Protein Translation




Protein translation is the process of synthesizing proteins from amino acids. This series of reactions translates the code provided to messenger ribonucleic acid or RNA (mRNA) by deoxyribonucleic acid or DNA into a sequence of amino acids that makes up the active protein molecule. Protein synthesis begins with a strand of mRNA synthesized in response to the genetic code located in a gene on a strand of DNA. The process of translation is slightly different in eukaryotic cells from that in prokaryotic cells; for the sake of simplicity, translation in prokaryotes will be discussed here.

Proteins are made up of combinations of the twenty common amino acids. Placement of each amino acid is specified by a three-nucleotide codon. Four different nucleotides are available to code; taken three at a time, they can form sixty-four combinations. Some amino acids have more than one codon (a phenomenon known as redundancy). Translation also requires a start codon and stop codon.

Four general actions comprise protein translation: The mRNA binds to a ribosome ; amino acids are carried to the ribosome by transfer RNA (tRNA); the amino acids are joined, forming peptide bonds; and the completed protein molecule is released from the ribosome. The synthetic processes are called initiation, elongation, and termination and are assisted by protein factors essential for each step.

Adenosine triphosphate (ATP) provides energy for the attachment of amino acids by specific enzymes to tRNA. For reference, the ends of DNA and RNA strands are designated 5′ and 3′. Translation begins with attachment of the 5′-end of mRNA to the small unit of a ribosome. A tRNA molecule carrying formylmethionine attaches to the ribosome at a location matching the initiation codon (AUG) on mRNA. The large subunit of the ribosome then attaches to this complex, forming a complete, active ribosome. During periods of rapid translation, a number of ribosomes may cluster together with mRNA, forming polyribosomes. A second tRNA brings another amino acid to the ribosome, matching the codon on mRNA for the second amino acid. A peptide bond forms between the two amino acids, and the first tRNA is released.

Guanosine triphosphate (GTP) provides energy to move the growing peptide chain along as mRNA passes through the ribosome. As more amino acids are added, the polypeptide chain grows longer until the ribosome reaches the stop codon of mRNA.

When the growing peptide chain approaches the carboxylic acid end (at the stop codon on mRNA), protein termination factors help to disassemble the ribosome and release the completed protein chain. Protein translation is interrupted by antibiotics such as tetracycline or chloramphenicol that interfere with protein synthesis.

SEE ALSO Codon ; Proteins ; Protein Synthesis ; Ribonucleic Acid .

Dan M. Sullivan

Bibliography

Devlin, Thomas M., ed. (2002). Textbook of Biochemistry: With Clinical Correlations, 5th edition. New York: Wiley-Liss.

McKee, Trudy, and McKee, James R. (2003). Biochemistry: The Molecular Basis of Life, 3rd edition. Boston: McGraw-Hill.



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