CHEM 109C – Zhang
Coenzymes IV: Carbon Transfer Reactions - KEY
CLAS, F10
1. (Ch 24, #23) Answer the following questions about coenzymes/cofactors.
b. What are the coenzymes (cofactors) that donate one-carbon groups?
c. What three one-carbon groups do various THFs donate to substrates?
Tetrahydrofolates (THFs)
H
H2N
N
N
N
H
N
5
CH3
O
Methyl, CH3: N5-methyl-THF
NHR
10
H
H2N
N
N
N
H
N
5
H2C
O
Methylene, CH2: N5, N10-methylene-THF
NR
10
Formyl, CHO:
H
H2N
N
N
N
+
H
N
5
O
N5, N10-methenyl-THF
HC
NR
10
H
H2N
N
N
N
H
N
5
O
HC
NHR
O
N5-formyl-THF
10
H
H2N
N
N
N
H
N
5
O
N10-formyl-THF
H
RN
10
CH
O
Page 1 of 4
CHEM 109C – Zhang
Coenzymes IV: Carbon Transfer Reactions - KEY
CLAS, F10
H
H2N
N
N
N
H
N
5
CH
O
N5-formimino-THF
HN
NHR
10
2. Which THF would you use to complete each of the following reactions?
b.
c.
d.
Page 2 of 4
CHEM 109C – Zhang
Coenzymes IV: Carbon Transfer Reactions - KEY
CLAS, F10
3. What coenzyme is required by enzymes that transfer two-carbon pieces? Can you
give an example of one of these reactions? (Hint: The substrate can be pyruvate.)
Thiamine pyrophosphate (TPP)
Examples:
CH2CO piece of pyruvate transferred to an H to form acetaldehyde.
CH2CO piece from one pyruvate transferred to the -C of a second pyruvate
to form acetolactate.
CH2CO piece from pyruvate transferred to the -C of -ketobutyrate to
form -aceto--hydroxybutyrate.
CH2CO piece from pyruvate transferred to CoASH to form acetyl-CoA.
Page 3 of 4
CHEM 109C – Zhang
Coenzymes IV: Carbon Transfer Reactions - KEY
Pyruvate dehydrogenase complex/system:
CLAS, F10
pyruvate → acetyl-CoA + CO2
Mechanism
1. pyruvate + TPP-E1 → hydroxyethyl-TPP-E1 + CO2
2. hydroxyethyl-TPP-E1 + lipoamide-Lys-E2 → -TPP-E1 + acetyllipoamideLys-E2
3. acetyllipoamide-Lys-E2 + HS-CoA → dihydrolipoamide-Lys-E2 + acetylCoA
+ CoASH →
4. dihydrolipoamide-Lys-E2 + FAD-E3 → lipoamide-Lys-E2 + FADH2-E3
→
FAD-E3 +
E3
+ FADH2-
5. FADH2-E3 + NAD+ → FAD-E3 + NADH
Enzyme
Abbreviation
Coenzyme
Reaction Catalyzed
Pyruvate dehydrogenase component
E1
Oxidative decarboxylation of pyruvate
Dihydrolipoyl transacetylase
E2
TPP
Lipoamide/lipoate bound to
Lys-E2
Coenzyme A
Dihydrolipoyl dehydrogenase
E3
FAD
NAD+
Transfer of acetyl group to coenzyme A
Transfer of acetyl group to coenzyme A
Regeneration of oxidized form of
lipoamide
Regeneration of FAD
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