United Strength of Biochemical Structures

The forces that hold biomolecules together in three dimensions are small, on the order of a few kJ/mole, and much weaker than a covalent bond (formed through sharing of electrons between two atoms), which has an energy of formation a hundred times larger. Would life be possible if these molecules were held together only by covalent bonds? Probably not. For example, muscle contraction involves movement of the protein myosin relative to a filament composed of another protein, actin. This movement does not involve the breakage or formation of covalent bonds in the protein. A single contraction cycle requires about 60 kJ/mole; which is about 3% to −5% of the energy captured during the complete combustion of a mole of glucose. If the energy required for contraction were the same as that of forming a carbon‐ carbon covalent bond, almost all the energy of combustion of a molecule of glucose would be required for a single contraction. This would place a much higher demand for energy on the cell, which would require a similarly high demand for food on an organism.

The forces that hold biomolecules together in three dimensions are small, on the order of a few kJ/mole, and much weaker than a covalent bond (formed through sharing of electrons between two atoms), which has an energy of formation a hundred times larger. Would life be possible if these molecules were held together only by covalent bonds? Probably not. For example, muscle contraction involves movement of the protein myosin relative to a filament composed of another protein, actin. This movement does not involve the breakage or formation of covalent bonds in the protein. A single contraction cycle requires about 60 kJ/mole; which is about 3% to −5% of the energy captured during the complete combustion of a mole of glucose. If the energy required for contraction were the same as that of forming a carbon- carbon covalent bond, almost all the energy of combustion of a molecule of glucose would be required for a single contraction. This would place a much higher demand for energy on the cell, which would require a similarly high demand for food on an organism.

If the forces holding them together are so small, how can biomolecules have any sort of stable structure? Because these small forces are summed over the entire molecule. For example, consider a double-stranded DNA a thousand base pairs long. The energy of an average base pair, about 0.5 kJ/mole, is not great, but the energy of 1,000 base pairs equals 500 kJ/mole, equivalent to the energy of several covalent bonds. This also has important consequences for the dynamics of individual base pairs: They can be opened easily while the molecule as a whole is held together.