Metabolism ii-chp-17-bioc-361-version-dec-2012 - Glycolysis
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Transcript of Metabolism ii-chp-17-bioc-361-version-dec-2012 - Glycolysis
Chapter 17Glycolysis
Mary K. CampbellShawn O. Farrellhttp://academic.cengage.com/chemistry/campbell
Paul D. Adams • University of Arkansas
The Overall Pathway of Glycolysis
• Glycolysis is the first stage of glucose metabolism
• One molecule of glucose is converted to fructose-1,6-bisphosphate, which gives rise to two molecules of pyruvate
• It plays a key role in the way organisms extract energy from nutrients
• Once pyruvate is formed, it has one of several fates
Glycolysis Overview
• The anaerobic oxidation of glucose to give two molecules of pyruvate
• Glucose + 2 ADP + 2 Pi + 2 NAD+ 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O
• Pyruvate used in follow-up reactions to sustain glycolysis
• NADH must be reoxidized so that glycolysis can continue
Fates of Pyruvate From Glycolysis
The Reactions of Glycolysis
• Phosphorylation of glucose to give glucose-6-phosphate
• Isomerization of glucose-6-phosphate to give fructose-6-phosphate
• Phosphorylation of fructose-6-phosphate to yield fructose-1,6-bisphosphate
• Cleavage of fructose-1,6,-bisphosphate to give glyceraldehyde-3-phosphate and dihyroxyacetone phosphate
• Isomerization of dihyroxyacetone phosphate to give glyceraldehyde-3-phosphate
The Reactions of Glycolysis (Cont’d)
• Oxidation of glyceraldehyde-3-phosphate to give 1,3-bisphosphoglycerate
• Transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP to give 3-phosphoglycerate
• Isomerization of 3-phosphoglycerate to give 2-phosphoglycerate
• Dehydration of 2-phosphoglycerate to give phosphoenolpyruvate
• Transfer of a phosphate group from phosphoenolpyruvate to ADP to give pyruvate
Conversion of Glucose to Glyceraldehyde-3-Phosphate (Cont’d)• Fructose-1,6-bisphosphate is split into two 3-carbon
fragments• Reaction catalyzed by aldolase• Side chains of an essential Lys and Cys play key
roles in catalysis
Conversion of Glucose to Glyceraldehyde-3-Phosphate (Cont’d)• In step 5, dihydroxyacetone phosphate (DHAP) is converted
to glyceraldehyde-3-phosphate• These compounds are trioses• This reaction has small +G (0.58kcal/mol-1)• Remember that glycolysis has several reactions that have
very negative G values, and drive other reactions to completion, so that the overall process is negative
Glycolysis Reactions 1 and 2
Reaction 1 (INVESTMENT 1)• Substrate glucoseglucose is phosphorylated by hexokinase• Product is glucose-6-phosphate
• Source of the phosphoryl group is ATP• Expenditure of ATP early in the pathway works as energy
“debt” necessary to get the pathway started• Not reversible
Reaction 2• Product of reaction 1 is rearranged to the structural
isomer fructose-6-phosphate by enzyme phosphoglucose isomerase
• Product has an “exposed” C-1, no longer part of the ring structure• Converts and aldose to a ketose
+ ADP
OCH2
HH
OHH
OH
OH
HOH
HOPO3
2-
Glycolysis: Step 1, 2
Hexokinase
Glucose
+ ATP
OCH2
HH
OHH
OH
OH
HOH
HOH
O
CH2
H
OH
H
H
OHOH
CH2OHOPO3
2-
Phosphoglucoseisomerase
Glucose-6-phosphate
Fructose-6-phosphate
Glycolysis: Step 2
Glycolysis Reaction 3
Reaction 3 (INVESTMENT 2)
• Substrate fructose-6-phosphatefructose-6-phosphate is phosphorylated by phosphofructokinase
• Product is fructose-1,6-bisphosphate• Source of the phosphoryl group is ATP• Again the expenditure of ATP early in the pathway
works as energy “debt” necessary to get the pathway started
• Step 1 and 3 considered the first committed steps of glycolysis, not reversible
Glycolysis: Step 3
+ ATP
O
CH2
H
OH
H
H
OHOH
CH2OOPO3
2-
PO32-
PhosphofructokinaseO
CH2
H
OH
H
H
OHOH
CH2OHOPO3
2-
+ ADP
Fructose-1,6-bisphosphate
Fructose-6-phosphate
Phosphofructokinase is a Key Regulatory Enzyme in glycolysis• Phosphofructokinase (PFK):Phosphofructokinase (PFK):• Exists as a tetramer and subject to allosteric feedback• The tetramer is composed of L and M subunits
• M4, M3L, M2L2, ML3, and L4 all exist. Combinations of these subunits are called isozymes
• Muscles are rich in M4; the liver is rich in L4
• ATP is an allosteric effector; high levels inhibit the enzyme, low levels activate it
• Fructose-1,6-bisphosphate is also an allosteric effector
PFK can exist in Many Isozyme Forms
Glycolysis Reactions 4 and 5
Reaction 4• Product of reaction 3 is split into two 3-carbon
intermediates by the enzyme aldolase forming:• Glyceraldehyde-3-phosphate (substrate of next reaction)• Dihydroxyacetone phosphate
Reaction 5• Dihydroxyacetone phosphateDihydroxyacetone phosphate is rearranged into a
second glyceraldehyde-3-phosphate by the enzyme triose phosphate isomerase• Glyceraldehyde-3-phosphate is the only substrate for the
next reaction
Glycolysis: Steps 4 and 5
O
CH2
H
OH
H
H
OHOH
CH2OOPO3
2-
PO32-
Dihydroxyacetone
phosphate
D-glyceraldehyde-3-phosphate
Aldolase
CH2CCH2
OOOH
PO32-
CHCCH2
OOHO
HPO3
2-
+
Triosephosphate isomerase
CHCCH2
OOHO
HPO3
2-D-glyceraldehyde-3-phosphate
Fructose-1,6-bisphosphate
Glycolysis: Steps 4 and 5
Glycolysis Reaction 6
Reaction 6 (remember this reaction and 7,8,9,10 all happen twice per glucose)
• Substrate glyceraldehyde-3-phosphateglyceraldehyde-3-phosphate is oxidized to a carboxylic acid by glyceraldehyde-3-phosphate dehydrogenase• Reduces NAD+ to NADH (substrate is oxidized, but not oxidative phosphorylation|)
• Transfers an inorganic phosphate group to the carboxyl group
• First step in glycolysis to “harvest” energy (x2)• Product is 1,3-Bisphosphoglycerate
• New phosphate group attached with a “high-energy” bond• This and all subsequent steps occur twice for each G-3-P
Glycolysis: Step 6
CH
C
CH2
O
OH
OPO32-
H
+ NAD+ + HPO4
2-
C
C
CH2
O
OH
OPO32-
H
OPO32-
+ NADH + H+
Glyceraldehyde 3-phosphate
dehydrogenase
Glycerate-1,3-bisphosphate
Glyceraaldehyde-Glyceraaldehyde-3-phosphate3-phosphate
Glycolysis: Step 6
Glycolysis Reactions 7 and 8
Reaction 7• Harvest energy in the form of ATP• 1,3-Bisphosphoglycerate1,3-Bisphosphoglycerate high energy phosphate
group is transferred to ADP by phosphoglycerate kinase:• 3-Phosphoglycerate• ATP
• This is the first substrate level phosphorylation of glycolysis (we are now out of debt!)
Reaction 8• 3-Phosphoglycerate3-Phosphoglycerate is isomerized into 2-
phosphoglycerate by the enzyme phosphoglycerate mutase• Moves the phosphate group from carbon-3 to carbon-2
Glycolysis: Steps 7 and 8
C
C
CH2
O
OH
OPO32-
H
OPO32-
+ ADP
C
C
CH2
O
OH
OPO32-
H
O + ATP
C
C
CH2
O
OPO32-
OH
H
O
Phosphoglyceratekinase
Phosphoglyceratemutase
2-Phosphoglycerate
3-Phosphoglycerate1,3-Bisphosphoglycerate1,3-Bisphosphoglycerate
Glycolysis Reactions 9 and 10
Reaction 9• The enzyme enolase catalyzes dehydration of 2-2-
phospholgyceratephospholgycerate• Phosphoenolpyruvate
• Energy rich – highest energy phosphorylated compound in metabolism
Reaction 10• Final substrate-level dehydration in the pathway• PhosphoenolpyruvatePhosphoenolpyruvate serves as donor of the
phosphoryl group transferred to ADP by pyruvate kinase making ATP and releasing water• Pyruvate is the final product of glycolysis• A coupled reaction in which hydrolysis of the
phosphoester bond provides energy for the formation of the phosphoanhydride bond of ATP
Glycolysis: Steps 9 and 10
C
C
CH2
O
O
O
PO32-
+ H2O
Enolase
“High energy bond”
CC
CH2
O
OPO32-
OH
HO
CCCH3
O
OO
+ ATP
Pyruvate
Phosphoenolpyruvate
Pyruvatekinase
2-Phosphoglycerate2-Phosphoglycerate
Summary
• In the first stages of glycolysis, glucose is converted to two molecules of glyceraldehyde-3-phosphate
• The key intermediate in this series of reactions is fructose-1,6-bisphosphate. The enzyme that catalyzes this reaction, phosphofructokinase, is subject to allosteric control
Anaerobic Metabolism of Pyruvate
• Under anaerobic conditions, the most important pathway for the regeneration of NAD+ is reduction of pyruvate to lactate
• Lactate dehydrogenase (LDH) is a tetrameric isoenzyme consisting of H and M subunits; H4 predominates in heart muscle, and M4 in skeletal muscle
NAD+ Needs to be Recycled to Prevent Decrease in Oxidation Reactions
Fermentation
• From glycolysis pyruvate remains for further degradation and NADH must be reoxidized
• In aerobic conditions, both will occur in cellular respiration (mitochondria, TCA, Ox-phos)
• Under anaerobic conditions, fermentation reactions accomplish this• Fermentation reactions are catabolic reactions
occurring with no net oxidation• Major fermentation pathways:• Lactate fermentation • Alcohol fermentation
Lactate Fermentation
• Lactate fermentation is the anaerobic metabolism that occurs in exercising muscle• Bacteria also use lactate fermentation• Production of yogurt and cheese
• This reaction produces NAD+ and degrades pyruvate (allowing glycolysis to continue)
Alcohol Fermentation
• Yeast ferment sugars of fruit and grains anaerobically, using pyruvate from glycolysis• Pyruvate decarboxylase removes CO2 from the
pyruvatepyruvate producing acetaldehyde• Alcohol dehydrogenase catalyzes reduction of
acetaldehydeacetaldehyde to ethanol, releasing NADH in the process