So today, we're going to talk about enzymes
and all the different kinds of reactions
that enzymes can catalyze.
But before we do that, let's review the idea
that enzymes make biochemical reactions go faster.
And if you look at a reaction coordinate diagram,
you'd notice that enzymes speed up reactions
by lowering their activation energy.
Now, enzymes are generally named for their reactions, which
is convenient because it makes it a lot easier to remember
what an enzyme does if someone gives you its name.
And a great example of this is that one
of the enzymes involved in DNA replication
is called DNA polymerase, which is named as such because it
acts on DNA and specifically makes polymers of DNA.
Now, the suffix "ase" is usually just one
that you find at the end of most enzyme names.
Now, another great example is that the enzyme that
catalyzes the first step of glycolysis, which you may
remember is the reaction between glucose and ATP
to form glucose-6-phosphate and ADP, is called hexokinase.
And "hexo" refers to the number 6,
which is a reference to glucose being a six-carbon sugar.
And "kinase" is a term referring to enzymes
that add phosphate functional groups to different substrates.
So overall, hexokinase adds phosphates
to six-carbon sugars like glucose.
Now generally, every enzyme has a very specific name
that gives insight into the specific reaction
that that enzyme can catalyze.
So we can actually divide most enzymes
into six different categories based off
the kinds of reactions that they catalyze.
Now, our first group is the transferase group.
And the basic reaction that transferases catalyze
are ones where you move some functional group, X,
from molecule B to molecule A. And a great example
of one of these reactions occurs during protein translation,
where amino acids bound to tRNA molecules
are transferred over to the growing polypeptide chain.
So in this case, A refers to our amino acid chain,
B refers to our tRNA, and X refers to this lysine residue,
which is being transferred from B to A.
And this reaction in particular is
catalyzed by an enzyme called peptidyl transferase, which
is an appropriate name since it is a transferase involved
in making peptides.
Next we have the ligase group, which
catalyzes reactions between two molecules,
A and B, that are combining to form a complex between the two,
or AB.
And an example of a reaction using a ligase
that you might be familiar with occurs during DNA replication,
where two strands of DNA are being joined together.
So in this reaction, A and B represent the two separated DNA
polymers, which are being joined to form a single strand.
And this reaction in particular is
catalyzed by an enzyme called DNA
ligase, which is named since it's
a ligase that works on DNA strands.
Now our third group as the oxidoreductase group,
which is a little different from the others
since it actually includes two different types of reactions.
And these reactions involve transferring electrons
from either molecule B to molecule A
or from molecule A to molecule B. Now,
we say that an oxidase is directly involved
in oxidizing or taking electrons away from a molecule,
while a reductase is involved in reducing or giving electrons
to a molecule.
And we call these enzymes oxidoreductases together
because they can usually catalyze
both the forward and reverse reactions, which
is why I used equilibrium arrows here instead of just
a normal single-headed arrow.
Now a great example of an oxidation reduction reaction
occurs during lactic acid fermentation, where electrons
are either passed from NADH to pyruvate or from lactic acid
to NAD.
Now, this reaction is catalyzed by an enzyme
called lactate dehydrogenase.
Remember that the word "dehydrogenase"
refers to the removal of a hydride functional group.
And that's the same as saying the removal of electrons,
since hydrides are basically just hydrogen
atoms with two electrons on them instead of just one.
Now, this enzyme is given its name
since it's able to remove a hydride, or remove electrons,
from a molecule of lactic acid.
Next, we have the isomerase group.
And enzymes in this group are typically
involved in reactions where a molecule, like molecule A,
is being converted to one of its isomers.
And an example of this type of a reaction
is the conversion of glucose-6-phostate
to fructose-6-phosphate, which is
one of the steps of glycolysis that you may remember.
Now, this reaction is catalyzed by an enzyme called
phosphoglucose isomerase, which is appropriately
named since it creates isomers of glucose molecules
that are phosphorylated.
Now, our next category is the hydrolase category.
And hydrolases use water to cleave
a molecule, like molecule A, into two other molecules, B
and C. And a great example of one of these reactions
is the hydrolysis reaction that can occur to peptide bonds.
And if we have this lysine-alanine dipeptide here,
it could be reacted with water to form
two individual amino acids that are no longer bound.
And this particular hydrolysis reaction
can be catalyzed by a class of enzymes
that we call serine hydrolases, which
some people call serine proteases.
And they are named this way because they are hydrolases
that use a serine residue as the key catalytic amino acid that
is responsible for breaking the peptide bond.
Now, our last category is a little more complicated
than the others.
And it's the lyase group.
Now, lyases catalyze the dissociation of a molecule,
like molecule A, into molecule B and C,
without using water like hydrolases would,
and without using oxidation or reduction
like an oxidoreductase would.
And one example of a reaction catalyzed by a lyase
is the cleavage of argininosuccinate
into arginine and succinate.
And this reaction takes place during the urea cycle,
which you also might be familiar with.
Now this specific reaction is catalyzed by an enzyme called
argininosuccinate lyase, which is appropriately
named because it is a lyase that catalyzes
the breakdown of an argininosuccinate molecule.
Now, it's important to recognize that since lyases don't
use water or oxidation to break a bond,
they need to generate either a double bond between two atoms
or a ring structure in a molecule in order to work.
So what did we learn?
Well, first we learned that enzymes are sometimes
named for their reactions.
And next we learned about the six different types of enzymes.
We have transferases, which transfer functional groups
from one molecule to another; ligases, which ligate or join
two molecules together; oxidoreductases,
which move electrons between molecules;
isomerases, which convert a molecule
from one isomer to another; hydrolases,
which break bonds using water; and lyases, which break bonds
without using water and without using oxidation.