Question:
β-hydroxybutyrate dehydrogenase catalyzes the last step in ketogenesis, the reduction of acetoacetate to b-hydroxybutyrate.
(a) Write a balanced equation for this reaction.
(b) Calculate the standard free energy (ΔG°′) associated with this reaction, using the data in Table 14.1. Is this reaction favorable in the direction written under standard conditions? If not, what cellular conditions would be required for this reaction to continuously produce β-hydroxybutyrate at a high rate?
Table 14.1.
Transcribed Image Text:
TABLE 14.1 Standard reduction potentials of interest in biochemistry
Oxidant
Acetate + CO₂ + 2H+ + 2e-
Succinate + CO₂ + 2H+ + 2e
Acetate + 3H+ + 2e¯
Ferredoxin (oxidized) + e
2H+ + 2e™
a-Ketoglutarate + CO₂ + 2H+ + 2e
Acetoacetate + 2H+ + 2e
Pyruvate + CO₂ + H+ + 2e
NAD+ + H+ + 2e™
NADP+ +H+ + 2e™
Lipoate (oxidized) + 2H+ + 2e™
1,3-Bisphosphoglycerate + 2H+ + 2e
Glutathione (oxidized) + 2H+ + 2e
FAD (free coenzyme) + 2H+ + 2e-
Acetaldehyde + 2H+ + 2e™
Pyruvate + 2H+ + 2e™
Oxaloacetate + 2H+ + 2e
O₂ + e
a-ketoglutarate + NH + 2H+ + 2e™
FAD (enzyme-bound) + 2H+ + 2e
Methylene blue (oxidized) + 2H+ + 2e
Fumarate + 2H+ + 2e
Q + 2H+ + 2e™
Dehydroascorbate + 2H+ + 2e™
Cytochrome b (+3) + e
Cytochrome c₁ (+3) + e
Cytochrome c (+3) + e
Cytochrome a (+3) + e
O₂ + 2H+ + 2e™
Ferricyanide + 2e™
NO (Nitrate) +2H+ + 2e
Cytochrome a3(+3) + e¯
Fe (+3) + e
O₂ + 2H+ + 2e™
Reductant
Pyruvate + H₂O
a-Ketoglutarate + H₂O
Acetaldehyde + H₂O
Ferredoxin (reduced)
H₂
Isocitrate
B-Hydroxybutyrate
Malate
NADH
NADPH
Lipoate (reduced)
Glyceraldehyde-3-phosphate + P
2 Glutathione (reduced)
FADH₂2
Ethanol
Lactate
Malate
O₂ (superoxide)
Glutamate + H₂O
FADH₂ (enzyme-bound)
Methylene blue (reduced)
Succinate
QH₂
Ascorbate
Cytochrome b (+2)
Cytochrome c₁ (+2)
Cytochrome c (+2)
Cytochrome a (+2)
H₂O₂
Ferrocyanide
NO₂ (Nitrite) + H₂O
Cytochrome a3 (+2)
Fe (+2)
H₂O
n
2
2
2
1
2
2
2
2
2
2
2
2
2
2
2
2
2
1
2
2
2
2
2
2
1
1
1
1
2
2
2
1
1
2
E° (V)
-0.70
-0.67
-0.60
-0.43
-0.42
-0.38
-0.35
-0.33
-0.32
-0.32
-0.29
-0.29
-0.23
-0.22
-0.20
-0.19
-0.17
-0.16
-0.14
-0 to -0.30
0.01
0.03
0.04
0.06
0.07
0.23
0.25
0.29
0.30
0.36
0.42
0.55
0.77
0.82