Transcript Alcohol / Ethanol / Booze - University of California, Irvine

Alcohol / Ethanol / Booze
Making Alcohol
• The enzyme alcohol dehydrogenase plays a central role in the most
ancient form of biotechnology: alcoholic fermentation.
• Yeast and many bacteria produce alcohol dehydrogenases. These
microbial enzymes catalyze the last step in the conversion of food into
metabolic energy, creating ethanol.
• Sugars are broken down and used for energy, forming ethanol as the
waste product, which is excreted into the liquid surrounding the cell.
• We have harnessed this process to produce alcoholic beverages: yeast is
allowed to ferment grain sugars to form beer, and yeast is allowed to
ferment grape juice to form wine.
Microbial ADH
• Tetramer
• 4 x 352 amino acid residues
• 4 zinc ions (Zn++)
• 4 NAD cofactors
Making Alcohol
• Alcohol dehydrogenases in microbes function as tetramers.
• They are zinc-containing enzymes that utilize glucose.
• Each glucose molecule is broken down in a 10-step process called
glycolysis. The product of glycolysis is two three-carbon sugars, called
pyruvates, and ATP (adenosine triphosphate).
• The two pyruvates are then converted into ethanol and carbon dioxide.
The overall process of fermentation is to convert glucose sugar to alcohol and carbon dioxide gas:
C6H12O6
sugar
(glucose)
 2 CH3CH2OH
alcohol
(ethyl alcohol or ethanol)
+
2 CO2
carbon dioxide gas
Making Alcohol
Making Alcohol
Breaking Down Alcohol
In 1997, Americans drank an average of 2 gallons (7.6 liters) of
alcohol per person. This translates roughly into one six-pack of
beer, two glasses of wine and three or four mixed drinks per wee.
So while recovering from the excesses at the Anthill Pub [last
night] after it reopens next fall, we might ponder the human
alcohol dehydrogenase enzyme, which ceaselessly battles all the
beer & wine that we have consumed.
Breaking Down Alcohol
• Alcohol dehydrogenase is our primary defense against alcohol, a
toxic molecule that compromises the function of our nervous system.
• The high levels of alcohol dehydrogenase in our liver and stomach
detoxify about one drink each hour.
• The alcohol is converted to acetaldehyde, an even more toxic
molecule and the main cause of hangovers!
• Acetaldehyde in turn is converted to acetate and other molecules
that are easily processed by our cells.
Human ADH
• Homodimer (two molecules)
• 2 x 373 amino acid residues
• 6 zinc ions (Zn++)
• 2 NAD cofactors
Human
ADH
Microbial
Breaking Down Alcohol
Alcohol dehydrogenase
CH3CH2OH
+ 2 NAD
alcohol
(ethanol)

cofactor
CH3CHO
+ 2 NADH
aldehyde
(acetaldehyde)
cofactor
Acetaldehyde dehydrogenase 2
CH3CHO
aldehyde
(acetaldehyde)
+ H2O

CH3COOH
acid
(acetic acid or vinegar)
Breaking Down Alcohol
Acetaldehyde dehydrogenase 2
CH3CHO
aldehyde
(acetaldehyde)
+ H2 O

CH3COOH
acid
(acetic acid or vinegar)
The acetic acid can be used to form fatty acids (watch that
waistline!), or it can be further broken down into CO2 and water.
Dangers of Alcohol
Alcohol dehydrogenase provides a line of defense against a common
toxin in our environment.
• But alcohol dehydrogenase also modifies other alcohols, sometimes
producing even more dangerous products:
• Methanol, which is commonly used to “denature” ethanol rendering it
undrinkable, is converted to formaldehyde by alcohol dehydrogenase.
• The formaldehyde then causes severe damage, attacking proteins and
embalming them.
• Small amounts of methanol cause blindness, as the sensitive proteins in
the retina are attacked, and larger amounts, perhaps a glassful, lead to
widespread damage and death.
Breaking Down Methanol
Alcohol dehydrogenase
CH3CH2OH
+ 2 NAD
alcohol
(ethanol)

cofactor
CH3CHO
+ 2 NADH
aldehyde
(acetaldehyde)
cofactor
Alcohol dehydrogenase
CH3OH
alcohol
(methanol)
+
2 NAD
cofactor

CH3CHO
aldehyde
(formaldehyde)
+ 2 NADH
cofactor
Structure (Form) & Function
• Our bodies create at least nine different forms of alcohol
dehydrogenase, each with slightly different properties.
• Most of these are found primarily in the liver, including the b3 form
• The s form is found in the lining of the stomach.
• Each enzyme is composed of two subunits.
• Ethanol is not the only target or substrate of these enzymes, they also
make important modifications to retinol, steroids, and fatty acids.
Structure (Form) & Function
• Human alcohol dehydrogenases use two “helpers” to perform their
reaction on ethanol.
• The first are zinc ions (Zn++), which are used to hold and position the
alcohol group on ethanol.
• The second is the NAD cofactor (constructed using the vitamin niacin),
which actually performs the chemical reaction.
• The zinc atom, shown in light blue, is cradled by three amino acids
from the protein: cysteine 46 to the left, cysteine 174 to the right, and
histidine 67 above. The ethanol, shown in green and magenta, binds to
the zinc and is positioned next to the NAD cofactor, which extends
below the ethanol molecule in this illustration.