Protein synthesis is an essential life process that builds the important large amino acid macromolecules that function as enzymes, antibodies, and cellular structural components. Although synthesis begins with the transcription of DNA into RNA, this pathway depicts the reactions that occur during translation. Transcribed messenger RNA (mRNA), which contains the genetic code to direct protein synthesis, is transported out of the nucleus and becomes bound to ribosomes in the cytoplasm or endoplasmic reticulum. The amino acids required to assemble polypeptide chains are delivered to the ribosomes using transfer RNA (tRNA). Each tRNA molecule has both a binding site for a specific amino acid and a three-nucleotide sequence called the anticodon that forms three complementary base pairs with an mRNA codon. Charging or loading the appropriate amino acid onto its tRNA is carried out by an aminoacyl-tRNA synthetase (aaRS or ARS), also called tRNA-ligase. This enzyme catalyzes the esterification of an amino acid to one of all its compatible tRNAs to form an aminoacyl-tRNA. Each of the twenty amino acids has a corresponding aa-tRNA made by a specific aminoacyl-tRNA synthetase. Ribosomes match the anticodons of the charged tRNA molecules with successive codons of the mRNA. After a match is found, the ribosome transfers the amino acid from the matching tRNA onto the growing peptide chain via a reaction termed peptide condensation, and the tRNAs, no longer carrying amino acids, are released.

Protein Synthesis: Threonine References