Consider the mechanism of the aldolase reaction given in figure 9.25. In chapter 12, we saw that the same enzyme was used to catalyze the reverse reaction, DHAP + glyceraldehyde-3-phosphatefructose-1,6-bisphosphate, in the first step of stage 3 in the Calvin Cycle. Using arrows and structures similar to what is shown in 9.25, propose a mechanism for this reverse reaction (which is an aldol condensation EnamineintermediateHProtonatedSchiff baseintermediateCH₂OPO3²HIsomerization 3OHHCH₂OPO3²-Fructose-1,6-bisphosphateCH₂OPO3²C=NH*H-C-OHCH₂OPO3²-C=OHO-C-HH-C-OHH-C-OH2Aldol cleavageand first product(glyceraldehyde-3-P)is releasedHO C-HCH₂OPO3²-Glyceraldehyde-3-phosphate4Aldolase enzymeactive siteH₂OHydrolysis of the Schiff baseintermediate, and secondproduct (dihydroxyacetone-P)is releasedO=H₂OH-C-OHH-O-C-HHO-C-HCH₂OPO3²-HOCH₂OPO3²-H CHO-C-H2-Nucleophilic attack bylysine residue of enzymgenerates a covalentlybound intermediateCH₂OPO3²NH₂C-HCH₂OPO3²-C=OC-HCH₂OPO3²-Protonated Schiff baseintermediateHDihydroxyacetonephosphateH-OHFigure 9.25 The four-step reaction mechanism offructose-1,6-bisphosphate cleavage by the enzymealdolase in reaction 4 of glycolysis. Reaction 4: Cleavage of Fructose-1,6-BP intoGlyceraldehyde-3-P and Dihydroxyacetone-P by AldolaseThe splitting of fructose-1,6-BP into the triose phosphatesglyceraldehyde-3-P and dihydroxyacetone-P is the reactionthat puts the lysis in glycolysis (lysis means "splitting"), asshown in Figure 9.24. The enzyme responsible for thiscleavage reaction between C-3 and C-4 in fructose-1,6-BP isaldolase (also called fructose bisphosphate aldolase). In thecontext of the glycolytic pathway, aldolase performs thereverse of an aldol condensation. The mechanism ofcleavage by aldolase (Figure 9.25) involves the formation ofa covalent enzyme-substrate complex through the generationof a Schiff base requiring a lysine residue in the enzymeactive site. The four-step reaction can be summarized asfollows:1. The open-chain form of fructose-1,6-BP binds to thealdolase active site, and a lysine residue in the activesite initiates a nucleophilic attack on the ketose carbon,generating a covalently bound protonated Schiff baseintermediate.2. Base abstraction by an active site carboxyl group leadsto C—C (aldol) cleavage between C-3 and C-4 offructose, and a covalent enamine intermediate isformed. The first product, glyceraldehyde-3-P, isreleased.3. Isomerization leads to the formation of a secondprotonated Schiff base intermediate.4. The Schiff base intermediate is hydrolyzed, releasingthe second product, dihydroxyacetone-P.

Answered: Consider the mechanism of the aldolase…

Biochemistry

9th Edition

ISBN:9781319114671

Author:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.

Publisher:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.

Chapter1: Biochemistry: An Evolving Science

Section: Chapter Questions

Problem 1P

See similar textbooks

Related questions

Question

Consider the mechanism of the aldolase reaction given in figure 9.25. In chapter 12, we saw that
the same enzyme was used to catalyze the reverse reaction, DHAP + glyceraldehyde-3-
phosphatefructose-1,6-bisphosphate, in the first step of stage 3 in the Calvin Cycle. Using arrows and
structures similar to what is shown in 9.25, propose a mechanism for this reverse reaction (which is an
aldol condensation

Enamine
intermediate
H
Protonated
Schiff base
intermediate
CH₂OPO3²
H
Isomerization 3
OH
H
CH₂OPO3²-
Fructose-1,6-bisphosphate
CH₂OPO3²
C=N
H*
H-C-OH
CH₂OPO3²-
C=O
HO-C-H
H-C-OH
H-C-OH
2
Aldol cleavage
and first product
(glyceraldehyde-3-P)
is released
HO C-H
CH₂OPO3²-
Glyceraldehyde-
3-phosphate
4
Aldolase enzyme
active site
H₂O
Hydrolysis of the Schiff base
intermediate, and second
product (dihydroxyacetone-P)
is released
O=
H₂O
H-C-OH
H-O-C-H
HO-C-H
CH₂OPO3²-
HO
CH₂OPO3²-
H C
HO-C-H
2-
Nucleophilic attack by
lysine residue of enzym
generates a covalently
bound intermediate
CH₂OPO3²
NH₂
C-H
CH₂OPO3²-
C=O
C-H
CH₂OPO3²-
Protonated Schiff base
intermediate
H
Dihydroxyacetone
phosphate
H
-OH
Figure 9.25 The four-step reaction mechanism of
fructose-1,6-bisphosphate cleavage by the enzyme
aldolase in reaction 4 of glycolysis.

Reaction 4: Cleavage of Fructose-1,6-BP into
Glyceraldehyde-3-P and Dihydroxyacetone-P by Aldolase
The splitting of fructose-1,6-BP into the triose phosphates
glyceraldehyde-3-P and dihydroxyacetone-P is the reaction
that puts the lysis in glycolysis (lysis means "splitting"), as
shown in Figure 9.24. The enzyme responsible for this
cleavage reaction between C-3 and C-4 in fructose-1,6-BP is
aldolase (also called fructose bisphosphate aldolase). In the
context of the glycolytic pathway, aldolase performs the
reverse of an aldol condensation. The mechanism of
cleavage by aldolase (Figure 9.25) involves the formation of
a covalent enzyme-substrate complex through the generation
of a Schiff base requiring a lysine residue in the enzyme
active site. The four-step reaction can be summarized as
follows:
1. The open-chain form of fructose-1,6-BP binds to the
aldolase active site, and a lysine residue in the active
site initiates a nucleophilic attack on the ketose carbon,
generating a covalently bound protonated Schiff base
intermediate.
2. Base abstraction by an active site carboxyl group leads
to C—C (aldol) cleavage between C-3 and C-4 of
fructose, and a covalent enamine intermediate is
formed. The first product, glyceraldehyde-3-P, is
released.
3. Isomerization leads to the formation of a second
protonated Schiff base intermediate.
4. The Schiff base intermediate is hydrolyzed, releasing
the second product, dihydroxyacetone-P.

Expert Solution

Step 1

Aldolase are class of enzyme that performs an aldol reaction (formation of an aldol) or its reverse (breaking of an aldol). Fructose-1,6-bisphosphate aldolase catalyze the reversible reaction, DHAP + glyceraldehyde-3-phosphate $\leftrightarrow$ fructose-1,6-bisphosphate, in the first step of stage 3 in the Calvin Cycle.

Calvin cycle is a light-independent reactions or dark reactions, which is basically a biosynthetic phase or photosynthetic carbon reduction (PCR) cycle occurs in photosynthesis. It contains a series of chemical reactions that converts CO2 and hydrogen-carrier compounds (like ribulose 1,5 bisphosphate) into glucose.