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.

00:02:41,040 --> 00:02:43,140 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.