Naturally occurring proteins consist of L-amino acids. Let's understand what L-amino acids are and why they are present in proteins.

L-Amino Acids:

- Amino acids are the building blocks of proteins.
- They have a central carbon atom called alpha carbon, which is bonded to a hydrogen atom, an amino group (-NH2), a carboxyl group (-COOH), and a side chain (R-group).
- The R-group varies in different amino acids, giving them unique properties.
- When two amino acids are joined together through a peptide bond, the carboxyl group of one amino acid reacts with the amino group of the other amino acid, releasing a water molecule and forming a peptide bond.
- The resulting chain of amino acids is called a polypeptide chain, which folds into a three-dimensional structure to form a protein.

L-Amino Acids in Proteins:

- In naturally occurring proteins, only L-amino acids are present.
- L-amino acids have the amino group (-NH2) on the left side and the carboxyl group (-COOH) on the right side of the alpha carbon.
- This arrangement is called the L-configuration.
- The reason for the presence of L-amino acids in proteins is due to the origin of life on Earth.
- Life on Earth originated from the homochiral selection of L-amino acids and D-sugars.
- Chiral molecules have mirror-image isomers, which are non-superimposable.
- In the early stages of life, only L-amino acids were selected because they were more stable and had a lower energy state.
- Therefore, only L-amino acids were used to form proteins, and this homochirality has been conserved throughout evolution.

In conclusion, naturally occurring proteins consist of L-amino acids due to the homochiral selection of L-amino acids in the origin of life on Earth.