The first step in metabolism of ethanol in human is the oxidation of ethanol to
ID: 178093 • Letter: T
Question
The first step in metabolism of ethanol in human is the oxidation of ethanol to acetaldehyde by the enzyme alcohol dehydrogenase (ADH). At 298 K and pH = 7.0 ethanol is incubated with ADH and the concentration of ethanol and acetaldehyde is measured over time.
a. Using the results above, calculate the standard free energy change (G°’) for this reaction. Explain your calculations.
b. After dinner and a few drinks the concentration of ethanol in a cell is measured to be 3412 mol/L and the concentration of acetaldehyde is measured to be 1.4 mol/L. Calculate G’ and explain what this value means.
c. Identify the subsequent steps in the pathway of ethanol metabolism. Does your body ultimately gain energy from the consumption of ethanol?
d. What would you expect to happen if these steps could not be completed? (For instance, a molecule was present that inhibited the next enzyme in the pathway.)
e. Ethanol is sometimes used as an antidote for antifreeze poisoning. Explain why.
Time Point Ethanol Acetaldehyde 1 6515.7 0.1 2 6514.6 1.2 3 6513.0 2.8 4 6512.3 3.5 5 6512.3 3.5Explanation / Answer
a. The standard free energy of a substance represents the free energy change associated with the formation of the substance from the elements in their most stable forms as they exist under standard conditions.
CH3CH2OH àCH3CHO + 2H
Gf = Gfp Gfo
Ethanol: 174.8 kJ/mol
Acetaldehyde: 127.6 kJ/mol
Gf = 127.6 + 174.8 = 47.2 kJ/mol
Gf = 47.2 kJ/mol
If the value is +ve ---- the reaction is endothermic
If it is –ve ---- the reaction is exothermic
The Reaction is therefore endergonic.
c.Energy calculations for catabolism of alcohol:
The reaction from ethanol to CO2 and H2O is a complex one that proceeds in three steps. The Gibbs Free Energy of Formation for each step is shown with Gf values.
Complete reaction: C2H6O(Ethanol)C2H4O(Acetaldehyde)C2H4O2(acetic Acid) Acetyl-CoA3H2O+2CO2.
Gf = Gfp Gfo
Step One
Ethanol: 174.8 kJ/mol
Acetaldehyde: 127.6 kJ/mol
Gf1 = 127.6 + 174.8 = 47.2 kJ/mol (Endergonic)
Gf = 47.2 kJ/mol (Endergonic)
Step Two
Acetaldehyde: 127.6 kJ/mol
Acetic acid: 389.9 kJ/mol
Gf2 = 389.9 + 127.6 = 262.3 kJ/mol (Exergonic)
Gf = = 262.3 + 47.2 = 215.1 kJ/mol (Exergonic)
Step Three
(Because the Gibbs energy is a state function, we can skip the Acetyl-CoA (step 3), for which thermodynamic values are not known).
Acetic Acid: 389.9 kJ/mol
3H2O+2CO2: 1500.1 kJ/mol
Gf4 = 1500 + 389.6 = 1110.5 kJ/mol (Exergonic)
Gf = = 1110.5 - 215.1 = 1325.6 kJ/mol (Exergonic)
If catabolism of alcohol goes all the way to completion, then, we have a very exothermic event yielding some 1325 kJ/mol of energy.
d. Glucose is converted to pyruvate by glycolysis. When animal tissues cannot be supplied with sufficient oxygen to support aerobic oxidation of the pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by the reduction of pyruvate to lactate. Some tissues and cell types (such as erythrocytes, which have no mitochondria that cannot oxidize pyruvate to CO2) produce lactate from glucose even under aerobic conditions and therefore leads to accumulation of lactate. The reduction of pyruvate is catalyzed by lactate dehydrogenase.
PyruvateLactate dehydrogenaseàLactate
e. A medical therapy based on competition at the active site is used to treat patients who have ingested methanol, a solvent found in gas-line antifreeze. The liver enzyme alcohol dehydrogenase converts methanol to formaldehyde, which is damaging to many tissues. Blindness is a common result of methanol ingestion, because the eyes are particularly sensitive to formaldehyde. Ethanol competes effectively with methanol as an alternative substrate for alcohol dehydrogenase. The effect of ethanol is much like that of a competitive inhibitor, with the distinction that ethanol is also a substrate for alcohol dehydrogenase and its concentration will decrease over time as the enzyme converts it to acetaldehyde. The therapy for methanol poisoning is slow intravenous infusion of ethanol, at a rate that maintains a controlled concentration in the bloodstream for several hours. This slows the formation of formaldehyde, lessening the danger while the kidneys filter out the methanol to be excreted harmlessly in the urine.
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