Enzymes MiraCosta Biology

To illustrate how enzymes work we’re going to focus on hexokinase, and this enzyme’s
name can be broken into two parts—hexose and kinase. “Hexose” refers to glucose,
which the six-carbon sugar that we’re starting out with, and this makes sense because this
enzymatic action that we’re detailing here is the very first step in glycolysis. “Kinase”
means “activating enzyme”—basically this says that we’re activating the glucose
in order to send it on down the pathway of glycolysis. Remember that glycolysis is something
that happens before the fueling process gets to the mitochondrion, so hexokinase would
be one of the many different enzymes that are present in your cell’s cytoplasm.
In order to complete its job, the enzyme hexokinase must bind to an ATP molecule and also to a
glucose. We would call the ATP and the glucose the reactants in this chemical reaction. The
role of the enzyme is to facilitate the rearrangement of the covalent bonds, so that in the end
the third phosphate group on ATP gets broken off of ADP and attached to the glucose. Recall
from an earlier lesson that that ATP looks like this (pause) there’s the adenosine,
which is a ribose sugar attached to the base adenine, and this is attached to three phosphates
(pause), while ADP—or adenosine diphosphate—is the same thing but with only two phosphates.
In this reaction that third phosphate is now covalently bonded to the glucose which is
now called “glucose-phosphate,” and instead of the ATP we now have ADP—and this new
orientation of bonds is favored because it’s more stable. (pause) What the enzyme does
is it holds the two reactants, ATP and glucose, in exactly the right way to allow this bond
rearrangement to occur.
At this point you might say that this new bond arrangement that you end up with—when
you have this new molecule glucose-phosphate and ADP—this is the more stable arrangement
that is favored by the natural laws of the universe, so even without the enzyme this
reaction would just naturally happen if you started with ATP and glucose, and technically
you’d be correct—the problem is that if you started out with ATP and glucose, you’d
have to wait years before even just a few of the molecules bumped into each other in
exactly the right way with exactly the right energy in order to permit this change to occur.
The enzyme hexokinase makes this reaction happen just as soon as it binds to the two
reactants—which is very fast.
This is basically the way that all enzymes work—they don’t really provide any impetus
to push a chemical reaction along—it’s always some kind of covalent bond rearrangement
that would occur naturally if the reactants happened to get into just the right position,
but since that right position is so unlikely to occur without any help from the enzyme
it just appears like the enzyme is sort of magically making a reaction take place that
otherwise would not be happening.
So think of the reaction as something that does happen naturally but at an abysmally
slow rate. The enzyme is just speeding things up—and this is why the enzyme is a catalyst.
By definition a catalyst is a substance that speeds up the rate of a chemical reaction
without being consumed in any way by the reaction itself.
The enzyme hexokinase releases the ADP and glucose-phosphate, at which point it is free
to bind to another ATP and glucose.
Another way that people like to talk about the way that enzymes work (and this is the
way it’s explained in your text book) is to focus on the activation energy of a reaction
using a chart like this [*].
The reactants are on the left and are put at a higher position relative to the products
shown on the right—this is deliberate to represent the amount of potential energy that’s
there in the reactants compared to the products. The products have less potential energy (because
of the covalent bond structure), and therefore the reaction from reactant to product is actually
favored.
However, in order to get from point A (the reactant side) to the more stable point B
(the product side), you have to go through an intermediate state that has a much higher
level of instability—this represents a huge increase in potential energy that the reactants
need to overcome before they can get to the lower-energy side of the reaction. And this
is what we call the activation energy.
In the absence of an enzyme it would be practically impossible for the reactants to achieve this
intermediate state because of the unlikelihood of them achieving this level of energy—this
relates in my previous explanation to the extreme unlikelihood of the ATP and the glucose
to bump into each other in exactly the right way for the bonds to rearrange on their own.
What the enzyme does is it makes the intermediate state much, much more likely to occur—or
in other words the enzyme lowers the activation energy. By making the activation energy very
easy to overcome, the enzyme is (again) dramatically speeding up a reaction that is really something
that happens naturally (albeit at an excruciatingly slow rate).
Another enzyme I have mentioned before—salivary amylase—works in essentially the same way.
The reaction starts with large, branching chains of glucoses in the form of a starch
molecule. Amylase breaks apart the chain—liberating smaller carbohydrates—and it does so by
hydrolysis (which is the splitting of water—just like glycolysis is the splitting of sugar).
Basically the amylase enzyme grabs hold of the starch and holds two adjacent glucoses
in exactly the right orientation to allow a water molecule to come in have its bonds
rearrange with the one between the atoms that are holding the glucoses together. And after
the covalent bonds have shifted over into a more stable arrangement the chain of glucoses
has been broken where the two parts of the split water molecule inserted themselves.
Alternatively you could say that the broken chain represents a lower energy state relative
to the whole starch molecule, but it doesn’t get there without overcoming an activation
energy. The enzyme amylase helps this catabolic reaction along by lowering the activation
energy and making it very likely for the water molecule to come in, split, and sever the
chain.
Enzymes are a really, really important part of biology because they’re catalyzing all
of the chemical reactions that make cells and organisms do all the things they do. Glycolysis
is a pathway, made up of ten enzyme-catalyzed steps—each of ten enzymes has its own reactants
and products. The steps have a sequence in the sense that the products of one step become
the reactants for the next step, for example the glucose-phosphate we end up with after
the hexokinase-catalyzed first step is the reactant for the next enzyme in the sequence.
At the end of the ten glycolysis steps we end up with our two pyruvic acids, which in
our cells move along next to the mitochondrion and those parts of the fueling reactions that
require oxygen.
Also, as you’re thinking about the context here of Glycolysis, don’t forget that in
the end we’re producing a net yield of 2 ATP from 2(ADP plus phosphate). Given that
glycolysis is a fueling reaction, the net production of ATP is really the whole point.
We're going to continue with fueling reactions in the next video where we get into all the
things that go on inside the mitochondrion.