Protein synthesis Energy cost

In addition to the direct energy cost for the synthesis of a protein, there are indirect energy costs—those required for the cell to make the necessary enzymes for protein synthesis. Compare the magnitude of the indirect costs to a eukaryotic cell of the biosynthesis of linear (al—>4) glycogen chains and the biosynthesis of polypeptides, in terms of the enzymatic machinery involved. 

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. 

Reflect and Apply What is the energy cost per amino acid in prokaryotic protein synthesis Relate this to low entropy. 

Although osmo-conformation allows the clam to survive in highly saline environments, the processes of protein synthesis and repair are energy-intensive, and are estimated to cost 20%-25% of total energy expenditure under nonstressful conditions. The energy used to accumulate amino acids in response to environmental elevated salinities is not available for other survival and reproductive purposes, and probably makes the clam susceptible to succumb to other stresses. 

There is another way to compare the two probabilities. This is to compare the energy required to combine amino acids into a protein with that recovered when the protein of 99 peptide bonds is hydrolyzed to return to amino acid residues. In short, 1,618 ATP equivalents were required to form 99 peptide bonds of the 100 residue protein, which is just over 16 ATPs per peptide bond. This is a free energy cost of 8 x 16 = 128 kcal/mole. As hydrolysis of a peptide bond yields no more than 3 kcal/mole, the efficiency of energy conversion would be about 2%. Protein synthesis without considering production of the required DNA and RNA, but only the positioning of a single amino acid in the correct position, could properly be called extravagant as well. 

Other estimates of the energy cost of protein synthesis lie within the range 12-17 mmol O2 g protein (Jobling 1985) Jobling has calculated for the data of Hogendoorn (1983) that the energy cost of protein deposition was 31 mmol G2 g protein in the African catfish Clarias. Recent estimates from simultaneous measurements of oxygen consumption and protein synthesis of trout hepatocytes have... 

Table 4. Aerobic energy costs of protein synthesis from a variety of sources ...

The assumptions used for the minimal theoretical cost have been slightly variable and discussed frequently (e.g. Reeds et al. 1985 Aoyagi et al. 1988). From the stoichiometry of peptide bond formation it is difficult to estimate what other energy demanding processes may be associated with protein synthesis (Aoyagi et al. 1988). 

Estimates of the energy cost of protein synthesis by these correlation methods, as demonstrated here, give higher values than those based upon the minimal stoichiometry of peptide bond formation (Table 4 Waterlow 1984 Aoyagi et al. 

Another approach to estimating energy costs of protein synthesis is to inhibit synthesis and to measure the decline in synthesis and oxygen consumption. In chickens, heat production declined by 37% while protein synthesis declined by 87% of control values when cycloheximide was used to inhibit protein synthesis (Aoyagi et al. 1988). Using cycloheximide to inhibit protein synthesis in trout hepatocytes resulted in a 75% decline in oxygen consumption and a 87% decline in protein synthesis (Pannevis and Houlihan 1990). 

Data from Mytilus using minimal theoretical costs suggest that protein synthesis accounts for approximately 20% of the heat loss at energy equilibrium (Hawkins et al. 1986,1989). These values are very similar to the proportions of the metabolic rate accounted for by protein synthesis using the same assumptions in a variety of mammals (e.g. Reeds et al. 1985). 

In Summerfelt RC, Hall GE (eds) Age and growth of fish. Iowa State Univ Press, Ames Aoyagi Y, Tasaki I, Okumura J-I, Muramatsu T (1988) Energy cost of whole-body protein synthesis measured in vivo in chicks. Comp Biochem Physiol 91A 765-768 Ashford AJ, Pain VM (1986) Effects of diabetes on the rates of synthesis and degradation of ribosomes in rat muscle and liver in vivo. J Biol Chem 261 4059-4065 Bates PC, Millward DJ (1981) Characteristics of skeletal muscle growth and protein turnover in a fast-growing rat strain. Br J Nutr 46 7-13... 

Protein synthesis requires activation of amino acids, initiation of chain formation, chain elongation and termination, all of which have an energy cost. 

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