Discussion Exercise 9: Mechanisms for Key Enzymatic Reactions
Skill 1: Aldol/Retroaldol reaction
 The bond between the  and  carbons of a-hydroyketone is made in an aldol
reaction/broken in a retroaldol reaction.

An aldolase catalyzes the reaction by forming a Schiff base, which then stabilizes the
carbanion that is involved in the aldol reaction
 Draw a mechanism for aldolase to show how the aldehyde forms.
Example: Aldolase from glycolysis. In the figure below, the full mechanism for Schiff base
formation is not given. The key step in this reaction is step 3, where a base pulls the proton off of
the hydroxyl group that is b to the Shiff base, and the electrons are stabilized by the Schiff base.
Problem 1: Without referring to the figure provided, draw a mechanism for the retroaldol
reaction of fructose-1,6-bisphosphate with aldolase. A tyrosine residue acts as the general base.
Problem 2. What are the products of the retroaldol reaction of the enzyme transaldolase when it
catalyzes the reaction of sedoheptulose-7-P?
Skill 2: Recognize three types of decarboxylation reactions and tell which cofactor is
needed, if any. Then draw the decarboxylation product.




A b-ketoacid does not need a cofactor; it will decarboxylate spontaneously
An a-keto acid needs the cofactor thiamine pyrophosphate (TPP)
An a-amino acid needs the cofactor pyridoxal phosphate (PLP)
Decarboxylation is loss of CO2, replaced with a proton.
-ketoacid
O
-
ketoacid
O
aminoacid
O
O
R
O
R
-
O
O
CO2 +
R
-
O
O
H
R
CO2 +
NH3
H
R
O
CO2 +
H
R
NH3
Problem 3: For each reaction, draw the product of a decarboxylation reaction, and tell which
cofactor is necessary.
O
H2N
CH
C
last step in the
production of
serotonin, a
neurotransmitter
OH
CH2
OH
HN
serotonin
Problem 4: Oxaloacetate can be decarboxylated without a cofactor. Which carboxylate group
will be lost? Draw the product of the reaction:
O
-
O
-
O
O
decarboxylation
CO2 +
O
Skill 3: Understand the role of TPP in decarboxylation


TPP is a cofactor that forms a reversible covalent bond to the substrate so that the
decarboxylation can be stabilized.
This stabilization is in the form of resonance.
Problem 5: Fill in the mechanistic arrows and resonance arrows for the formation of
acetaldehyde from pyruvate. In words, explain the role of TPP. Why would the reaction be very
slow without TPP?