>>Dr. Paul Sims: Welcome to this segment of the chemistry of beer. In this segment we'll
be discussing alcohol metabolism. As you know, in this country the legal drinking age is
21, and at that time people make a decision whether or not they want to drink alcohol.
I think it's important, therefore, that people understand how alcohol is metabolized in our
bodies, and therefore, I think this topic will be relevant and interesting for everyone.
To outline what we'll be talking about, I want to first of all talk about the chemical
properties of the alcohol that's in alcoholic beverages. Then we'll give a few preliminaries
about blood alcohol concentration. Finally, we'll discuss enzyme-catalyzed reactions,
those that are responsible for for metabolizing the alcohol. And then at the end we'll try
to tie together a few loose ends and give a summary of what we discussed.
As some of you may already know, the particular type of alcohol that's present in alcoholic
beverages is ethanol. Ethanol has the formula CH3CH2OH, and it is introduced into beer through
the action of yeast. Ethanol is a relatively small molecule. It has a molecular weight
of 46. It's amphiphilic in that it has a nonpolar end and a polar end, and it is completely
miscible in water. Sometimes people refer to it as being infinitely soluble in water.
Another interesting feature of ethanol is that it passes through membranes readily,
and it can penetrate the blood-brain barrier.
The next topic I wanted to discuss was blood alcohol concentration, which I'll be referring
to as BAC. There are several factors that determine what a person's BAC will be after
he or she consumes an alcoholic beverage. Some of these factors include: one, how rapidly
the alcoholic beverage was consumed; two, how much food the person has in their stomach;
three, the size of the person; and four, how rapidly a person is able to metabolize ethanol.
That is the subject we're going to move into next--metabolizing ethanol. Continuing with
our topic of blood alcohol concentration, or BAC, we want to think about properties
of enzymes, and in particular, some of the properties of enzymes that metabolize ethanol.
Because enzyme parameters such as Km, or the Michaelis constant, which we'll discuss shortly,
because these terms are given in in terms of concentration, such as millimolar, but
alcohol concentrations are typically given in terms of percent volume, it is important
for you to be able to make some basic conversions so that you can answer questions, such as:
"What is the millimolar concentration of alcohol in the aqueous portion of the human body after
a person has consumed a 12 ounce beer that contains 6% volume volume of alcohol? Assume
that alcohol is fully distributed and is not begun to be metabolized or excreted, and here's
a hint: you can consider that the human body contains about 40 L of water." And please
note that there are several ways to solve this problem, but one thing you might want
to do is look up the density of ethanol.
Our next topic is to answer the question: where is the ethanol metabolized? The liver
is the primary site of ethanol metabolism, although some ethanol metabolism also occurs
in the gastrointestinal tract, the pancreas, and the brain. There are three enzyme systems
that are responsible for metabolizing ethanol. The first enzyme system is alcohol dehydrogenase,
which I will abbreviate as ADH, and aldehyde dehydrogenase, which I will abbreviate as
ALDH. Combined, ADH and ALDH process about 80 to 90% of the ethanol that a person consumes.
Another enzyme system is the cytochrome P450 enzyme complex. This is comprised of cytochrome
P450 reductase and CYP2E1. This enzyme system processes about 10 to 20% of the ethanol that
a person consumes. The last enzyme system that metabolizes ethanol is catalase. This
is fairly minor except in the fasted state.
We're now going to talk about each of these enzymes in a little more detail. And so the
first enzyme that we're going to start with is alcohol dehydrogenase, or ADH. If we consider
the ADH reaction, we see that we start with ethanol and something called NAD+, which I'll
talk more about in a minute. And we have an equilibrium arrow, a double-headed arrow.
And underneath that, we've written ADH, and that's to signify alcohol dehydrogenase. We
see that we form a molecule called acetaldehyde, and then we form NADH, and we also form H+,
or a hydrogen ion, or if you want, a proton. There are a few interesting features of ADH
that you should be aware of. First of all, ADH is found in the cytosol of liver cells
or sometimes called hepatic cells, and there are many isoforms of this enzyme. The NAD+
and the NADH stand for nicotinamide adenine dinucleotide, oxidized and reduced forms,
respectively. And since that's kind of a mouthful, you can see why we just refer to them as NAD+
or NADH.
Then finally the acetaldehyde that is produced by the ADH reaction is very toxic, and that
points to the need for another reaction that can further metabolize acetaldehyde. And that
is the aldehyde dehydrogenase reaction, or the ALDH reaction. So the next enzyme that
we're considering is the aldehyde dehydrogenase enzyme, or ALDH. As you can see from the reaction
scheme, we start with the product of the alcohol dehydrogenase reaction, which is acetaldehyde.
That's that toxic molecule that needs to be further metabolized before it can do significant
damage. So we start with acetaldehyde and NAD+ again and water, and this time the aldehyde
dehydrogenase catalyzes the formation of acetate and NADH and liberates two hydrogen ions,
or two protons. You should be aware that there are cytosolic and mitochondrial forms of aldehyde
dehydrogenase. Similar to what we saw with alcohol dehydrogenase, aldehyde dehydrogenase
also converts NAD+ to NADH as it converts the substrate (acetaldehyde) to the product
(acetate). We're going to be seeing NAD+ and NADH quite a bit, and so I want you to key
in on those two molecules.
Finally, the acetate that is produced can be metabolized further to carbon dioxide and
water, or it can serve as a precursor for fatty acid biosynthesis. In addition to abbreviating
nicotinamide adenine dinucleotide as NAD+ or NADH, I also want to explain why I'm not
going to be showing the chemical structure of these molecules every time. As you can
tell from this figure, NAD+ and NADH are reasonably complicated molecules, and it would take too
long to draw them every time, so that's why we're going to stick with the abbreviation
NAD+ and NADH.
The second enzyme system that metabolizes ethanol is the cytochrome P450, and that will
be the topic of this discussion. As you can see from the diagram, we start with ethanol.
This time we start with, in addition to ethanol, NADPH. This is very similar to NADH. There's
just the difference of whether or not a phosphoryl group is attached to the molecule. We also
include a hydrogen ion and oxygen, and then the enzyme catalyzes the formation of acetaldehyde.
And remember that's toxic, so it needs to be metabolized further by aldehyde dehydrogenase.
We also form NADP+ and two waters as products. The CYP2E1 is located in the lumen of the
endoplasmic reticulum of cells, and what's interesting is it uses a heme cofactor.
Now depending on your study of biology and biochemistry, you may or may not have encountered
heme before. But if you've talked about or or read about the proteins hemoglobin and
myoglobin, then you might be familiar with what a heme cofactor is. For those proteins,
hemoglobin and myoglobin, the heme cofactor functions to carry oxygen, or or bind oxygen
I should say. But in the cytochrome P450 enzyme, the heme cofactor is actually used to help
catalyze the reaction. Now CYP2E1 has a higher Km, a higher Michaelis constant, for ethanol
than does alcohol dehydrogenase. Thus, this enzyme doesn't really come into play until
the ethanol concentration is relatively high.
I've mentioned Km a couple of times now, and before I get too much further, I think it's
important to take a minute and to to try to illustrate what we mean by Km. If we look
at this graph, we see a plot of velocity on the y-axis as a function of substrate concentration.
That's the S in brackets on the x-axis. We see that as we start to increase the substrate
concentration, the velocity rapidly increases, but as we go to higher substrate concentrations,
the velocity begins to taper off and approach an asymptote. Now that asymptote is the vmax,
the maximal velocity of the reaction. What the Km is is, it's the substrate concentration
that allows the enzyme to achieve half of its maximal velocity. Graphically, we can
see that by going to 25 micromolar per minute, that's half of our vmax, which is 50 micromolar
per minute. And we move out from 25 micromolar per minute until we intersect the the trace
of the enzyme activity, and then we move straight down until we intersect the x-axis, which
reflects the substrate concentration. And so we see that the Km is the substrate concentration
that allows the enzyme to achieve half of its maximal velocity.
So if we go back to our discussion about cytochrome P450, I mentioned that it has a relatively
high Km, and that means it's not very active until the ethanol concentration is relatively
high. So that's an explanation for why ADH, alcohol dehydrogenase, metabolizes most of
the ethanol, because it has a lower Km. So that means it can be more active at lower
alcohol concentrations, rather than the cytochrome P450, which needs higher ethanol concentrations
to be active. The last enzyme that we'll consider in the metabolism of ethanol is catalase.
As you can see from the diagram, catalase catalyzes the reaction between ethanol and
H2O2. H2O2, as you probably know, is hydrogen peroxide. In the process acetaldehyde is formed,
again that toxic molecule that is metabolized further by aldehyde dehydrogenase, and we
also form two waters. Catalase is found in peroxisomes. And similar to cytochrome P450
or CYP2E1, catalase also contains a heme cofactor. As noted earlier, this reaction plays a very
minor role in alcohol metabolism, except if someone is in a fasted state. We've seen that
the alcohol dehydrogenase catalyzes the conversion of ethanol into acetaldehyde, and acetaldehyde
is toxic, so it is further metabolized by the enzyme aldehyde dehydrogenase to the molecule,
the product acetate.
Our next topic is to consider what happens to the acetate that's produced. As you can
see from this diagram, the acetate reacts with ATP, adenosine triphosphate, that's referred
to sometimes as the energy currency of the cell, and CoASH, and that stands for coenzyme
A, and the SH is to indicate that it ends with a thiol. The enzyme that catalyzes this
reaction is called acetyl-CoA synthetase, and the products of the reaction are acetyl-CoA,
AMP (adenosine monophosphate), and PPI, which stands for inorganic pyrophosphate. Much of
the acetate that is formed as a result of the aldehyde dehydrogenase reaction leaves
the liver. It can then be taken up by the heart, the brain, and the skeletal muscle,
where it is further metabolized, initially by the acetyl-CoA synthetase reaction. The
acetyl-CoA that's produced as a result of the acetyl-CoA synthetase reaction can then
enter the Tricarboxylic Acid cycle, or it can serve as a precursor for fatty acid biosynthesis
and cholesterol biosynthesis. We are going to come back to the Tricarboxylic Acid cycle
in a future video.
So we're pretty much done with our discussion of ethanol metabolism, just have a few loose
ends to to wrap up and then to kind of give a brief summary. There are a few nonoxidative
pathways that occur, but I opted not to mention these pathways. There's also something you
should be aware of. The cytochrome P450 enzyme reaction, it can generate reactive oxygen
species, which are sometimes abbreviated as ROS. These reactive oxygen species can damage
cells by forming, for example, adducts with proteins. And by the way, acetaldehyde can
also form adducts with proteins. That's part of its dangerous, toxic effect. To summarize
what we talked about, the liver is the primary organ that metabolizes ethanol. Alcohol dehydrogenase
is the primary enzyme that reacts with ethanol, but CYP2E1, or cytochrome P450, and catalase
also play a role. The aldehyde dehydrogenase reaction converts toxic acetaldehyde to acetate,
which can be broken down further to carbon dioxide and water. Both the ADH (alcohol dehydrogenase)
and ALDH (aldehyde dehydrogenase) reaction form NADH. Please keep this in mind as we
revisit this cofactor several times.