Rates and Energetics of Translation

At 37C, an E. coli ribosome can synthesize a 300-residue polypeptide chain in about 20 seconds. This rate is almost exactly the same as that calculated for prokaryotic transcription. mRNA can be translated as fast as it is transcribed because it is possible for many ribosomes to simultaneously translate a single message. Such a complex is called a polyribosome. Under some conditions, as many as 50 ribosomes may be packed onto a single mRNA.

Given this high rate of synthesis and the fact that E. coli contains about 15,000 ribosomes, each cell can synthesize about 750 protein molecules of 300 residues in length per second, at full capacity.

For a polypeptide of N residues, a minimum of 4N high energy phosphates (such as ATP or GTP) must be hydrolyzed. This corresponds to about 50,000 kJ/mol for a typical protein of 300 residues. Per peptide bond, therefore, it costs the cell about 160 kJ of free energy. Note, however, that the free energy change required for peptide bond synthesis in dilute aqueous solution is about 20 kJ/mol. The difference between cellular peptide bond synthesis (160 kJ/mol) and peptide bond synthesis in dilute solutions (20 kJ/mol) is that cellular peptide bond synthesis is not random. That is, making a defined sequence of amino acids comes with an energy cost.

See also: Translation, Translation Overview