The synthesis of a polypeptide chain in E. coli requires energy in the form of adenosine triphosphate (ATP) equivalents. The energetic cost can be estimated by considering the different steps involved in protein synthesis, including amino acid activation, mRNA translation, and peptide bond formation. Here's a breakdown of the estimated ATP equivalents required for each step:
- Amino acid activation: Before incorporating into the growing polypeptide chain, each amino acid needs to be activated by attaching an ATP molecule. The process of amino acid activation consumes one ATP equivalent per amino acid.
For a polypeptide chain of 100 amino acids, this step would require 100 ATP equivalents.
- mRNA translation: The translation process involves the movement of ribosomes along the mRNA template, which requires energy. Each amino acid added to the growing polypeptide chain consumes four high-energy phosphate bonds (equivalent to four ATP molecules) for peptide bond formation, translocation, and ribosome recycling.
Since there are 99 peptide bonds formed in a 100-residue polypeptide chain, this step would require 99 x 4 = 396 ATP equivalents.
- Additional ATP equivalents: In addition to the steps mentioned above, there are other ATP equivalents required for various cellular processes involved in protein synthesis, such as tRNA charging and protein elongation factors.
The exact number of ATP equivalents required for these additional processes can vary, but for estimation purposes, an additional 50 ATP equivalents can be considered.
Therefore, the total estimated energetic cost in ATP equivalents for the synthesis of a polypeptide chain of 100 residues in E. coli would be:
100 ATP equivalents (amino acid activation) + 396 ATP equivalents (mRNA translation) + 50 ATP equivalents (additional processes) = 546 ATP equivalents.
It's important to note that this estimation provides a rough approximation, and the actual energy expenditure during protein synthesis can vary depending on various factors, including the specific sequence and context of the polypeptide chain, the efficiency of translation machinery, and the cellular conditions.