Enzymes and Activation Energy part 2

Exergonic Reactions and Spontaneity

Exergonic reactions release free energy and are energetically favorable, described as spontaneous reactions.
Spontaneity doesn't imply instantaneousness; reactions can be slow.
Example: Removing arginine from 600 polypeptides, where half take 7 years, illustrates slow spontaneous reaction.
Even exergonic reactions need an initial energy input to start, known as activation energy.

Even though the products of a exergonic reaction have less free energy than reactants, there is an energy barrier to overcome for the reaction to proceed.
Activation Energy (E_a): The energy required to contort reactant molecules so bonds can break. Alternatively, the energy necessary to break the bonds of the reactants.
The height of the barrier corresponds to the difficulty of breaking bonds.
Body temperature alone isn't enough to provide the kinetic energy for reactions to occur spontaneously.
Adding heat isn't a solution, as it speeds up all reactions, including destructive ones like protein denaturation.

To facilitate reactions, a different approach is needed to selectively break bonds and form new ones.

Enzymes lower the activation energy of a reaction without changing the overall free energy change (\Delta G).
Enzymes remain unchanged, allowing them to catalyze multiple reactions sequentially.

Substrates: Specific reactants for an enzyme.
Enzyme specificity is often compared to a key fitting a lock; enzymes catalyze one specific reaction.
Enzymes exhibit shape, size, and charge specificity for their substrates.

The lock-and-key model describes the shape specificity between an enzyme and its substrate. After binding, the enzyme undergoes a conformational change which stresses the bonds needing to be broken.
The induced fit favors bond breakage and formation of new bonds during transformation of reactants to products.

Enzymes bring specific reactants together in the proper orientation in their active site, where catalysis happens.
The active site positions reactants to favor the reaction, stabilizing the high-energy transition state.
Components of the active site participate in the reaction, favoring bond breakage and formation.
Active sites have specific sizes, shapes, and charge distributions, ensuring high specificity for substrates.

A slight change in an active site, such as replacing one amino acid with a similar one, can significantly impact enzyme activity.
The amino acids lining the active site pocket are crucial for the reaction's progress because enzymes are proteins.
Active sites contain precisely positioned atoms that alter electron distribution, stressing substrates and stabilizing the transition state.
Enzymes can donate or accept electrons during the reaction, returning to their original state after the process.

Enzymes sometimes have coenzymes (organic) or cofactors (e.g., metal ions) that participate in the catalytic function.
These non-protein molecules can be permanently stationed or temporarily bind in the active site.
Many vitamins act as coenzymes, aiding enzymes in catalyzing reactions.

Enzymes orient substrates precisely, rearrange electrons, and strain substrates to lower activation energy.
These activities—redistribution of electrons, precise orientation, and substrate straining—enable enzymes to facilitate reactions.