CHAPTER 16Glycolysis

Hexosestage

Triosestage

Net reaction of Glycolysis

Converts: 1 Glucose

2 pyruvate

- Two molecules of ATP are produced

- Two molecules of NAD+ are reduced to NADH

Glucose + 2 ADP + 2 NAD+ + 2 Pi

2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Hexosestage

Triosestage

Glycolysis can be divided into two stages

4 ATP are produced per glucose

2 ATP are consumed per glucose

NET: 2 ATP produced per glucose

Enzymes are needed to speed up the reactions.

Note the near equilibrium rxns concentration affects flux

Vs.Those that are irreversible

points of regulation

Hexosestage

Triosestage

Enzymes are needed to speed up The reactions.

Note the near equilibrium rxns concentration affects flux

Vs.Those that are irreversible

points of regulation

Hexosestage

Triosestage

Two molecule of NADH are produced in triose stage and are equivalent to several ATP in terms of redox potential.

Enzyme 1 Hexokinase- Transfers the g-phosphoryl of ATP to glucose C-6 oxygen to

generate glucose 6-phosphate

- Mechanism: Mg2+ is an important cofactor, attack of C-6 hydroxyloxygen of glucose on the g-phosphorous of MgATP2- displacing MgADP-

This is an irreversible reaction

Figure 16.2: Enzyme 1 Hexokinase2 lobes show induced fit model

Enzyme 2 Phosphoglucose Isomerase

- Converts glucose 6-phosphate (an aldose) to fructose 6-phosphate (a ketose)

- Enzyme converts glucose 6-phosphate to open chain form of fructose 6-phosphate in the active site.

A near equilibrium reaction

Enzyme 3 Phosphofructokinase-1 (PFK-1)

- Catalyzes transfer of a phosphoryl group from ATP to theC-1 hydroxyl group of fructose 6-phosphate to form fructose 1,6-bisphosphate

- PFK-1 is metabolically irreversible and a critical regulatorypoint for glycolysis in most cells (PFK-1 is the first committedstep of glycolysis.

Enzyme 4 Aldolase

- Aldolase cleaves the hexose, fructose 1,6-bisphosphateinto two triose phosphates: glyceraldehyde 3-phosphate (GAP)and dihydroxyacetone phosphate (DHAP)

This reaction is also near equilibriumbut products are favored

Enzyme 5 Triose Phosphate Isomerase

- Conversion of dihydroxyacetone phosphate (DHAP) into glyceraldehyde 3-phosphate (GAP)

This reaction is also near equilibriumbut products are favored due to GAP metabolism

Fate of carbon atoms from hexose stage to triose stage

Consumed 2 ATP in theprocess - per one glucose Figure 11.7

Enzyme 6 Glyceraldehyde 3-phosphateDehydrogenase

- Conversion of glyceraldehyde 3-phosphate (GAP) to1,3-bisphosphoglycerate (1,3-BPG)

- One molecule of NAD+ is reduced to NADH

NADH can be reoxidized elsewhere, such asthe membrane associated electron transport chain

Oxidation and phosphorylation,yielding a high energy mixedacid hydride (NADH)

Enzyme 6 Glyceraldehyde 3-phosphateDehydrogenase

The coupling of these two reactions make the overall conversionnear equilibrium

Very unfavorablereaction

Enzyme 7 Phosphoglycerate Kinase

- Uses the high-energy compound 1,3-bisphosphoglycerateto transfer a phosphoryl group to ADP to form ATP

- 3-phosphoglycerate is formed in the process

1,3-BPG has more stored chemical energy than ATP

Enzyme 8 Phosphoglycerate Mutase

- Catalyzes transfer of a phosphoryl group from one part of of the substrate molecule to another

- Reaction occurs without input of ATP

Enzyme 8 Phosphoglycerate Mutase

A different phosphate is placedon carbon-2.

Enzyme 9 Enolase: 2-phosphoglycerateto phosphoenolpyruvate (PEP)

- Elimination of water (dehydration) yields PEP

- PEP has a very high phosphoryl group transfer potentialbecause it exists in its unstable enol form

Enzyme 10 Pyruvate Kinase:

- Metabolically irreversible reaction

- This reaction is another site of regulation

- Pyruvate kinase is regulated both allosterically andby covalent modification with a phosphate group

unstable

Triose Stage

4 ATP generatedper one glucose

2 NADH generatedper one glucose

2 glyceraldehyde 3-phosphate molecules

generatedper one glucose

Table 16.1: Irreversible reactions have large negative free energy (DG)

When DG = 0, equilibrium is established

Fates of pyruvate

- For centuries, bakeriesand breweries have exploited the conversionof glucose to ethanol andCO2 by glycolysis in yeast

Metabolism of Pyruvate

1. Aerobic conditions: pyruvate is oxidized to acetyl CoAwhich enters the citric acid cycle for further oxidation to CO2

2. Anaerobic conditions: (microorganisms): pyruvate is convertedto ethanol

3. Anaerobic conditions: (muscles, red blood cells): pyruvateis converted to lactate

4. Amino acid synthesis: pyruvate is a precursor to alanine

Anaerobic Conversion: Pyruvate to Ethanol- Yeast cells convert pyruvate to ethanol and CO2 and oxidize NADH to NAD+.

- Two steps (and enzymes) are required:- Decarboxylation to form acetaldehyde by pyruvate decarboxylase. - Coenzyme thiamine pyrophosphate(TPP) is also needed

- Reduction of acetaldehyde to ethanol by alcohol dehydrogenase. This is coupled with the oxidation of NADH

Redox isbalance

Reduction of Pyruvate to Lactatemuscles - anaerobic

- Muscles lack pyruvate dehydrogenase and cannot produceethanol from pyruvate

- Muscle have lactate dehydrogenase to convert pyruvateto lactate

Reduction of Pyruvate to Lactatemuscles - anaerobic

This reaction regenerates NAD+ for use by glyceraldehyde 3-phosphate dehydrogenase in glycolysis. This will maintain glycolytic flux.

- Lactate formed in skeletal muscles during exercise is transported to the liver via the bloodstream.

- Liver lactate dehydrogenase can convert lactate to pyruvate

- Lactic acidosis can result from insufficient oxygen (an increase in lactic acid and decrease in blood pH)

Figure 16.7 Entry of other sugars into glycolysis

From ingestedfructose

From ingestedfructose

Figure 16.8:Fructose is converted to Glyceraldehyde 3-phosphate

Conversion requires2 ATP molecules sameas the hexose stagefor glucose.

Figure 16.9: Gal-1-P is activated by attachment of a UMP.

Activatedform

phosphoglucomutase

Glu-6P

Regulation of Glycolysis

- Enzymes that are not reversible:Reaction 1 – hexokinaseReaction 3 – phosphofructokinaseReaction 10 – pyruvate kinase

- These steps are metabolically irreversible, and these enzymes are regulated. - All other steps of glycolysis are near equilibrium in cells and not regulated

1. When ATP is needed, glycolysis is activated- Inhibition of phosphofructokinase-1 is relievedby accumulation of AMP or fructose 2,6-bisphosphate- Pyruvate kinase is activated by fructose 1,6-bisphosphate

2. When ATP levels are sufficient, glycolysis activity decreases- Phosphofructokinase-1 is inhibited by ATP and Citrate- Hexokinase is inhibited by glucose 6-phosphate

Glucose transport intothe cell- the first stepin regulation

Transporters are proteins consistingof ~500 AA single chain polypeptide (Integral membrane proteins)

Insulin binding activates GLUT 4 embedded in plasma membrane.

Glucose uptake into skeletal and heart muscles

Regulation of Hexokinase

- Hexokinase reaction is metabolically irreversible

- Glucose 6-phosphate (product) levels increase whenglycolysis is inhibited at sites further along in the pathway

- Glucose 6-phophate inhibits hexokinase

Regulation of Phosphofructokinase-1 (PFK-1)- ATP is a substrate and an allosteric inhibitor of PFK-1

- High concentrations of AMP (and ADP) allosterically activatePFK-1 by relieving the ATP inhibition. ([ATP] does not vary much in cells. [ADP] and [AMP]<< [ATP])

- Elevated levels of citrate in the liver (indicate ample substrates for citric acid cycle) also inhibit phosphofructokinase-1

Figure 16.12

Figure 16.13: Allosteric regulation of hexokinase and PFK

Glycolysis regulation in the LiverPFK-1 by Fructose 2,6-bisphosphate (F-2,6-BP)

Fructose 2,6-bisphosphate is formed form fructose 6-phosphateand allosterically activates PFK-1 to increase its affinity for fructose6-phosphate.

Figure 16.14

PFK-2

PFK-1

Regulation of Liver Pyruvate Kinaseallosteric effectors and covalent modification

The hormone glucagon stimulates a protein kinase A, which phosphorylates pyruvate kinase, converting it to a less active form

ProteinKinase A

Assignment

Read Chapter 15Read Chapter 16