Difference Between L-tyrosine and Tyrosine        

L-tyrosine vs Tyrosine

The key difference between l-tyrosine and tyrosine is the ability to rotate plane polarized light. Tyrosine is a biologically active naturally occurring non-essential α-amino acid. It can occur in two forms of isomers, due to forming of two different enantiomers around the chiral carbon atom. These are known as L- and D- forms or equivalent to left-handed and right-handed configurations, respectively. These L- and D- forms are said to be optically active, and rotate plane polarized light in a different directions such as clockwise or anticlockwise. If the plane polarized light rotates the tyrosine anticlockwise, then the light reveals levorotation, and it is known as l-tyrosine. However, it should be carefully noted here that the D- and L- labeling of the isomers is not identical as the d- and l- labeling.

What is Tyrosine?

Tyrosine is a non-essential amino acid, which is synthesized in our body from an amino acid called phenylalanine. It is a biologically important organic compound composed of amine (-NH2) and carboxylic acid (-COOH) functional groups with chemical formula C6H4(OH)-CH2-CH(NH2)-COOH. The key elements of tyrosine are carbon, hydrogen, oxygen, and nitrogen. Tyrosine is considered as an (alpha-) α-amino acid because a carboxylic acid group and an amino group are attached to the same carbon atom in the carbon skeleton. The molecular structure of tyrosine is given in figure 1.

l-tyrosine vs tyrosine

Figure 1: Molecular structure of tyrosine (* carbon atom is a chiral or asymmetric carbon atom and also represent the alpha- carbon atom)

Tyrosine plays a vital role in photosynthesis of plants. It acts as a building block for the synthesis of several important neurotransmitters also known as brain chemicals such as epinephrine, norepinephrine, and dopamine. In addition, tyrosine is essential to producing melanin pigment, which is responsible for human skin hue. Moreover, tyrosine also helps in the functions of adrenal, thyroid, and pituitary glands for production and regulating their hormones.

What is l- tyrosine?

Tyrosine has four different groups around 2nd carbon, and it is an asymmetric configuration. Also, tyrosine is considered as an optically active amino acid due to the presence of this asymmetric or chiral carbon atom. These asymmetric carbon atoms in the tyrosine are shown in figure 1. Thus, tyrosine can produce stereoisomers, which are isomeric molecules that have the similar molecular formula, but vary in the three-dimensional (3-D) directions of their atoms in space. In biochemistry, enantiomers are two stereoisomers that are non-superimposable mirror images of each other. Tyrosine is available in two enantiomer forms known as L- and D- configuration and the enantiomers of tyrosine are given in figure 2.

difference between l-tyrosine and tyrosine

Figure 2: Enantiomers of tyrosine amino acid. L-form of tyrosine enantiomers, the COOH, NH2, H, and R groups are arranged around the asymetric C atom in a clockwise direction whereas D-form they are arranged in anticlockwise direction. The L- and D- forms of tyrosine are chiral molecules which can rotate the plane of polarized light into different directions such as L-forms and D-forms can rotate plane polarized light either to the left (l- form) or to the right (d- form).

L- tyrosine and D- tyrosine are enantiomers of each other and have identical physical characteristics, apart from the direction in which they rotate polarized light. However, the nomenclature of D and L is not common in amino acids including tyrosine. Also, they have non-superimposable mirror image relationship, and these mirror images can rotate the plane-polarized light in the similar degree but in different directions. The D and L-isomer of tyrosine which rotates the plane polarized light in the clockwise direction is called as dextrorotatory or d-lysine that enantiomer is labeled (+). On the other hand, the D and L-isomer of tyrosine which rotates the plane polarized light in the anticlockwise direction is called as laevorotatory or l-tyrosine that enantiomer is labeled (-). These, l- and d- forms of tyrosine are known as optical isomers (Figure 2).

l-tyrosine is the most available stable form of tyrosine and d- tyrosine is a synthetic form of tyrosine which can be synthesized from l-tyrosine by racemization. l- tyrosine plays a significant role in the human body in the synthesis of neurotransmitters, melamine, and hormones. Industrially, l-tyrosine is produced by a microbial fermentation process. It is mainly utilized in pharmaceutical and food industry as either a dietary supplement or food additive.

What is the difference between l-tyrosine and Tyrosine?

Tyrosine and l- tyrosine have identical physical properties, but they rotate plane polarized light in different direction. As a result, l- tyrosine 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,


l-tyrosine: l-forms of amino acids are considered as tasteless,

Tyrosine: d-forms tend to have a sweet taste.

Therefore, l- tyrosine may be less/no sweeter than tyrosine.


l-tyrosine: The l-forms of amino acid including l-tyrosine is the most abundant form in nature. As an example, nine of the nineteen L-amino acids commonly found in proteins are dextrorotatory, and the rest are levorotatory.

Tyrosine: The d-forms of amino acids observed experimentally were found to occur very rarely.


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Solomons, T. W. G. and Graig, B. F. (2004). Organic Chemistry (8thEd). Hoboken: John Wiley and Sons, Inc.
Webster, D. and Wildgoose, J. (2010). Review – Tyrosine supplementation for phenylketonuria. Cochrane Database Syst Rev. 4(8): 1507.