What is the Difference Between Lysine and Arginine?

Lysine and arginine are both positively charged amino acids that play important roles in proteins. They have some differences in their properties and functions:

1. Essentiality: Lysine is an essential amino acid, meaning it cannot be synthesized by the human body and is required through diet. In contrast, arginine is a non-essential amino acid, as it can be synthesized in the human body.
2. Codon Encoding: Lysine is encoded by codons AAA and AAG, while arginine is encoded by codons CGA, CGC, CGG, CGU, AGA, and AGG.
3. Side Chain Structure: Lysine has a lysyl side chain, while arginine has a side chain consisting of a 3-carbon aliphatic straight-chain ending in a guanidino group.
4. Stability: Arginine provides more stability to proteins due to its geometric structure, while lysine provides less stability.
5. pKa Value: The pKa value of the side chain of lysine is 10.54, while the pKa value of the side chain of arginine is 12.48.
6. Electrostatic Interactions: Lysine forms fewer electrostatic interactions in proteins and in one direction only, while arginine forms a larger number of electrostatic interactions in proteins in three possible directions.
7. Functions: Lysine plays a role in proteinogenesis, epigenetic histone modifications, and helps in the production of carnitine, crosslinking of collagen polypeptides, and uptake of arginine. Arginine is the precursor for the biosynthesis of nitric oxide and is involved in various roles in membrane proteins.

Comparative Table: Lysine vs Arginine

Lysine and arginine are both positively charged amino acids with some similarities and differences. Here is a table comparing their characteristics:

Lysine and arginine are both basic amino acids with high pKa values, making them positively charged and hydrophilic. They are both found on protein surfaces and participate in ionic interactions and hydrogen bonds in proteins. However, lysine is an essential amino acid that cannot be synthesized by the human body, while arginine is non-essential and can be synthesized by the human body. Lysine provides less stability to proteins due to its geometric structure, while arginine provides more stability. Additionally, arginine forms a larger number of electrostatic interactions in proteins compared to lysine.