Key Difference – L vs D Amino Acids
L amino acids and D amino acids are two forms of amino acids. The key difference between L and D amino acids is that L amino acid is the enantiomer of an amino acid that is capable of rotating plane polarized light anticlockwise (left-hand side) whereas D amino acid is the enantiomer of an amino acid that is capable of rotating plane polarized light clockwise (right-hand side).
An amino acid is simply an organic molecule with a basic amino group (-NH2), an acidic carboxyl group (-COOH), a proton and a variable ‘R’ group bound to an sp3 hybridized central carbon atom. Differences in the R group convey different chemical properties to amino acids and the possible vast array of chemical groups that could bind to the central carbon atom as R groups yields a remarkable diversity of amino acids with a massive versatility. Amino acids are well known as structural subunits that make up proteins. They also work as intermediates of cellular metabolism.
CONTENTS
1. Overview and Key Difference
2. What is Chirality
3. What is an L Amino Acid
4. What is a D Amino Acid
5. Relationship Between L and D Amino Acids
6. Side by Side Comparison – L vs D Amino Acids in Tabular Form
7. Summary
What is Chirality?
Chirality of an organic compound is a result of one or more chiral carbons being present in that particular organic molecule. A ‘chiral carbon’ is an asymmetric carbon atom attached to four different types of atoms or chemical groups. Now, all alpha-amino acids – except glycine which have two indistinguishable hydrogen atoms bound to the alpha carbon – have chiral alpha carbons. These chiral alpha carbons allow stereoisomerism, and as, a consequence, all physiological alpha amino acids except glycine can form two stereoisomers each, which are mirror images of each other. These non-superimposable mirror images as termed ‘enantiomers’ and named as ‘L’ or ‘D’ (L/D nomenclature) or ‘N’ or ‘S’ (N/S nomenclature). Regardless of the nomenclature, this enantiomeric difference is of high biological significance because amino acids interact with highly sensitive molecules which can recognize only of the two possible enantiomers.
What is an L Amino Acid?
An L amino acid is the enantiomer which, when in a solution, rotates plane polarized light anticlockwise. The letter ‘L’ indicates the Latin word ‘Laevus’, which means ‘left’. This rotation is termed ‘optical activity’ and is measured using an equipment called ‘polarimeter’. Despite the existence of both L and D forms, surprisingly in most physiological proteins, only L amino acids are found, and as a result, most amino acids naturally exhibit L-enantiomeric excess in biological systems.
What is a D Amino Acid?
A D-amino acid is the enantiomer of a particular amino acid which is capable of rotating the plane polarized light clockwise. Incorporating the Latin word ‘Dexter’ – meaning ‘right’ – these enantiomers are termed D-enantiomers. Generally, D-amino acids are not manufactured and incorporated into proteins by cellular systems. Yet, some D-amino acids can be found in bacterial cell walls but interestingly, not in bacterial proteins.
Figure 1: L and D Alanine
Though D-amino acids are scarce in biological systems, there are many instances where D forms play crucial roles. One example is the activity of racemase enzyme of Vibro cholera which, during slow growth, produce D forms of methionine and leucine from their L counterparts which reduce the peptidoglycan production.
What is the Relationship Between L and D Amino Acids?
- L and D amino acids are enantiomers of each other.
What is the Difference Between L and D Amino Acids?
L vs D Amino Acids |
|
| L amino acid is the enantiomer of an amino acid which is capable of rotating plane polarized light anticlockwise. | D amino acid is the enantiomer of an amino acid which is capable of rotating plane polarized light clockwise. |
| Nomenclature | |
| Letter “L” of L amino acid means “Laevus’”. | Letter “D” of L amino acid means “Dexter’”. |
| Occurrence | |
| In most physiological proteins, only L amino acids are found. | Some D-amino acids can be found bacterial cell walls, not in bacterial proteins. |
Summary – L vs D Amino Acids
Amino acids are the building blocks of proteins. There are two forms of enantiomers of amino acids: L amino acids and D amino acids. The key difference between L and D amino acids is that L amino acid is the enantiomer of an amino acid which is capable of rotating plane polarized light anticlockwise or to the left-hand side whereas D amino acid is the enantiomer of an amino acid which is capable of rotating plane polarized light clockwise or to the right-hand side.
Reference:
1. “Amino Acid.” Wikipedia, Wikimedia Foundation, 16 Mar. 2018, Available here.
2. “L And D Amino Acids .” Pearson – The Biology Place, Available here.
Image Courtesy:
1. “Figure 02 03 06” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia