8-glycolysis.ppt
DESCRIPTION
Glycolysis Pathway.TRANSCRIPT
![Page 1: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/1.jpg)
Glycolysis
Copyright © 1998-2007 by Joyce J. Diwan. All rights reserved.
Biochemistry of Metabolism
![Page 2: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/2.jpg)
Glycolysis takes place in the cytosol of cells.
Glucose enters the Glycolysis pathway by conversion to glucose-6-phosphate.
Initially there is energy input corresponding to cleavage of two ~P bonds of ATP.
H O
OH
H
OHH
OH
CH2OPO32
H
OH
H
1
6
5
4
3 2
glucose-6-phosphate
![Page 3: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/3.jpg)
H O
O H
H
O HH
O H
CH 2O H
H
O H
H H O
O H
H
O HH
O H
CH 2O PO 32
H
O H
H
23
4
5
6
1 1
6
5
4
3 2
A T P A D P
M g 2+
glucose g lucose -6 -phosphate
H ex ok inase
1. Hexokinase catalyzes:
Glucose + ATP glucose-6-P + ADP
The reaction involves nucleophilic attack of the C6 hydroxyl O of glucose on P of the terminal phosphate of ATP.
ATP binds to the enzyme as a complex with Mg++.
![Page 4: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/4.jpg)
Mg++ interacts with negatively charged phosphate oxygen atoms, providing charge compensation & promoting a favorable conformation of ATP at the active site of the Hexokinase enzyme.
N
NN
N
NH2
O
OHOH
HH
H
CH2
H
OPOPOP O
O
O O
O O
O
adenine
ribose
ATP adenosine triphosphate
![Page 5: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/5.jpg)
The reaction catalyzed by Hexokinase is highly spontaneous.
A phosphoanhydride bond of ATP (~P) is cleaved.
The phosphate ester formed in glucose-6-phosphate has a lower G of hydrolysis.
H O
O H
H
O HH
O H
CH 2O H
H
O H
H H O
O H
H
O HH
O H
CH 2O PO 32
H
O H
H
23
4
5
6
1 1
6
5
4
3 2
A T P A D P
M g 2+
glu co se g lu co se -6 -p h osp h ate
H ex ok in ase
![Page 6: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/6.jpg)
the C6 hydroxyl of the bound glucose is close to the terminal phosphate of ATP, promoting catalysis.
water is excluded from the active site.
This prevents the enzyme from catalyzing ATP hydrolysis, rather than transfer of phosphate to glucose.
glucose
Hexokinase
H O
O H
H
O HH
O H
CH 2O H
H
O H
H H O
O H
H
O HH
O H
CH 2O PO 32
H
O H
H
23
4
5
6
1 1
6
5
4
3 2
A T P A D P
M g 2+
glucose g lucose -6 -phosphate
H ex ok inase
Induced fit:
Glucose binding to Hexokinase stabilizes a conformation in which:
![Page 7: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/7.jpg)
It is a common motif for an enzyme active site to be located at an interface between protein domains that are connected by a flexible hinge region.
The structural flexibility allows access to the active site, while permitting precise positioning of active site residues, and in some cases exclusion of water, as substrate binding promotes a particular conformation.
glucose
Hexokinase
![Page 8: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/8.jpg)
2. Phosphoglucose Isomerase catalyzes:
glucose-6-P (aldose) fructose-6-P (ketose)
The mechanism involves acid/base catalysis, with ring opening, isomerization via an enediolate intermediate, and then ring closure. A similar reaction catalyzed by Triosephosphate Isomerase will be presented in detail.
H O
O H
H
O HH
O H
CH 2 O PO 32
H
O H
H
1
6
5
4
3 2
CH 2 O PO 32
O H
CH 2 O H
H
O H H
H HO
O6
5
4 3
2
1
g lu c o s e - 6 - p h o s p h a te f r u c to s e - 6 - p h o s p h a te P h o s p h o g lu c o s e Is o m e r a s e
![Page 9: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/9.jpg)
3. Phosphofructokinase catalyzes: fructose-6-P + ATP fructose-1,6-bisP + ADP
This highly spontaneous reaction has a mechanism similar to that of Hexokinase.
The Phosphofructokinase reaction is the rate-limiting step of Glycolysis.
The enzyme is highly regulated, as will be discussed later.
CH 2 O PO 32
O H
CH 2 O H
H
O H H
H HO
O6
5
4 3
2
1 CH 2 O PO 32
O H
CH 2 O PO 32
H
O H H
H HO
O6
5
4 3
2
1
A T P A D P
M g 2 +
f r u c t o s e - 6 - p h o s p h a t e f r u c t o s e - 1 , 6 - b i s p h o s p h a t e
P h o s p h o f r u c t o k i n a s e
![Page 10: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/10.jpg)
4. Aldolase catalyzes: fructose-1,6-bisphosphate dihydroxyacetone-P + glyceraldehyde-3-P
The reaction is an aldol cleavage, the reverse of an aldol condensation.
Note that C atoms are renumbered in products of Aldolase.
6
5
4
3
2
1 CH 2 O PO 32
C
C
C
C
CH 2 O PO 32
O
HO H
H O H
H O H
3
2
1
CH 2 O PO 32
C
CH 2 O H
O
C
C
CH 2 O PO 32
H O
H O H+
1
2
3
f ru c to s e -1 ,6 - b is p h o s p h a te
A ld o la s e
d ih y d ro x y a c e to n e g ly c e ra ld e h y d e -3 - p h o s p h a te p h o s p h a te
T rio s e p h o s p h a te Is o m e ra s e
![Page 11: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/11.jpg)
A lysine residue at the active site functions in catalysis.
The keto group of fructose-1,6-bisphosphate reacts with the -amino group of the active site lysine, to form a protonated Schiff base intermediate.
Cleavage of the bond between C3 & C4 follows.
CH2OPO3
2
C
CH
C
C
CH2OPO32
NH
HO
H OH
H OH
(CH2)4 Enzyme
6
5
4
3
2
1
+
Schiff base intermediate of Aldolase reaction
H3N+ C COO
CH2
CH2
CH2
CH2
NH3
H
lysine
![Page 12: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/12.jpg)
5. Triose Phosphate Isomerase (TIM) catalyzes:
dihydroxyacetone-P glyceraldehyde-3-P
Glycolysis continues from glyceraldehyde-3-P. TIM's Keq favors dihydroxyacetone-P. Removal of glyceraldehyde-3-P by a subsequent spontaneous reaction allows throughput.
6
5
4
3
2
1 CH 2 O PO 32
C
C
C
C
CH 2 O PO 32
O
HO H
H O H
H O H
3
2
1
CH 2 O PO 32
C
CH 2 O H
O
C
C
CH 2 O PO 32
H O
H O H+
1
2
3
f ru c to s e -1 ,6 - b is p h o s p h a te
A ld o la s e
d ih y d ro x y a c e to n e g ly c e ra ld e h y d e -3 - p h o s p h a te p h o s p h a te
T rio s e p h o s p h a te Is o m e ra s e
![Page 13: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/13.jpg)
The ketose/aldose conversion involves acid/base catalysis, and is thought to proceed via an enediol intermediate, as with Phosphoglucose Isomerase.
Active site Glu and His residues are thought to extract and donate protons during catalysis.
C
C
CH 2 O PO 32
O
C
C
CH 2 O PO 32
H O
H O H
C
C
CH 2 O PO 32
H O H
O H
H
H O H H + H + H + H +
d i h y d r o x y a c e t o n e e n e d i o l g l y c e r a l d e h y d e - p h o s p h a t e i n t e r m e d i a t e 3 - p h o s p h a t e
T r i o s e p h o s p h a t e I s o m e r a s e
![Page 14: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/14.jpg)
C
CH 2 O PO 32
O O
C
CH 2 O PO 32
HC O
O H
p r o p o s e d e n e d i o l a t e
i n t e r m e d i a t e
p h o s p h o g l y c o l a t e t r a n s i t i o n s t a t e
a n a l o g
2-Phosphoglycolate is a transition state analog that binds tightly at the active site of Triose Phosphate Isomerase (TIM).
This inhibitor of catalysis by TIM is similar in structure to the proposed enediolate intermediate.
TIM is judged a "perfect enzyme." Reaction rate is limited only by the rate that substrate collides with the enzyme.
![Page 15: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/15.jpg)
TIM
Triosephosphate Isomerase structure is an barrel, or TIM barrel.
In an barrel there are 8 parallel -strands surrounded by 8 -helices.
Short loops connect alternating -strands & -helices.
![Page 16: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/16.jpg)
TIM
TIM barrels serve as scaffolds for active site residues in a diverse array of enzymes.
Residues of the active site are always at the same end of the barrel, on C-terminal ends of -strands & loops connecting these to -helices.
There is debate whether the many different enzymes with TIM barrel structures are evolutionarily related.
In spite of the structural similarities there is tremendous diversity in catalytic functions of these enzymes and little sequence homology.
![Page 17: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/17.jpg)
TIM
Explore the structure of the Triosephosphate Isomerase (TIM) homodimer, with the transition state inhibitor 2-phosphoglycolate bound to one of the TIM monomers.
Note the structure of the TIM barrel, and the loop that forms a lid that closes over the active site after binding of the substrate.
C
CH 2 O PO 32
O O
C
CH 2 O PO 32
HC O
O H
p r o p o s e d e n e d i o l a t e
i n t e r m e d i a t e
p h o s p h o g l y c o l a t e t r a n s i t i o n s t a t e
a n a l o g
![Page 18: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/18.jpg)
C
C
CH 2 O PO 32
H O
H O H
C
C
CH 2 O PO 32
O O PO 32
H O H+ P i
+ H +
N A D + N A D H 1
2
3
2
3
1
g l y c e r a l d e h y d e - 1 , 3 - b i s p h o s p h o - 3 - p h o s p h a t e g l y c e r a t e
G l y c e r a l d e h y d e - 3 - p h o s p h a t e D e h y d r o g e n a s e
6. Glyceraldehyde-3-phosphate Dehydrogenase catalyzes:
glyceraldehyde-3-P + NAD+ + Pi
1,3-bisphosphoglycerate + NADH + H+
![Page 19: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/19.jpg)
C
C
CH 2 O PO 32
H O
H O H
C
C
CH 2 O PO 32
O O PO 32
H O H+ P i
+ H +
N A D + N A D H 1
2
3
2
3
1
g l y c e r a l d e h y d e - 1 , 3 - b i s p h o s p h o - 3 - p h o s p h a t e g l y c e r a t e
G l y c e r a l d e h y d e - 3 - p h o s p h a t e D e h y d r o g e n a s e
Exergonic oxidation of the aldehyde in glyceraldehyde- 3-phosphate, to a carboxylic acid, drives formation of an acyl phosphate, a "high energy" bond (~P).
This is the only step in Glycolysis in which NAD+ is reduced to NADH.
![Page 20: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/20.jpg)
A cysteine thiol at the active site of Glyceraldehyde-3-phosphate Dehydrogenase has a role in catalysis.
The aldehyde of glyceraldehyde-3-phosphate reacts with the cysteine thiol to form a thiohemiacetal intermediate.
H3N+ C COO
CH2
SH
H
cysteine
C
C
CH2OPO32
H O
H OH
1
2
3
glyceraldehyde-3- phosphate
![Page 21: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/21.jpg)
The “high energy” acyl thioester is attacked by Pi to
yield the acyl phosphate (~P) product.
CH CH2OPO32
OHEnz-Cys SH
Enz-Cys S CH CH CH2OPO32
OHOH
Enz-Cys S C CH CH2OPO32
OHO
HC
NAD +
NADH
Enz-Cys SH
Pi
C CH CH2OPO32
OHO
O3PO2
O
glyceraldehyde-3-phosphate
1,3-bisphosphoglycerate
thiohemiacetal intermediate
acyl-thioester intermediate
Oxidation to a carboxylic acid (in a ~ thioester) occurs, as NAD+ is reduced to NADH.
![Page 22: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/22.jpg)
Recall that NAD+ accepts 2 e plus one H+ (a hydride) in going to its reduced form.
N
R
H
CN H 2
O
N
R
CN H 2
OH H
+
2 e + H
+
N A D + N A D H
![Page 23: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/23.jpg)
C
C
CH 2 O PO 32
O O PO 32
H O H
C
C
CH 2 O PO 32
O O
H O H
A D P A T P
1
22
3 3
1
M g 2+
1 , 3 - b i s p h o s p h o - 3 - p h o s p h o g l y c e r a t e g l y c e r a t e
P h o s p h o g l y c e r a t e K i n a s e
7. Phosphoglycerate Kinase catalyzes: 1,3-bisphosphoglycerate + ADP 3-phosphoglycerate + ATP
This phosphate transfer is reversible (low G), since one ~P bond is cleaved & another synthesized.
The enzyme undergoes substrate-induced conformational change similar to that of Hexokinase.
![Page 24: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/24.jpg)
C
C
CH 2 O H
O O
H O PO 32
2
3
1C
C
CH 2 O PO 32
O O
H O H2
3
1
3 - p h o s p h o g l y c e r a t e 2 - p h o s p h o g l y c e r a t e
P h o s p h o g l y c e r a t e M u t a s e
8. Phosphoglycerate Mutase catalyzes: 3-phosphoglycerate 2-phosphoglycerate
Phosphate is shifted from the OH on C3 to the OH on C2.
![Page 25: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/25.jpg)
C
C
CH 2 O H
O O
H O PO 32
2
3
1C
C
CH 2 O PO 32
O O
H O H2
3
1
3 - p h o s p h o g l y c e r a t e 2 - p h o s p h o g l y c e r a t e
P h o s p h o g l y c e r a t e M u t a s e
C
C
CH2OPO32
O O
H OPO32
2
3
1
2,3-bisphosphoglycerate
An active site histidine side-chain participates in Pi transfer, by donating & accepting phosphate.
The process involves a 2,3-bisphosphate intermediate.
View an animation of the Phosphoglycerate Mutase reaction.
H3N+ C COO
CH2
CHN
HC NH
CH
H
histidine
![Page 26: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/26.jpg)
9. Enolase catalyzes:
2-phosphoglycerate phosphoenolpyruvate + H2O
This dehydration reaction is Mg++-dependent.
2 Mg++ ions interact with oxygen atoms of the substrate carboxyl group at the active site.
The Mg++ ions help to stabilize the enolate anion intermediate that forms when a Lys extracts H+ from C #2.
C
C
C H 2 O H
O O
H O P O 32
C
C
C H 2 O H
O O
O P O 32
C
C
C H 2
O O
O P O 32
O H
2
3
1
2
3
1
H
2 -p h o s p h o g ly c e r a t e e n o la t e in t e r m e d ia te p h o s p h o e n o lp y r u v a te
E n o la s e
![Page 27: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/27.jpg)
10. Pyruvate Kinase catalyzes:
phosphoenolpyruvate + ADP pyruvate + ATP
C
C
CH 3
O O
O2
3
1A D P A T PC
C
CH 2
O O
O PO 32
2
3
1
p h o s p h o e n o l p y r u v a t e p y r u v a t e
P y r u v a t e K i n a s e
![Page 28: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/28.jpg)
This phosphate transfer from PEP to ADP is spontaneous.
PEP has a larger G of phosphate hydrolysis than ATP.
Removal of Pi from PEP yields an unstable enol, which spontaneously converts to the keto form of pyruvate.
Required inorganic cations K+ and Mg++ bind to anionic residues at the active site of Pyruvate Kinase.
C
C
CH 3
O O
O2
3
1A D P A T PC
C
CH 2
O O
O PO 32
2
3
1 C
C
CH 2
O O
O H2
3
1
p h o s p h o e n o l p y r u v a t e e n o l p y r u v a t e p y r u v a t e
P y r u v a t e K i n a s e
![Page 29: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/29.jpg)
Hexokinase
Phosphofructokinase
glucose Glycolysis
ATP
ADP glucose-6-phosphate
Phosphoglucose Isomerase
fructose-6-phosphate
ATP
ADP fructose-1,6-bisphosphate
Aldolase
glyceraldehyde-3-phosphate + dihydroxyacetone-phosphate
Triosephosphate Isomerase Glycolysis continued
![Page 30: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/30.jpg)
Glyceraldehyde-3-phosphate Dehydrogenase
Phosphoglycerate Kinase
Enolase
Pyruvate Kinase
glyceraldehyde-3-phosphate
NAD+ + Pi
NADH + H+
1,3-bisphosphoglycerate
ADP
ATP
3-phosphoglycerate
Phosphoglycerate Mutase
2-phosphoglycerate H2O
phosphoenolpyruvate
ADP
ATP pyruvate
Glycolysis continued.
Recall that there are 2 GAP per glucose.
![Page 31: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/31.jpg)
Glycolysis
Balance sheet for ~P bonds of ATP:
How many ATP ~P bonds expended? ________
How many ~P bonds of ATP produced? (Remember there are two 3C fragments from glucose.) ________
Net production of ~P bonds of ATP per glucose: ________
2
4
2
![Page 32: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/32.jpg)
Balance sheet for ~P bonds of ATP: 2 ATP expended 4 ATP produced (2 from each of two 3C fragments
from glucose) Net production of 2 ~P bonds of ATP per glucose.
Glycolysis - total pathway, omitting H+: glucose + 2 NAD+ + 2 ADP + 2 Pi 2 pyruvate + 2 NADH + 2 ATP
In aerobic organisms: pyruvate produced in Glycolysis is oxidized to CO2
via Krebs Cycle NADH produced in Glycolysis & Krebs Cycle is
reoxidized via the respiratory chain, with production of much additional ATP.
![Page 33: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/33.jpg)
They must reoxidize NADH produced in Glycolysis through some other reaction, because NAD+ is needed for the Glyceraldehyde-3-phosphate Dehydrogenase reaction.
Usually NADH is reoxidized as pyruvate is converted to a more reduced compound.
The complete pathway, including Glycolysis and the reoxidation of NADH, is called fermentation.
C
C
CH 2 O PO 32
H O
H O H
C
C
CH 2 O PO 32
O O PO 32
H O H+ P i
+ H +
N A D + N A D H 1
2
3
2
3
1
g l y c e r a l d e h y d e - 1 , 3 - b i s p h o s p h o - 3 - p h o s p h a t e g l y c e r a t e
G l y c e r a l d e h y d e - 3 - p h o s p h a t e D e h y d r o g e n a s e
Fermentation:
Anaerobic organisms lack a respiratory chain.
![Page 34: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/34.jpg)
C
C
CH 3
O
O
O
C
H C
CH 3
O
O H
ON A D H + H + N A D +
L a c t a t e D e h y d r o g e n a s e
p y r u v a t e l a c t a t e
E.g., Lactate Dehydrogenase catalyzes reduction of the keto in pyruvate to a hydroxyl, yielding lactate, as NADH is oxidized to NAD+.
Lactate, in addition to being an end-product of fermentation, serves as a mobile form of nutrient energy, & possibly as a signal molecule in mammalian organisms.
Cell membranes contain carrier proteins that facilitate transport of lactate.
![Page 35: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/35.jpg)
C
C
CH 3
O
O
O
C
H C
CH 3
O
O H
ON A D H + H + N A D +
L a c t a t e D e h y d r o g e n a s e
p y r u v a t e l a c t a t e
Skeletal muscles ferment glucose to lactate during exercise, when the exertion is brief and intense.
Lactate released to the blood may be taken up by other tissues, or by skeletal muscle after exercise, and converted via Lactate Dehydrogenase back to pyruvate, which may be oxidized in Krebs Cycle or (in liver) converted to back to glucose via gluconeogenesis
![Page 36: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/36.jpg)
C
C
CH 3
O
O
O
C
H C
CH 3
O
O H
ON A D H + H + N A D +
L a c t a t e D e h y d r o g e n a s e
p y r u v a t e l a c t a t e
Lactate serves as a fuel source for cardiac muscle as well as brain neurons.
Astrocytes, which surround and protect neurons in the brain, ferment glucose to lactate and release it.
Lactate taken up by adjacent neurons is converted to pyruvate that is oxidized via Krebs Cycle.
![Page 37: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/37.jpg)
C
C
CH 3
O
O
O
C
CH 3
O HC
CH 3
OH H
H
N A D H + H + N A D +CO 2
P y r u v a t e A l c o h o l D e c a r b o x y l a s e D e h y d r o g e n a s e
p y r u v a t e a c e t a l d e h y d e e t h a n o l
Some anaerobic organisms metabolize pyruvate to ethanol, which is excreted as a waste product.
NADH is converted to NAD+ in the reaction catalyzed by Alcohol Dehydrogenase.
![Page 38: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/38.jpg)
Glycolysis, omitting H+:
glucose + 2 NAD+ + 2 ADP + 2 Pi
2 pyruvate + 2 NADH + 2 ATP
Fermentation, from glucose to lactate:
glucose + 2 ADP + 2 Pi 2 lactate + 2 ATP
Anaerobic catabolism of glucose yields only 2 “high energy” bonds of ATP.
![Page 39: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/39.jpg)
Glycolysis Enzyme/ReactionGo'
kJ/molG
kJ/mol
Hexokinase -20.9 -27.2
Phosphoglucose Isomerase +2.2 -1.4
Phosphofructokinase -17.2 -25.9
Aldolase +22.8 -5.9
Triosephosphate Isomerase +7.9 negative
Glyceraldehyde-3-P Dehydrogenase& Phosphoglycerate Kinase
-16.7 -1.1
Phosphoglycerate Mutase +4.7 -0.6
Enolase -3.2 -2.4
Pyruvate Kinase -23.0 -13.9*Values in this table from D. Voet & J. G. Voet (2004) Biochemistry, 3rd Edition, John Wiley & Sons, New York, p. 613.
![Page 40: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/40.jpg)
Flux through the Glycolysis pathway is regulated by control of 3 enzymes that catalyze spontaneous reactions: Hexokinase, Phosphofructokinase & Pyruvate Kinase.
Local control of metabolism involves regulatory effects of varied concentrations of pathway substrates or intermediates, to benefit the cell.
Global control is for the benefit of the whole organism, & often involves hormone-activated signal cascades.
Liver cells have major roles in metabolism, including maintaining blood levels various of nutrients such as glucose. Thus global control especially involves liver.
Some aspects of global control by hormone-activated signal cascades will be discussed later.
![Page 41: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/41.jpg)
Hexokinase is inhibited by product glucose-6-phosphate: by competition at the active site by allosteric interaction at a separate enzyme site.
Cells trap glucose by phosphorylating it, preventing exit on glucose carriers.
Product inhibition of Hexokinase ensures that cells will not continue to accumulate glucose from the blood, if [glucose-6-phosphate] within the cell is ample.
H O
O H
H
O HH
O H
CH 2O H
H
O H
H H O
O H
H
O HH
O H
CH 2O PO 32
H
O H
H
23
4
5
6
1 1
6
5
4
3 2
A T P A D P
M g 2+
glu co se g lu co se -6 -p h osp h ate
H ex ok in ase
![Page 42: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/42.jpg)
Glucokinase has a high KM for glucose. It is active only at high [glucose].
One effect of insulin, a hormone produced when blood glucose is high, is activation in liver of transcription of the gene that encodes the Glucokinase enzyme.
Glucokinase is not subject to product inhibition by glucose-6-phosphate. Liver will take up & phosphorylate glucose even when liver [glucose-6-phosphate] is high.
H O
O H
H
O HH
O H
CH 2O H
H
O H
H H O
O H
H
O HH
O H
CH 2O PO 32
H
O H
H
23
4
5
6
1 1
6
5
4
3 2
A T P A D P
M g 2+
glu co se g lu co se -6 -p h osp h ate
H ex ok in ase
Glucokinase is a variant of Hexokinase found in liver.
![Page 43: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/43.jpg)
Glucokinase is subject to inhibition by glucokinase regulatory protein (GKRP).
The ratio of Glucokinase to GKRP in liver changes in different metabolic states, providing a mechanism for modulating glucose phosphorylation.
![Page 44: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/44.jpg)
Glucose-6-phosphatase catalyzes hydrolytic release of Pi from glucose-6-P. Thus glucose is released from the liver to the blood as needed to maintain blood [glucose].
The enzymes Glucokinase & Glucose-6-phosphatase, both found in liver but not in most other body cells, allow the liver to control blood [glucose].
Glycogen Glucose
Hexokinase or Glucokinase
Glucose-6-Pase Glucose-1-P Glucose-6-P Glucose + Pi Glycolysis Pathway
Pyruvate Glucose metabolism in liver.
Glucokinase, with high KM for glucose, allows liver to store glucose as glycogen in the fed state when blood [glucose] is high.
![Page 45: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/45.jpg)
High [glucose] within liver cells causes a transcription factor carbohydrate responsive element binding protein (ChREBP) to be transferred into the nucleus, where it activates transcription of the gene for Pyruvate Kinase.
This facilitates converting excess glucose to pyruvate, which is metabolized to acetyl-CoA, the main precursor for synthesis of fatty acids, for long term energy storage.
C
C
CH 3
O O
O2
3
1A D P A T PC
C
CH 2
O O
O PO 32
2
3
1
p h o s p h o e n o l p y r u v a t e p y r u v a t e
P y r u v a t e K i n a s e
Pyruvate Kinase, the last step Glycolysis, is controlled in liver partly by modulation of the amount of enzyme.
![Page 46: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/46.jpg)
Phosphofructokinase is usually the rate-limiting step of the Glycolysis pathway.
Phosphofructokinase is allosterically inhibited by ATP.
At low concentration, the substrate ATP binds only at the active site.
At high concentration, ATP binds also at a low-affinity regulatory site, promoting the tense conformation.
CH 2 O PO 32
O H
CH 2 O H
H
O H H
H HO
O6
5
4 3
2
1 CH 2 O PO 32
O H
CH 2 O PO 32
H
O H H
H HO
O6
5
4 3
2
1
A T P A D P
M g 2 +
f r u c t o s e - 6 - p h o s p h a t e f r u c t o s e - 1 , 6 - b i s p h o s p h a t e
P h o s p h o f r u c t o k i n a s e
![Page 47: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/47.jpg)
The tense conformation of PFK, at high [ATP], has lower affinity for the other substrate, fructose-6-P. Sigmoidal dependence of reaction rate on [fructose-6-P] is seen.
AMP, present at significant levels only when there is extensive ATP hydrolysis, antagonizes effects of high ATP.
0
10
20
30
40
50
60
0 0.5 1 1.5 2[Fructose-6-phosphate] m M
PFK
Act
ivity
high [A T P]
low [A T P]
![Page 48: 8-glycolysis.ppt](https://reader035.vdocument.in/reader035/viewer/2022062318/552a6ab84a7959756d8b45fa/html5/thumbnails/48.jpg)
Inhibition of the Glycolysis enzyme Phosphofructokinase when [ATP] is high prevents breakdown of glucose in a pathway whose main role is to make ATP.
It is more useful to the cell to store glucose as glycogen when ATP is plentiful.
Glycogen Glucose
Hexokinase or Glucokinase
Glucose-6-Pase Glucose-1-P Glucose-6-P Glucose + Pi Glycolysis Pathway
Pyruvate Glucose metabolism in liver.