Key Difference – Lysine vs L-lysine
Lysine and L-lysine are both types of amino acids, sharing same physical properties though, there is some difference between them. The key difference between Lysine and L-lysine is in the ability to rotate plane polarized light. Lysine is a biologically active naturally occurring essential α-amino acid. It can occur in two isomeric forms due to the possibility of forming two different enantiomers around the chiral carbon atom. These are known as L- and D- forms, analogous to left-handed and right-handed configurations. These L- and D- forms are said to be optically active and rotate plane polarized light in a different sense; clockwise or anticlockwise. If the light rotates lysine anticlockwise, then the light exhibits levorotation, and it is known as L-lysine. However, it should be carefully noted here that the D- and L- labeling of the isomers is not the same as the d- and l- labeling.
What is Lysine?
Lysine is an essential amino acid which is not synthesized in our body and has to be supplied by the regular diet. Therefore, lysine is an essential amino acid for humans. It is a biologically important organic compound composed of amine (-NH2) and carboxylic acid (-COOH) functional groups with chemical formula NH2-(CH2)4-CH(NH2)-COOH. The key elements of lysine are carbon, hydrogen, oxygen, and nitrogen. In biochemistry, amino acids having both the amine and the carboxylic acid groups attached to the first (alpha-) carbon atom are known as α-amino acids. Thus lysine is also considered as α-amino acids. The structure of lysine is given in figure 1.
Figure 1: Molecular structure of Lysine (* carbon atom is a chiral or asymmetric carbon atom and also represent the alpha- carbon atom)
Lysine is basic in nature because it contains two basic amino groups and one acidic carboxylic acid group. Hence, it also forms extensive hydrogen bonding due to the presence of two amino groups. Good sources of lysine are protein rich animal sources such as eggs, red meat, lamb, pork, and poultry, cheese, and certain fish (such as cod and sardines). Lysine is also rich in plant proteins like soy, beans, and peas. However, it is a limiting amino acid in most cereal grains but is abundant in most pulses and legumes.
What is L-lysine?
Lysine has four different groups around the 2nd carbon, and it is asymmetric structure. Also, lysine is an optically active amino acid due to the presence of this asymmetric or chiral carbon atom. Thus, lysine can create stereoisomers that are isomeric molecules having the same molecular formula, but differ in the three-dimensional orientations of their atoms in space. Enantiomers are two stereoisomers that are related to each other by a reflection or they are mirror images of each other that are non-superimposable. Lysine is available in two enantiomeric forms known as L- and D- and the enantiomers of lysine are given in figure 2.
Figure 2: Enantiomers of lysine amino acid. The groups COOH, H, R and NH2 are arranged around the C atom in a clockwise manner, the enantiomer is called L- form and D-form otherwise. L- and D- only refer to spatial arrangement around the carbon atom and do not refer to optical activity. While the L- and D- forms of a chiral molecule do rotate the plane of polarized light in different directions, some L-forms(or D-forms) rotate light to the left (levo or l- form) and some to the right (dextro or d- form). l- and d- forms are called optical isomers.
L-lysine and D-lysine are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light. They have non-superimposable mirror image relationship. However, the nomenclature of D and L is not common in amino acids including lysine. They rotate the plane-polarized light in the same magnitude but in different directions. The D and L-isomer of lysine which rotates the plane polarized light in the clockwise direction is called as dextrorotatory or d-lysine and the one which rotates the plane polarized light in the anticlockwise direction is called as laevorotatory or L-lysine (Figure 2).
L-Lysine is the most available stable form of lysine. D-Lysine is a synthetic form of lysine and can be synthesized from l-lysine by racemization. It is utilized in the processing of poly-d-lysine, which is used as a coating material to enhance the cell attachment. L-Lysine plays a significant role in the human body, in the calcium absorption, muscle protein development, and synthesis of hormones, enzymes, and antibodies. Industrially, L-lysine is produced by a microbial fermentation process using Corynebacterium glutamicum.
What is the difference between Lysine and L-lysine?
Lysine and L-lysine have the same physical properties, except for the direction in which they rotate polarized light. As a result, L-lysine may have substantially different biological effects and functional properties. However, very limited research has been done in order to distinguish these biological effects and functional properties. Some of these differences may include,
Taste
L-lysine: L-forms of amino acids tend to be tasteless.
D-lysine: D-forms of amino acids tend to taste sweet.
Therefore l-lysine may be less/no sweeter than lysine.
Abundance
L-lysine: The l-forms of amino acid including l-lysine is the most abundant form in the nature. As an example, nine of the nineteen L-amino acids commonly found in proteins are dextrorotatory and the rest are levorotatory.
D-lysine: The D- forms of amino acids that were observed experimentally were found to occur very rarely.
References:Solomons, T.W. Graham, and Graig B. Fryhle (2004). Organic Chemistry (8th ed). Hoboken: John Wiley & Sons, Inc.Everhardus, A. (1984). Stereochemistry, a basis for sophisticated nonsense in pharmacokinetics and clinical pharmacology, European Journal of Clinical Pharmacology, 26, 663-668. Image Courtesy:“L-lysine-monocation-from-hydrochloride-dihydrate-xtal-3D-balls” by Ben Mills – Own work via Wikimedia Commons
