Explanation:

Amino acids are the building blocks of proteins. There are 20 different types of amino acids that can be found in proteins. Amino acids are characterized by the presence of an amino group (-NH2) and a carboxyl group (-COOH) attached to the same carbon atom, called the alpha carbon. The remaining group attached to the alpha carbon is called the R-group, which can vary depending on the amino acid.

Types of amino acids:

There are two types of amino acids, L-amino acids and D-amino acids, based on the orientation of the amino group relative to the carboxyl group.

- L-amino acids: In L-amino acids, the amino group is on the left side of the alpha carbon when viewed in a Fischer projection. The R-group can vary among L-amino acids.
- D-amino acids: In D-amino acids, the amino group is on the right side of the alpha carbon when viewed in a Fischer projection. D-amino acids are less common in nature and are typically found in small peptides and bacterial cell walls.

Naturally occurring amino acids in proteins:

All of the amino acids found in proteins are L-amino acids. This is because the enzymes responsible for protein synthesis in cells are highly specific and can only incorporate L-amino acids into the growing peptide chain. In addition, the L-amino acids found in proteins are almost exclusively in the L-configuration, with the exception of glycine, which has a hydrogen atom as its R-group.

Conclusion:

In conclusion, the naturally occurring form of amino acid in proteins is L-amino acids only. D-amino acids are less common in nature and are typically found in small peptides and bacterial cell walls.