Chapter 15 Flashcards
1
Q
glycolysis is a sequence of 10 enzyme-catalyzed rxns by which one molecule of … is converted to two molecules of …, with the net production of … and the reduction of 2 … to 2 …
A
glucose; pyruvate; 2 ATP; NAD+; NADH
2
Q
in the first stage of glycolysis, glucose is phosphorylated by …, isomerized by …, phosphorylated by …, and cleaved by … to yield the trioses … and … which are interconverted by … These reactions consume … ATP per glucose
A
hexokinase; phosphoglucose isomerase; phosphofructokinase; aldolase; glyceraldehyde-3-phosphate (GAP); dihydroxyacetone phosphate (DHAP); triose phosphate isomerase (TIM); 2
3
Q
in the second stage of glycolysis, GAP is oxidatively phosphorylated by …, dephosphorylated by … to produce ATP, isomerized by …, dehydrated by …, and dephosphorylated by … to produce a second … and … This stage produces … ATP per glucose for a net yield of … ATP per glucose
A
glyceraldehyde-3-phosphate dehydrogenase (GAPDH); phosphoglycerate kinase (PGK); phosphoglycerate mutase (PGM); enolase; pyruvate kinase; ATP; pyruvate kinase; ATP; pyruvate; 4; 2
4
Q
under anaerobic conditions, pyruvate is reduced to regenerate … for glycolysis. in homolactic fermentation, pyruvate is reversibly reduced to ..
A
NAD+; lactate
5
Q
in alcoholic fermentation, pyruvate is decarboxylated by a … mechanism, and the resulting acetaldehyde is reduced to …
A
thiamine pyrophosphate (TPP)-dependent; ethanol
6
Q
the glycolytic reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase are metabolically …
A
irreversible
7
Q
… is the primary flux control point for glycolysis. ATP inhibition of this allosteric enzyme is relieved by … and …, whose concentrations change more dramatically than those of ATP
A
phosphofructokinase; AMP; ADP
8
Q
the opposing reactions of the fructose-6-phosphate (F6P)/fructose 1,6-bisphosphate (FBP) substrate cycle allow large changes in ..
A
glycolytic flux
9
Q
.., …, and … are enzymatically converted to glycolytic intermediates for catabolism
A
fructose; galactose; mannose
10
Q
in the pentose phosphate pathway, … is oxidized and decarboxylated to produce two …, .., and ..
A
glucose-6-phosphate (G6P); NADPH; CO2; ribulose-5-phosphate (Ru5P)
11
Q
depending on the cell’s needs, ribulose-5-phosphate may be isomerized to … for nucleotide synthesis or converted, via ribose-5-phosphate and xylulose-5-phosphate (Xu5P), to .. and …, which can re-enter the glycolytic pathway
A
ribose-5-phosphate 9R5P); fructose-6-phosphate; glyceraldehyde-3-phosphate
12
Q
the 10-rxn sequence of glycolysis is divided into two stages: … and …
A
energy investment; energy recovery
13
Q
glucose usually appears in the blood as a result of the breakdown of polysaccharides or from its synthesis from noncarbohydrate precursors (…)
A
gluconeogenesis;
14
Q
glycolysis converts glucose to two … (…). the free energy released in the process is harvested to synthesize … from … and l… Thus, glycolysis is a pathway of chemically coupled … reactions
A
C3 units; pyruvate; ATP; ADP; Pi
15
Q
glycolysis can be divided into two stages:
stage 1–> …: in this preparatory stage, the hexose glucose is phosphorylated and cleaved to yield two molecules of the triose … this process consumes …
A
energy investement; glyceraldehyde-3-phosphate; 2 ATP
16
Q
glycolysis can be divided into two stages:
stage 2–> …: the two molecules of glyceraldehyde-3-phosphate are converted to pyruvate, with concomitant generation of … glycolysis therefore has a net profit of … per glucose: stage 1 consumes 2; stage 2 produces ..
A
energy recovery; 4 ATP; 2 ATP; 4
17
Q
Overall rxn of glycolysis is:
glucose + 2 NAD+ + 2 ADP + 2 Pi –>
A
2 pyruvate + 2 NADH + 2 ATP + 2 H2O + 4 H+
18
Q
the NADH formed in glycolysis must be continually … to keep the pathway supplied with its primary oxidizing agent, …
A
reoxidized; NAD+
19
Q
rxn 1 of glycolysis is the transfer of a phosphoryl group from ATP to glucose to form … in a reaction catalyzed by ..
A
glucose-6-phosphate (G6P); hexokinase
20
Q
a kinase is an enzyme that transfers … groups between … and a …
A
ATP; metabolite
21
Q
… is a ubiquitous, relatively nonspecific enzyme that catalyzes the phosphorylation of hexoses such as D-glucose, D-mannose, and D-fructose. Liver cells also contain the isozyme …, which catalyzes the same rxn but which is primary involved in maintaining blood glucose levels
A
hexokinase; glucokinase
22
Q
although we do not always explicitly mention the participation of …, it is essential for kinase activity. it shields the negative charges of the ATP’s alpha and beta or beta and gamma phosphate oxygen atoms, making the gamma phosphorus atom more accessible for nucleophilic attack
A
Mg2+
23
Q
glucose induces a large … in hexokinase
A
conformational change
24
Q
the two lobes that form hexokinase’s active site swing together to engulf the glucose in a manner that suggest the closing of jaws. this movement places the ATP close to the C6H2OH group of glucose and excludes … from the active site (catalysis by …). if the catalytic and reacting groups were in the proper position for reaction while the enzyme was in the open position, … (i.e. phosphoryl group transfer to water, which is thermodynamically favored) would almost certainly be the dominant reaction
A
water; proximity effects; ATP hydrolysis
25
reaction 2 of glycolysis is the conversion of … to … by … This is the isomerization of an aldose to a ketose
G6P; fructose-6-phosphate (F6P); phosphoglucose isomerase
26
a proposed rxn mechanism for step 2 of glycolysis involves general acid-base catalysis by the enzyme:
step 1: the substrate binds
step 2: an enzymatic acid, probably the e-amino group of a conserved Lys residue, catalyzes ..
ring opening
27
a proposed rxn mechanism for step 2 of glycolysis involves general acid-base catalysis by the enzyme:
step 3: a base, thought to be a His imidazole group, abstracts the acidic proton from C2 to form a … intermediate (the proton is acidic because it is alpha to a carbonyl group)
cis-enediolate
28
a proposed rxn mechanism for step 2 of glycolysis involves general acid-base catalysis by the enzyme:
step 4: the proton is replaced on C1 in an overall … Protons abstracted by bases rapidly exchange with solvent proton. this step, though, was confirmed
proton transfer
29
a proposed rxn mechanism for step 2 of glycolysis involves general acid-base catalysis by the enzyme:
step 5: the ring … to form the product, which is subsequently released to yield free enzyme, thereby completing the catalytic cycle
closes
30
in rxn 3 of the glycolysis, … phosphorylates F6P to yield …
the product is a bisphosphate rather than a diphosphate bc its two phosphate groups are not attached directly to each other.
phosphofructokinase; fructose-1,6-bisphosphate
31
phosphofructokinase plays a central role in control of glycolysis bc it catalyzes one of the pathways … rxns
rate-determining
32
aldolase catalyzes reaction 4 of glycolysis, the cleavage of FBP to form the two trioses … and …
glyceraldehyde-3-phosphate (GAP); dihydroxyacetone phosphate (DHAP)
33
rxn 4 is an … (…). the … intermediate is stabilized by resonance
aldol cleavage; retro aldol condensation; enolate
34
aldol cleavage of G6P would yield products of unequal carbon chain length, while aldol cleavage of FBP results in two interconvertible .. that can therefore enter a common … pathway
C3 compounds; degradative
35
(mechanism of aldol cleavage in rxn 4 of glycolysis) Step 1: the substrate FBP binds to the enzyme
Step 2: the FBP carbonyl group reacts with the epsilon-amino group of the active site lys to form an …, that is, a protonated …
iminium cation; Schiff base
36
(mechanism of aldol cleavage in rxn 4 of glycolysis) Step 3: the C3-C4 bond is cleaved, forming an … intermediate and releasing GAP. the iminium ion is a better electron-withdrawing group than the oxygen atom of the precursor carbonyl group. thus, catalysis occurs bc the enamine intermediate is more stable than the corresponding … intermediate of the base-catalyzed … rxn
enamine; enolate; aldol cleavage
37
(mechanism of aldol cleavage in rxn 4 of glycolysis) step 4: … and … of the enamine yield the iminium cation from the Schiff base
Step 5: … of the iminium cation releases DHAP and regenerates the free enzyme
protonation; tautomerization; hydrolysis
38
DHAP and GAP are ketose-aldose isomers that are interconverted by an isomerization rxn with an … intermediate … catalyzes this process in rxn 5 of glycolysis, the final rxn of stage 1 `
enediol (enediolate); triose phosphate isomerase (TIM)
39
Support for rxn 5 of glycolysis comes from the use of the transition state analogs … and …, stable compounds whose geometry resembles that of the proposed enediol or enediolate intermediate
phosphoglycohydroxamate; 2-phosphoglycolate
40
loop closure in the TIM reaction supplies a striking example of the so-called … that enzymes can exert on a rxn. in solution, the enediol intermediate readily breaks down with the elimination of the phosphate at C3 to form the toxic compound … but that rxn is prevented bc the phosphate group is held by a flexible loop. Thus, … ensures that substrate is efficiently transformed to product
stereoelectronic control; methylglyoxal; flexible loop closure
41
TIM was the first protein found to contain an ..
alpha/beta barrel
42
TIM has achieved …: the rate of the biomolecular rxn between enzyme and substrate is diffusion controlled, so product formation occurs as rapidly as enzyme and substrate can collide in solution
catalytic perfection
43
GAP and DHAP are interconverted so efficiently that the concentrations of the two metabolites are maintained at their equilibrium values. however, under steady state conditions in a cell, GAP is consumed in the succeeding rxns of the glycolytic pathway. As GAP is siphoned off in this manner, more DHAP is converted to GAP to ..
maintain the equilibrium ratio
44
rxn 6 of glycolysis is the oxidation and phosphorylation of GAP by NAD+ and Pi as catalyzed by …
In this rxn, aldehyde oxidation, an exergonic rxn, drives the synthesis of the "high energy" acyl phosphate …
glyceraldehyde-3-phosphate dehydrogenase; 1,3-bisphosphoglycerate (1,3-BPG)
45
(GAPDH rxn mechanism)
1. GAPDH is inactivated by … with stoichiometric amounts of iodoacetate. the presence of … in the hydrolysate of the resulting alkylated enzyme suggests that GAPDH has an active site Cys sulfhydryl group.
alkylation; carboxymethylcysteine
46
(GAPDH rxn mechanism) 2. GAPDH quantitatively transfers H from C1 of … to … thereby establishing that this rxn occurs via ...
GAP; NAD+; direct hydride transfer
47
(GAPDH rxn mechanism) 3. GAPDH catalyzes the exchange of … between … and the product analog … such isotope exchange rxns are indicative of an acyl-enzyme intermediate; that is, the acetyl group forms a covalent complex with the enzyme
32P; Pi; acetyl phosphate;
48
(GAPDH rxn mechanism)
Step 1: GAP binds to enzyme
Step 2: the essential sulfhydryl group, acting as a nucleophile, attacks the aldehyde to form a …
thiohemiacetal
49
(GAPDH rxn mechanism)
Step 3: the thiohemiacetal undergoes oxidation to an … by direct hydride transfer to NAD+. This intermediate has a large free energy of hydrolysis. Thus, the energy of aldehyde oxidation has not been dissipated but has been conserved through the synthesis of the … and the reduction of … to …
acyl thioester; thioester; NAD+; NADH
50
(GAPDH rxn mechanism)
Step 4: … binds to the enzyme-thioester-NADH complex
Step 5: the thioester intermediate undergoes nucleophilic attack by Pi to form the high energy mixed anhydride … product, which then dissociates from the enzyme followed by replacement of NADH by another molecule of NAD+ to regenerate the active enzyme
Pi; 1,3-BPG
51
Rxn 7 of the glycolytic pathway yield ATP together with … in a reaction catalyzed by … Physical measurements suggest that, on substrate binding, the two domains of PGK swing together to permit the substrates to react in a water-free environment, as occurs in hexokinase
3-phosphoglycerate; phosphoglycerate kinase
52
… is the common intermediate whose consumption in the pGK reaction "pulls" the … reaction forward
1,3-BPG; GAPDH
53
the production of ATP in the GAPDH rxn, which does not involve .., is an example of ...
O2; substrate-level phosphorylation
54
In rxn 8 of glycolysis, 3PG is converted to … by …
2-phosphoglycerate; phosphoglycerate mutase
55
a … catalyzes the transfer of a functional group from one position to another on a molecule
mutase
56
(catalysis by phosphoglycerate mutase) Step 1: 3PG binds to the phosphoenzyme in which … is phosphorylated
Step 2: the enzyme's phosphoryl group is transferred to the substrate, resulting in an intermediate …
Step 3: the enzyme is rephosphorylated by the substrate's … group
Step 4: release of the product … regenerates the phosphoenzyme
His 8; 2,3-bisphosphoglycerate-enzyme complex; 3-phospho; 2PG
57
in rxn 9 of glycolysis, 2PG is dehydrated to … in a rxn catalyzed by …
phosphoenolpyruvate (PEP); enolase
58
… ion inhibits glycolysis by blocking enolase activity
fluorid
59
in rxn 10 of glycolysis, its final rxn, … couples the free energy of PEP cleavage to the synthesis of ATP during the formation of pyruvate
pyruvate kinase
60
PK rxn occurs as follows:
| step 1: a beta phosphoryl oxygen of ADP nucleophilically attacks the …, thereby displacing … and forming ATP
PEP phosphorus; enolpyruvate
61
the high phosphoryl group transfer potential of PEP reflects the large … on converting the product enolpyruvate to its keto tautomer
release of free energy
62
(products of glycolysis) ATP: the initial invest of 2 ATP per glucose in stage I and the subsequent generation of 4 ATP by substrate-level phosphorylation give a net yield of .. per glucose
2 ATP
63
(products of glycolysis) NADH: during its catabolism by the glycolytic pathway, glucose is oxidized to the extent that … are reduced to …
Under aerobic conditions, electrons pass from reduced coenzymes through a series of electron carriers to the final oxidizing agent, O2, in a process known as … The free energy of electron transport drives the synthesis of ATP from ADP (…)
2 NAD+; 2 NADH; electron transport; oxidative phosphorylation
64
(products of glycolysis) pyruvate: the two pyruvate molecules produced through the partial oxidation of each glucose are still relatively … molecules. under aerobic conditions, complete oxidation of the pyruvate carbon atoms to CO2 is mediated by the … the energy released in that process drives the synthesis of much more ATP than is generated by the limited oxidation of glucose by the glycolytic pathway alone
reduced; citric acid cycle
65
under aerobic conditions, pyruvate is completely oxidized via the .. to … and ..
citric acid cycle; CO2; H2O
66
under anaerobic conditions, pyruvate must be converted to a reduced end product in order to reoxidize the … produced by the GAPDH rxn
NADH
67
anaerobic regeneration of NADH:
Under anaerobic conditions in muscle, pyruvate is reduced to … to regenerate NAD+ in a process known as ...(a fermentation is an anaerobic biological process).
lactate; homolactic fermentation
68
anaerobic regeneration of NADH:
In yeast and certain other microorganisms, pyruvate is decarboxylated to yield CO2 and ..., which is then reduced by NADH to yield NAD+ and ethanol. This process is known as ....
acetaldehyde; alcoholic fermentation
69
Thus, in aerobic glycolysis, NADH acts as a “high-energy” compound, whereas in anaerobic glycolysis, its free energy of oxidation is ....
dissipated as heat
70
In anaerobic conditions in muscle, … catalyzes the oxidation of NADH by pyruvate to yield NAD+ and lactate
lactate dehydrogenase (LDH)
71
the lactate dehydrogenase rxn is freely reversible, so … and … concentrations are readily equilibrated
pyruvate; lactate
72
overall process of anaerobic glycolysis in muscle can be represented as:
Glucose + 2ADP + 2Pi -->
2 lactate + 2 ATP + 2 H2O + 2 H+
73
yeast produces … and ..
ethanol; CO2
74
production of ethanol and CO2 occurs by the following:
1. the decarboxylation of pyruvate to form acetaldehyde and CO2 as catalyzed by …
2. the reduction of acetaldehyde to ethanol by … as catalyzed by … thereby regenerating NAD+ for use in the GAPDH rxn of glycolysis
pyruvate decarboxylase; NADH; alcohol dehydrogenase
75
pyruvate decarboxylase contains the coenzyme …
The enzyme uses TPP because uncatalyzed decarboxylation of an α-keto acid such as pyruvate requires the buildup of negative charge on the carbonyl carbon atom in the transition state, an unstable situation
thiamine pyrophosphate (TPP)
76
TPP's catalytically active functional group is the
thiazolium ring
77
…: dipolar carbanion
ylid
78
(pyruvate decarboxylase mechanism) Step 1: the ylid form of TPP, a nucleophile, attacks the carbonyl carbon of …
Step 2: … departs, generating a resonance-stabilized carbanion adduct in which the thiazolium ring of the coenzyme acts an electron sink
Step 3: the carbanion is …
Step 4: The TPP ylid is eliminated to form … and regenerate the active enzyme
pyruvate
CO2 protonated
acetaldehyde
79
intermediate of pyruvate decarboxylase mechanism: …
hydroxyethylthiamine pyrophosphate
80
The ability of TPP’s thiazolium ring to add to ...groups and act as an electron sink makes it the coenzyme most utilized in α-keto acid decarboxylation reactions.
carbonyl
81
..., the enzyme that converts acetaldehyde to ethanol, is a tetramer, each subunit of which binds one Zn2+ ion.
Yeast alcohol dehydrogenase (YADH)
82
... metabolizes the alcohols anaerobically produced by the intestinal fl ora as well as those from external sources (the direction of the alcohol dehydrogenase reaction varies with the relative concentrations of ethanol and acetaldehyde
Mammalian liver alcohol dehydrogenase (LADH)
83
Under physiological conditions, where the concentrations of reactants and products differ from those of the standard state, these reactions have thermodynamic efficiencies of ...
>50%.
84
This accounts for Pasteur’s observation that yeast consume far more sugar when growing anaerobically than when growing aerobically (the ...).
Pasteur effect
85
the rate of ATP production by anaerobic glycolysis can be up to … faster than that of oxidative phosphorylation. Consequently, when tissues such as muscle are rapidly consuming ATP, they regenerate it almost entirely by ....
100 times; anaerobic glycolysis
86
Homolactic fermentation does not really “waste” glucose since the ...can be aerobically reconverted to glucose by the liver
lactate
87
enzymes that function with large negative free energy changes are candidates for …
flux-control points
88
phosphofructokinase, the major regulatory point for glycolysis in muscle, is allosterically inhibited by … and activated by .. and...
ATP; AMP; ADP
89
(flux control mechanisms)
1. identification of the ...of the pathway by measuring the in vivo delta G for each rxn. enzymes that operate far from equilibrium are potential control pts
2. in vitro identification of … of the enzymes catalyzing the rate-determining rxns. the mechanisms by which these compounds act are determined from their effects on the enzymes' kinetics
3. measurement of the in vivo levels of the proposed … under various conditions to establish whether the concentration changes are consistent with the proposed control mechanism
rate-determining steps
allosteric modifiers
regulators
90
Only three reactions of glycolysis, those catalyzed by ..., ..., and ... function with large negative free energy changes in heart muscle under physiological conditions (Fig. 15-21). These nonequilibrium reactions of glycolysis are candidates for flux-control points.
hexokinase; phosphofructokinase; pyruvate kinase;
91
Consequently, at high concentrations, ATP acts as an ... of PFK by binding to the T state, thereby shifting the T ⇌ R equilibrium in favor of the T state and thus decreasing PFK’s affinity for F6P
allosteric inhibitor
92
Measurements of [ATP] in vivo at various levels of metabolic activity indicate that [ATP] varies ... between rest and vigorous exertion
<10%
93
such equilibrium-like conditions may be imposed on a nonequilibrium reaction if a second enzyme (or series of enzymes) catalyzes the ... from its product in a thermodynamically favorable manner.
regeneration of its substrate
94
Since two different enzymes catalyze the forward (f) and reverse (r) reactions, vf and vr may be ... and vr is no longer negligible compared to vf.
independently varied
95
...(FBPase), however, which is present in many mammalian tissues (and which is an essential enzyme in gluconeogenesis; Section 16-4B), catalyzes the exergonic hydrolysis of FBP
Fructose-1,6-bisphosphatase
96
…: set of opposing rxns that cycle a substrate to an intermediate and back again
substrate cycle
97
..: net result of rxns is the useless consumption of ATP
futile cycle
98
The combined effects of allosteric effectors on the opposing reactions of a substrate cycle can produce a much greater fractional effect on pathway flux (vf − vr) than is possible through ...
allosteric regulation of a single enzyme.
99
Substrate cycling does not increase the maximum flux through a pathway. On the contrary, it functions to … the minimum fl ux. In a sense, the substrate is put into a “holding pattern.”
decrease
100
(pathway for fructose --> glucose) 1. Fructokinase catalyzes the phosphorylation of fructose by ATP at C1 to form .... Neither hexokinase nor PFK can phosphorylate fructose-1-phosphate at C6 to form the glycolytic intermediate FBP.
fructose ; fructose-1-phosphate
101
(pathway for fructose --> glucose) 2. Aldolase (Section 15-2D) has several isozymic forms. Muscle contains Type A aldolase, which is specifi c for FBP. Liver, however, contains Type B aldolase, for which fructose-1-phosphate is also a substrate (Type B aldolase is sometimes called ...). In liver, fructose-1-phosphate therefore undergoes an aldol cleavage:
Fructose-1-phosphate ⇌ dihydroxyacetone phosphate + glyceraldehyde
fructose-1-phosphate aldolase
102
(pathway for fructose --> glucose) 3. Direct phosphorylation of ... by ATP through the action of ... forms the glycolytic intermediate GAP.
glyceraldehyde; glyceraldehyde kinase
103
(pathway for fructose --> glucose) 4. Alternatively, glyceraldehyde is converted to the glycolytic intermediate ..., beginning with its NADH-dependent reduction to glycerol as catalyzed by alcohol dehydrogenase.
DHAP
104
(pathway for fructose --> glucose) 5. ... catalyzes ATP-dependent phosphorylation to produce ...
Glycerol kinase; glycerol-3-phosphate
105
(pathway for fructose --> glucose) 6. DHAP is produced by NAD+-dependent oxidation catalyzed by ...
glycerol phosphate dehydrogenase.
106
(pathway for fructose --> glucose) 7. The DHAP is then converted to GAP by ...
triose phosphate isomerase.
107
galactose and glucose are .. that differ only in their configuration at C4
epimers
108
(galactose to glycolytic intermediate) 1. galactose is phosphorylated at C1 by ATP in a rxn catalyzed by …
2. .. transfers the uridylyl group of UDP-glucose to … to yield … and … by the reversible cleavage of UDP-glucose's pyrophosphoryl bond
galactokinase; galactose-1-phosphate uridylyl transferase; galactose-1-phosphate; glucose-1-phosphate; UDP-galactose
109
(galactose to glycolytic intermediate) 3. ...converts UDP–galactose back to UDP– glucose. This enzyme has an associated NAD+, which suggests that the reaction involves the sequential oxidation and reduction of the hexose C4 atom
UDP–galactose-4-epimerase
110
... is a genetic disease characterized by the inability to convert galactose to glucose.
Galactosemia
111
Mannose enters the glycolytic pathway after its conversion to F6P via a two reaction pathway (Fig. 15-29):
1. Hexokinase recognizes mannose and converts it to ...
2. ... then converts this aldose to the glycolytic intermediate F6P in a reaction whose mechanism resembles that of phosphoglucose isomerase (Section 15-2B).
mannose-6-phosphate; Phosphomannose isomerase;
112
Cells have a second currency besides ATP, ...
reducing power
113
NADPH and HADH are not … Whereas cells capture the free energy of metabolite oxidation as NADH to synthesize ATP (oxidative phosphorylation), cells capture free energy as NADPH for ...
metabolically interchangeable; reductive biosynthesis
114
NADPH is generated by the oxidation of glucose-6-phosphate via an alternative pathway to glycolysis, the ... (also called the ...
pentose phosphate pathway; hexose monophosphate shunt
115
the overall reaction of the pentose phosphate pathway is:
| 3 G6P + 6 NADP+ + 3H2O
6 NADPH + 6 H+ + 3CO2 + 2F6P + GAP
116
pentose phosphate pathway stages:
| Stage 1 Oxidative reactions (Fig. 15-30, Reactions 1–3), which yield ...and ... (Ru5P):
NADPH ; ribulose-5-phosphate
117
pentose phosphate pathway stages:
Stage 2 Isomerization and epimerization reactions (Fig. 15-30, Reactions 4 and 5), which transform Ru5P either to ... (R5P) or to ...(Xu5P)
ribose-5-phosphate; xylulose-5-phosphate
118
pentose phosphate pathway stages:
Stage 3 A series of ... reactions (Fig. 15-30, Reactions 6–8) that convert two molecules of Xu5P and one molecule of R5P to two molecules of F6P and one molecule of GAP.
C—C bond cleavage and formation
119
(steps of NADPH production in pentose phosphate) 1. ... (G6PD) catalyzes net transfer of a hydride ion to NADP+ from C1 of G6P to form …
G6P, a cyclic hemiacetal with C1 in the aldehyde oxidation state, is thereby oxidized to a cyclic ester (lactone). The enzyme is specifi c for NADP+ and is strongly inhibited by NADPH
Glucose-6-phosphate dehydrogenase; 6-phosphoglucono-𝛅lactone:
120
(steps of NADPH production in pentose phosphate) 2. ...increases the rate of hydrolysis of 6-phosphoglucono-δ-lactone to ... (the nonenzymatic reaction occurs at a signifi cant rate).
6-Phosphogluconolactonase ; 6-phosphogluconate
121
(steps of NADPH production in pentose phosphate) 3. ...catalyzes the oxidative decarboxylation of 6-phosphogluconate, a β-hydroxy acid, to Ru5P and CO2 (Fig. 15-31). This reaction is thought to proceed via the formation of a β-keto acid intermediate. The keto group presumably facilitates decarboxylation by acting as an electron sink.
6-Phosphogluconate dehydrogenase
122
(steps of NADPH production in pentose phosphate) Formation of Ru5P completes the oxidative portion of the pentose phosphate pathway. It generates ... for each molecule of G6P that enters the pathway
two molecules of NADPH
123
Ru5P is converted to R5P by ...(Fig. 15-30, Reaction 4) or to Xu5P by ... (Fig
ribulose-5-phosphate isomerase ; ribulose-5-phosphate epimerase
124
The relative amounts of R5P and Xu5P produced from Ru5P depend on the ...
needs of the cell
125
Every three G6P molecules that enter the pathway yield three Ru5P molecules in Stage 1. These three pentoses are then converted to one R5P and two Xu5P (Fig. 15-30, Reactions 4 and 5). The conversion of these three C5 sugars to two C6 sugars and one C3 sugar involves a remarkable “juggling act” catalyzed by two enzymes, ...and ...
transaldolase ; transketolase
126
Flux through the pentose phosphate pathway and thus the rate of NADPH production is controlled by the rate of the ...reaction
glucose-6-phosphate dehydrogenase
