Black_Chemist

 Amino acids are the building blocks of proteins. An α-amino acid
consists of a central carbon atom, called the α carbon, linked to an amino group, a carboxylic acid group, a hydrogen atom, and a
distinctive R group. The R group is often referred to as the side chain.
With four different groups connected to the tetrahedral α-carbon atom, α-amino acids are chiral; the two
mirror-image forms are called the L isomer and the D isomer.

Notation
for distinguishing stereoisomers—

The four different substituents of an asymmetric
carbon atom are assigned a priority according to atomic number. The
lowest-priority substituent, often hydrogen, is pointed away from the viewer.
The configuration about the carbon is called S, from the Latin sinis-ter
for “left,” if the progression from the highest to the lowest priority is
counterclockwise. The configuration is called R, from the Latin rectus
for “right,” if the progression is clockwise.

Only amino acids are constituents of proteins. For almost all amino
acids, the L isomer has S (rather than R) absolute configuration. Although considerable effort has gone into understanding why amino  acids in proteins have this absolute
configuration, no satisfactory explanation has been arrived at. It seems
plausible that the selection of  L over D
was arbitrary but, once made, was fixed early in evolutionary history.

Reference: Biochemistry Berg et al