What is the Difference Between Cysteine and Selenocysteine?

Cysteine and selenocysteine are two proteinogenic amino acids that are incorporated into proteins during translation. They have similar structures but different chemical properties due to the presence of sulfur in cysteine and selenium in selenocysteine. Here are the key differences between cysteine and selenocysteine:

• Element: Cysteine has sulfur in its structure, while selenocysteine has selenium in its structure.
• Genetic Codons: Cysteine is coded by the genetic codons UGU and UGC, while selenocysteine is coded by the genetic codon UGA.
• Reactivity: Selenocysteine is more reactive with halo acid derivatives than cysteine and reacts readily with iodoacetate even at pH values much below the pK of the selenohydryl group.
• Redox Potentials: Selenocysteine has an apparent half-wave potential of -0.212 V, compared to 0.021 V for cysteine.
• Stability and Solubility: Selenocysteine is unstable to acid hydrolysis and is more soluble in water than cysteine.

Both cysteine and selenocysteine are found in proteins across prokaryotes and eukaryotes and play essential roles in various biological processes. Selenocysteine is present in a variety of proteins and catalyzes the oxidation of thiols to disulfides and the reduction of disulfides to thiols. The use of selenium instead of sulfur in redox-related proteins can be used to tune electrode or redox potentials.

Comparative Table: Cysteine vs Selenocysteine

Cysteine and selenocysteine are both proteinogenic amino acids with similar structures, but they have different elements in their structures and are encoded by different genetic codons. Here is a table comparing the differences between cysteine and selenocysteine:

Cysteine has sulfur in its structure, while selenocysteine has selenium in its structure. This substitution of selenium for sulfur is the key difference between the two amino acids.