Except for cysteine, only S amino acids occur in proteins....

Except for cysteine, only $S$ amino acids occur in proteins. Several $R$ amino acids are also found in nature, however. $(R)$ -Serine is found in earthworms, and (R)-alanine is found in insect larvae. Draw Fischer projections of ( $R$ )-serine and (R)-alanine. Are these D or L amino acids?

Except for cysteine, only $S$ amino acids occur in proteins. Several $R$ amino acids are also found in nature, however. $(R)$ -Serine is found in earthworms, and
(R)-alanine is found in insect larvae. Draw Fischer projections of ( $R$ )-serine and
(R)-alanine. Are these D or L amino acids?

Key Concepts

Fischer Projection

A Fischer projection is a two-dimensional representation used to depict the stereochemistry of molecules with one or more chiral centers, such as amino acids. In these diagrams, vertical lines represent bonds going into the plane of the paper and horizontal lines represent bonds coming out. This projection simplifies the visualization of the spatial arrangement of substituents, making it a standard tool for comparing configurations, particularly in biochemistry.

Chirality and Stereoisomerism

Chirality refers to the geometric property of a molecule that makes it non-superimposable on its mirror image, due to the presence of a stereocenter, typically a carbon atom with four different substituents. Stereoisomerism is the broader concept encompassing molecules that have the same structural formula but differ in the spatial arrangement of atoms. In biological systems, the chirality of amino acids is crucial since the function and recognition by enzymes are highly stereospecific.

R/S Configuration

The R/S configuration system is a method used to assign absolute configurations to chiral centers following the Cahn–Ingold–Prelog priority rules. Each substituent attached to the chiral carbon is ranked in order of decreasing atomic number, and the molecule is oriented so that the lowest priority group is directed away. The configuration is then determined by the order of the remaining groups; a clockwise arrangement is assigned the R (rectus) configuration, while a counterclockwise arrangement is assigned the S (sinister) configuration.

D/L Configuration

The D/L system for amino acids is based on the configuration of the chiral carbon relative to a reference molecule, glyceraldehyde. In Fischer projections, if the amino group is on the right side, the amino acid is designated as D, and if it is on the left, it is designated as L. This nomenclature is distinct from the R/S system and is traditionally used to describe the stereochemistry of sugars and amino acids in biological contexts, with most naturally occurring ?-amino acids in proteins being in the L form.

Transcript

Please give Ace some feedback