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