4-The Importance of High Energy Bonds

H2
O2
H2O
Energy is released
A molecule has a larger amount of free energy if it contains weak covalent bonds instead of strong covalent bonds. When a strong bond forms, a large amount of free energy is released.
This is described by the formula: (A-B) + (C-D) → (A-D) + (C-B)
(A-B) + (C-D) → (A-D) + (C-B)
Keq = concA-D x concC-B concA-B x concC-D
where concA-B and concC-D are the concentrations of the reactants in moles per liter and concA-D and concC-B are the concentrations of the products in moles per liter.
Lock and key
In the lock and key model, the shape of the active site or binding sites fits the substrate. Another way to say this is that the binding site is complementary to the substrate.
induced fit
In the induced fit model the enzyme changes shape to fit the substrate.
Enzymes do not affect the equilibrium of a reaction, but they speed up the rate of the reaction. If a reaction is thermodynamically unfavorable, the presence of an enzyme will not cause it to occur.
Energies
Enzymes speed up the rate of the biochemical reactions by lowering the activation energies to amounts that can be supplied by the heat of motion. Enzymes are absolutely essential for life.
The energy required to break the old covalent bonds in a chemical reaction is called the activation energy. The activation energy is usually less than the energy of the original covalent bond because the atoms do not completely come apart.
We know that: ∆G = -RT lnKeq or Keq = e -∆G/RT
The energies of activation are generally 20 to 30 kcal/mol, so activation states only rarely occur at normal physiological temperatures. The high activation energies block spontaneous rearrangements of covalent bonds in cells.
C. Free Energy in Biomolecules
All biochemical reactions must have a decrease in free energy. This is clearly the case for degradation pathways in which food molecules are converted into carbon dioxide (CO2) and water (H20). Heat is released and some energy is used to produce ATP.
Generally a collision between 2 reacting molecules temporarily forms a molecular complex called an activated state. In the activated state the molecules are close to each other and this destabilizes their bonds so that less energy is required to break a bond.
Not all of the energy is converted into ATP. Some of the energy is lost as heat and entropy. In cells about 40% of the energy is converted into ATP and the rest is lost as heat or entropy.
For each chemical bond formation, we should consider, does the formation of this bond result in a gain or loss of free energy? Formation of a chemical bond is thermodynamically favored when there is a negative ΔG. This is true even when enzymes are involved in the formation of the chemical bond.
Enzyme + Substrate ↔ [ES] ↔Enzyme + Product
Most enzymes have an active site in which they bind the substrate. The substrate is changed into the product and then it is released, allowing the enzyme to bind another substrate and repeat the process.
For example, the bond between oxygen and hydrogen is much stronger than the bond between 2 hydrogen atoms or between 2 oxygen atoms. This means energy will be released when oxygen and hydrogen are mixed together and water is formed.
It is equally important for there to be a mechanism for lowering the activation energy so that these reactions can occur in living systems.
B. Enzymes Lower Activation
A. Molecules that Donate Energy
Specific molecules will differ in the amount of free energy they have. This is because different kinds of covalent bonds have different bond energies.
Lysozyme site
Enzyme + Substrate ↔ [ES]↔Enzyme + Product
Enzyme-substrate complex
Enzyme-substrate complex
active site
Enzyme
Product released
bond breaks
It is important for life that these reactions do not occur spontaneously. If they did, all atoms would move to the state of the smallest possible energy. This would mean living organisms could not store energy.
The Importance of High Energy Bonds
Chapter 4
A. Molecules that Donate Energy B. Enzymes Lower Activation Energies C. Free Energy in Biomolecules D. High Energy Bonds Drive Biosynthesis E. Activation of Precursors in Group Transfer Reactions
O=C=O
For a chemical reaction to occur, a covalent bond must be broken. This requires energy. Once the atoms are partially apart, they can bind to new atoms forming stronger bonds. Only a small number of reactions will occur at physiological temperatures in the absence of a catalyst.
Degradative pathways produce small molecules that can be used to produce larger molecules. They also convert some of the free energy in the food molecules into ATP, which can be used in the cell for work.
+
Cellular Respiration
+ +
glucose
The ring structure folds as shown.
The CO2 produced by the oxidation of glucose is not a good food molecule because of the strong covalent double bonds between the carbon and oxygen. Carbon dioxide cannot be converted into more complex organic molecules without the addition of energy. Plants do this with the energy provided by sunlight.
For example, glucose is an excellent food molecule because it contains a large decrease in free energy when it is oxidized by oxygen to produce CO2 and water.