>>Dr. Ketchum: Before we get into any specifics on enzymes, what I’d like to do is review
how chemical reactions actually occur. And I call this a review because you probably
learned this material in a prerequisite course, such as Chemistry or Introductory Biology.
And so first off, when we think about well, how do reactions occur? Let’s go ahead first
and look at this image here. And in this image you have two atoms, and these two different
atoms will eventually have to collide, right? And you also have electrons in red and I’ll
just put an “e” there for the electrons, right? And they’re orbiting around the nucleus.
The nucleus, if you remember, is in the center of the atom. Okay, so the electrons orbit
around the nucleus of an atom whose radius is determined by the number of positive protons
in the nucleus and the energy level of the electrons. In order for two or more atoms
to come together, in order for them to share a strong chemical bond, the atoms have to
share or transfer electrons. This means electrons in each atom have to be an orbital shell that
is far enough away from the nucleus to be able to be influenced by the other atom.
For example: In order for a covalent bond to occur, one or more negatively charged electrons
are pulled into the space between the positively charged nuclei and are attracted to both.
This sharing of electrons holds the two atoms together. So this takes you back to the concept
of ionic bonds and covalent bonds. Now in order for a chemical reaction to proceed,
the following must occur: So the reactants have to collide. So these two atoms have to
collide. In order for them to collide, they have to have kinetic energy. So recall what
kinetic energy is. Right? So not only do they have to be brought together, so in other words
not only do they have to collide, the binding sites have to collide in exactly the right
orientation relative to each other, to ensure that a reaction can occur. So what we’re
getting at here is specificity, okay? If the reaction sites do not collide, they cannot
form a bond by sharing electrons. And also, some energy must be invested to break or form
bonds within the reactants so that the reaction can proceed. Occasionally, the interaction
of your reactant molecules can spontaneously cause a reaction. Notice I said “occasionally.”
However, most reactions require additional energy to proceed. The amount of energy needed
to initiate a chemical reaction is called “activation energy.” So the activation
energy can be illustrated by a graph. And so let’s go ahead and write out what activation
energy is, and then we’ll further define it for you.
So activation energy: This is the amount of energy that is needed for a chemical reaction
to occur. So what we’re going to do is take a closer look at this and then we’re going
to think about this in terms of enzymes—we’ll relate it back to enzymes. So like I said
before, occasionally the interaction of those reactant molecules or their collisions, right,
can spontaneously cause a reaction. But most of the time that’s not true, and they actually
need energy to proceed. Okay, so here’s a typical energy graph. So let me define what
parts of this graph are and then I’ll ask you a question. So first off when you look
at the y-axis, this is the energy state of the reaction…so another way to put that
is that’s the potential energy of the reactants. So here on the graph we have an “r.” So
at this point, and I’ll circle it in green, all right, so that’s the energy state of
the reactants or the potential energy of your reactants. Now “p,” I’ll circle “p”
for you, this is the energy state, okay, or the potential energy of your products. So
here’s something to notice. Notice that the potential energy of those products is
much less than the energy state of the reactants. What this illustrates to you is an exergonic
reactant, because here energy is actually being released. So it’s an exergonic reaction.
Okay. Now the other thing on this graph that needs to be explained to you is Ea. Ea is
another way of saying activation energy or energy of activation. So between this top
line here and this line down here, the potential energy of the reactants, in between that—that’s
the energy of activation. That’s the amount of energy that’s needed for a chemical reaction
to actually occur. We know that enzymes actually lower the activation energy, and so by lowering
the activation energy, reactions can actually occur much quicker.
So how do we add this energy? Where does this energy come from for these reactions to occur?
So we can add heat, that’s one way. If you add heat, that’s going to increase your
kinetic energy. And by increasing kinetic energy, now your atoms are moving at a much
faster rate and if they’re moving faster, the chances of them colliding increases. The
other thing is that with heat, the electrons are going to expand their orbitals, and they
move away from the nucleus; so they’re less stable. Now they’re more likely to participate
in a reaction. So here we have not only heat increasing kinetic energy, heat is also going
to cause orbital expansion. And once again with orbital expansion, your reactants are
more likely to participate in that reaction. With this, if you think about increasing kinetic
energy, increasing the rate of collisions, and increasing the likelihood that the reactants
will participate in a reaction, those things can help us—or help provide the energy needed
for this reaction to proceed. So that concludes this segment on enzymes.