Every single second inside every living cell,
thousands of chemical reactions are taking place.
These reactions are
performed by enzymes. An enzyme is a
protein that catalyzes a chemical reaction.
It initiates the reaction,
speeds up the reactions progress, and
makes sure the outcome is always the same.
These enzymes often work together to
form longer pathways, such as the
Citric Acid Cycle, which is a series of
chemical reactions used by cells to generate
energy from carbohydrates.
The essential tasks of life such as metabolism,
protein synthesis, and cell renewal and growth
are all regulated by enzymes.
The life-sustaining power of enzymes lies in the fact
that they catalyze reactions in
mild conditions of pH, temperature,
and atmospheric pressure.
The rates of catalyzed reactions are millions to
trillions times faster than those of the
same reactions uncatalyzed.
To speed up a reaction in the absence of enzymes,
additional energy would need to be
provided as heat, which is jostles the
substrates and occasionally provides
enough energy to trigger a reaction.
In the course of most reactions,
an unstable and highly energetic
transition state is formed as the
substrates are transformed into products.
An enzyme acts as a template for
the reaction, binding to its substrate and
holding it in the proper position to
form the product. An enzyme also
surrounds the substrate with reactive groups
that stabilize the transition state, making
it easier for the reaction to occur.
To understand how enzymes work, let's take a
closer look at a reaction in the
Citric Acid Cycle that is catalyzed by the
enzyme aconitase. Aconitase binds to
its substrate citrate and removes a
hydroxyl group and a hydrogen atom to
form intermediate cis-aconitate.
It then adds the hydrogen in the hydroxyl back
in slightly different positions to form
the product isocitrate.
In the active site, some amino acids are
perfectly positioned to recognize the
substrate and hold it in the optimal
position for the reaction to begin.
Some amino acids are involved in
recognizing and holding the substrate.
Other amino acids are directly involved in catalysis.
Histidine 101 acts as an acid by donating its proton.
Thanks to the chemical environment of Serine 642,
it can act as a base by accepting the
proton from the substrate.
The active site of aconitase also contains
an iron-sulfur cluster that stabilizes the
substrate electrostatically and helps to
position it relative to the catalytic residues.
The first step in the reaction is dehydration.
In this step, histidine acts as an acid
and protonates the hydroxyl
on the substrate allowing it to
leave as a water molecule.
Serine then acts as a base by extracting
a hydrogen atom from the opposite side
of the substrate forming the intermediate cis-aconitate.
Cis-aconitate then flips
upside down, and the complementary
hydration reaction is performed.
In this step, histidine grabs a hydrogen atom
from a passing water molecule,
placing the resulting hydroxyl group
back onto the substrate. Serine then returns its
hydrogen atom and the final product
isocitrate is released.
Notice that the enzyme itself was not changed by the reaction.
It extracted the hydroxyl group
and a hydrogen atom and then put them back,
starting and ending in the same state.
This is the hallmark of a catalyst,
when it finishes a reaction, it is ready
for the next, so it can perform thousands
of reactions in a row.
Notice also that the shape of an active site is often flexible.
Many enzymes surround their
substrates, closing around them to form
the perfect environment for a reaction.
Enzymes are fundamental to life on Earth,
working every second of every day to
maintain life processes in every cell,
and in every living creature on the planet.