in nature. L-amino acids are the form used by the cells to synthesize proteins.

Key Areas Covered

1. What is L Amino Acid
     – Definition, Stereostructure, Importance
2. What is D Amino Acid
     – Definition, Stereostructure, Importance
3. What are the Similarities Between L and D Amino Acids
     – Outline of Common Features
4. What is the Difference Between L and D Amino Acids
     – Comparison of Key Differences

Key Terms

Amino Acids, D Amino Acids, Enantiomers, Fisher Projection, L Amino Acids, Proteins

What is L Amino Acid

L-amino acids are the form of stereoisomers used by the cells to produce proteins. They occur in all proteins produced by animals, plants, fungi, and bacteria. These proteins have both structural and functional roles inside the cell. They serve as enzymes, which catalyze biochemical reactions, as well as hormones, which regulate the biological process in higher organisms. Also, they are the form of amino acids produced by lightning reactions as well.

Figure 1: L and D Amino Acids

In the Fisher projection, the amine group of these amino acids occurs on the left side.

What is D Amino Acid

D-amino acids are the other form of stereoisomers that occur in nature. The amine group of these amino acids occurs in the right side in the Fisher projection.

Figure 2: General Structure of an Amino Acid

Generally, cells do not incorporate D-amino acids into proteins. But, some proteins are produced by enzyme posttranslational modifications in cone snails. On the other hand, some D-amino acids also occur in the peptidoglycan cell walls of bacteria. In addition, D-serine serves as a neurotransmitter in the brain.

Similarities Between L and D Amino Acids

• L- and D-amino acids are two possible orientations of a particular amino acid in nature.
• They are the mirror image of each other.
• Also, they can be considered as the isomeric forms, stereoisomers or enantiomers.
• However, the simplest amino acid, glycine, does not have stereoisomers.
• Both contain a carboxylic acid group, an amine group, a carbon chain, and a hydrogen atom bound to the central carbon atom of the amino acid.
• Further, this central carbon atom is called the alpha carbon or the chiral carbon.

Difference Between L and D Amino Acids

Definition

L-amino acid refers to a stereoisomer of a particular amino acid whose amino group is on the left side in its Fisher projection while D-amino acid refers to the other stereoisomer of the amino acid whose amino group is on the right side in its Fisher projection. This explains the basic difference between L and D amino acids.

Rotation of the Plane-Polarized Light

Also, while L-amino acids can rotate plane-polarized light counterclockwise in a process called levorotation, D-amino acids can rotate plane-polarized light clockwise in a process called dextrorotation.

R/S Notation

Another difference between L and D amino acids is that L-amino acids have been superseded by R notation while the D-amino acids have been superseded by S notation.

Importance

Moreover, L-amino acids are used by the cell to produce proteins while D-amino acids occur in the cell wall of bacteria.

Conclusion

L-amino acids are a form of stereoisomers whose amine group occurs in the left side of the Fisher projection. On the other hand, D-amino acids are the other form of stereoisomers whose amine group occurs on the right side of the Fisher projection. L-amino acids are a form of stereoisomer which is abundant in proteins. This is the difference between L and D amino acids.

Reference:

1. Kojo, Shosuke. “L-Amino Acids, Key for the Evolution of Life Came from Extraterrestrial Space?” Atlas of Science, Available Here
2. Genchi G. “An Overview on d-Amino Acids.” Amino Acids, vol. 49, no. 9, Sept. 2017, pp. 1521–1533., doi:https://doi.org/10.1007/s00726-017-2459-5. Available Here

Image Courtesy:

1. “Op isomer” (Public Domain) via Commons Wikimedia 
2. “AminoAcidball” By GYassineMrabetTalk✉This W3C-unspecified vector image was created with Inkscape. – Own work (Public Domain) via Commons Wikimedia