13 - Glucose Metabolism

primary fuel of the brain, in high demand

glucose

form glucose is stored in for plants

starch

form glucose is stored in for animals

glycogen

what is glycolysis

conversion of glucose to pyruvate

overall glycolysis equation

how many reactions in glycolysis? how many phases?

10 reactions, split into 2 phases

first phase of glycolysis rxns, name, what happens

reactions 1-5

energy investment phase

hexose is phosphorylated and cleaved in half

second phase of glycolysis rxns, name, what happens

reactions 6-10

energy payoff phase

three-carbon molecules are converted to pyruvate

glycolysis step 1 name

hexokinase reaction

what does hexokinase reaction look like

hexokinase reaction starting, ending compounds, how it gets there

glucose + ATP → glucose-6-phosphate + ADP

enzyme hexokinase transfers a phosphoryl group from ATP to the C6 OH group of glucose to form glucose-6-phosphate.

what is a kinase

an enzyme that transfers a phosphoryl group from ATP (or another nucleoside triphosphate) to another substance.

free energy change of hexokinase reaction

-16.7 kJ/mol

is hexokinase reaction reversible or irreversible?

metabolically irreversible → prevents glucose from backing out of glycolysis

glycolysis step 2 name

phosphoglucose isomerase reaction

what does phosphoglucose isomerase reaction look like

phosphoglucose isomerase reaction starting, ending compounds, how it gets there

glucose-6-phosphate → fructose-6-phosphate

via phosphoglycose isomerase

is phosphoglucose isomerase reaction reversible or irreversible, why

freely reversible → near-equilibrium reaction

free energy change for phosphoglucose isomerase reaction

2.2 kJ/mol but in vivo -1.4 kJ/mol → around 0, near equilibrium

glycolysis step 3 name

phosphofructokinase reaction

phosphofructokinase reaction starting, ending compounds, how it gets there

fructose-6-phosphate + ATP → fructose -1,6-biphosphate + ADP

via phosphofructokinase

is phosphofructokinase reaction reversible or irreversible

irreversible

what is free energy change for phosphofructokinase reaction

-17.2 kJ/mol

what compound activates phosphofructokinase (in mammals)? what does that do?

fructose-2,6-biphosphate

phosphofructokinase carries out step 3 of glycolysis to make fructose-1,6-biphosphate

in bacteria, how is phosphofructokinase activity regulated?

phosphoenolypyruvate (product of step 9) diminishes its catalytic activity, so when lots of phosphoenolpyruvate is being produced, it can act as a feedback inhibitor to slow down the pathway

citrate is also a feedback inhibitor

when is activity of phosphofructokinase-2 hormonally stimulated?

when the concentration of glucose in the blood is high

what does phosphofructokinase reaction look like

____ is the primary control point for glycolysis, why?

the phosphofructokinase reaction

slowest reaction of glycolysis therefore rate-determining reaction, and operates far from equilibrium (large negative free energy change)

the rate of the phosphofructokinase reaction can be altered by ___ but not by ___

allosteric effectors; fluctuations in the concentrations of its substrates or products

does kcat or Km indicate catalytic speed?

kcat

glycolysis step 4 name

aldolase reaction

aldolase reaction starting, ending compounds, how it gets there

fructose-1,6-biphosphate → dihydroxyacetone phosphate (& glyceraldehyde-3-phosphate)

via aldolase

what does aldolase reaction look like

aldolase reaction is the reverse of what kind of reaction? why does this matter?

it is the reverse of an aldol (aldehyde → alcohol) condensation

that’s why enzyme catalyzing reaction is called aldolase

under what conditions does the aldolase reaction actually occur, impact on free energy change

in vivo, delta G is actually less than zero (while technically it’s +22.8 kJ/mol) since products of the reaction are quickly whisked away by subsequent reactions

important aldolase residues

lysine forms a Schiffbase (imine) with the substrate

aspartate that acts as a base catalyst

cysteine residue

glycolysis step 5 name

triose phosphate isomerase reaction

triose phosphate isomerase reaction starting, ending products, how it gets there

dihydroxyacetone phosphate → glyceraldehyde-3-phosphate

via triose phosphate isomerase

triose phosphate isomerase is an example of a ____, why?

catalytically perfect enzyme; rate is limited only by the rate at which its substrates can diffuse to its active site

standard free energy change for triose phosphate isomerase reaction, why does reaction occur then?

it is slightly positive even under physio conditions (in vivo ~ +4.4 kJ/mol)

reactions proceeds because glyceraldehyde-3-phosphate is quickly consumed in the next reaction

what does triose phosphate isomerase reaction look like

ATP consumed/produced in energy investment & energy payoff

steps 1-5 consumed 2 ATP

steps 6-10 produces 4 ATP

net gain of 2 ATP

glycolysis step 6 name

GAP dehydrogenase reaction

GAP dehydrogenase reaction starting, ending products, how it happen

glyceraldehyde-3-phosphate + NAD+ + Pi → 1,3-biphosphoglycerate

via glyceraldehyde-3-phosphate dehydrogenase

GAP dehydrogenase reaction adds what to substrate?

inorganic phosphate

GAP dehydrogenase reaction is what kind of reaction, what happens?

oxidation-reduction reaction

glyceraldehyde-3-phosphate aldehyde group is oxidized and cofactor NAD+ is reduced to NADH

active site residue involved in GAP dehydrogenase reaction?

cysteine residue

is GAP dehydrogenase reaction reversible or irreversible

reversible

glycolysis step 7 name

phosphoglycerate kinase reaction

phosphoglycerate kinase reaction starting, ending materials, how it gets there

1,3-biphosphoglycerate + ADP → 3-phosphoglycerate + ATP

via phosphoglycerate kinase

free energy release of phosphoglycerate kinase reaction, and what it’s used for

large amount of free energy; used to drive formation of ATP as 1,3-biphosphoglycerate donates its phosphoryl group to ADP

phosphoglycerate kinase reaction looks like

glycolysis step 8 name

phosphoglycerate mutase reaction

phosphoglycerate mutase reaction starting, ending products, how it gets there

3-phosphoglycerate → 2-phosphoglycerate

via phosphoglycerate mutase

is phosphoglycerate kinase rev/irrev

reversible

is phosphoglycerate mutase reaction rev or irrev

reversible

phosphoglycerate mutase reaction what it looks like

phosphoglycerate mutase reaction mech requires enzyme active site that contains a ____

phosphorylated histidine residue

glycolysis step 9 name

enolase reaction

enolase reaction starting, ending materials, how it happens

2-phosphoglycerate → phosphoenolpyruvate + H2O

via enolase

enolase reaction enzyme active site includes ____, which can form a complex with ___ to inhibit the enzyme

Mg2+ ion; fluoride ion & Pi

enolase reaction what it looks like

glycolysis step 10 name

pyruvate kinase reaction

pyruvate kinase reaction starting, ending materials, how it gets there

phosphoenolpyruvate + ADP → pyruvate + ATP

via pyruvate kinase

2 steps of pyruvate kinase reaction

1) ADP attacks phosphoryl group of phosphoenolpyruvate to form ATP & enolpyruvate

2) tautomerization

what is tautomerization

isomerization through the shift of an H atom

which 3 of the 10 glycolysis reactions have large negative free energy changes

reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase

feed-forward activation

once a monosaccharide has entered glycolysis, fructose-1,6-biphosphate helps ensure rapid flux through the pathway

glycolysis reactions producing ATP

7 → phosphoglycerate kinase reaction

10 → pyruvate kinase

what substances can pyruvate be converted to

lactate

acetyl CoA

ethanol + CO2

oxaloacetate

which substance pyruvate is converted to depends on?

depends on the cell type and the need for metabolic free energy and molecular building blocks

During exercise pyruvate can be temporarily converted to

lactate (sort of metabolic dead end)

pyruvate in yeast

Regeneration of NAD+ via alcoholic fermentation

2 steps of alcoholic fermentation

First, pyruvate decarboxylase (an enzyme not present in animals) catabolizes the removal of pyruvate's carboxylate group to produce acetaldehyde.

Next, alcohol dehydrogenase reduces acetaldehyde to ethanol.

Pyruvate can be further oxidized and catabolized to___ via____

CO2 via intermediate of acetyl-CoA

pyruvate → acetyl CoA is substrate for __

citric acid cycle

acetyl CoA from pyruvate can be used to synthesize___

Fatty acids, the precursors of triacylglycerols and many membrane lipids, can be synthesized from the two-carbon units of acetyl-CoA derived from pyruvate

Pyruvate is also the precursor of ____, an intermediate in ____

oxaloacetate; the synthesis of amino acids.

oxaloacetate synthesized by action of ___?

pyruvate carboxylase

how is oxaloacetate synthesized by the action of pyruvate carboxylase?

The pyruvate carboxylase has a biotin prosthetic group that acts as a carrier of CO2. The biotin group is covalently linked to an enzyme lysine residue

what is gluconeogenesis

considered to be the reversal of glycolysis, that is, the conversion of two molecules of pyruvate to one molecule of glucose

gluconeogenesis used where/when?

used in liver to synthesize glucose; when all of liver’s glucose is exhausted, occurs in kidneys

which tissues rely on the liver to supply them with newly synthesized glucose

central nervous system and red blood cells, which burn glucose as their primary metabolic fuel

What enzymes play opposite role to pyruvate kinase, phosphofructokinase, and hexokinase?

Opposite to Hexokinase: Glucose-6-phosphatase

Opposite to Phosphofructokinase-1 (PFK-1): Fructose-1,6-bisphosphatase

Opposite to Pyruvate Kinase: Pyruvate carboxylase, Phosphoenolpyruvate carboxykinase

Pyruvate cannot be converted directly back to phosphoenolpyruvate because pyruvate kinase catalyzes an irreversible reaction, so how does this happen in gluconeogenesis?

pyruvate is carboxylated by pyruvate carboxylase to yield the four-carbon compound oxaloacetate.

Next, phosphoenolpyruvate carboxykinase catalyzes the decarboxylation of oxaloacetate to form phosphoenolpyruvate

pyruvate → oxaloacetate → phosphoenolpyruvate 2 reactions are energetically…? why?

costly

Pyruvate carboxylase consumes ATP, and phosphoenolpyruvate carboxykinase consumes GTP (which is energetically equivalent to ATP)

main sources of gluconeogenic precursors, why

Amino acids (except for leucine and lysine); they can all be converted to oxaloacetate

In mammals, can fatty acids serve as gluconeogenic precursors? why?

no; they cannot be converted to oxaloacetate

what happens in gluconeogenesis when it does glycolysis steps 4–9 in reverse order

Two molecules of phosphoenolpyruvate are converted to one molecule of fructose-1,6-bisphosphate in a series of six reactions that are all catalyzed by glycolytic enzymes

2 phosphoenolpyruvate → 1 fructose-1,6-biphosphate through 6 reactions, these reactions’ characteristics?

reversible because they are near equilibrium (ΔG ≈ 0), and the direction of flux is determined by the concentrations of substrates and products.

phosphoglycerate kinase reaction consumes ___ when it operates in the direction of gluconeogenesis

ATP

first of final 3 glyconeogenesis reactions

undoes the phosphofructokinase reaction (the irreversible reaction that is the major control point of glycolysis)

enzyme fructose bisphosphatase hydrolyzes the C1 phosphate of fructose-1,6- bisphosphate to yield fructose-6-phosphate.

reaction is thermodynamically favorable, with a ΔG value of −8.6 kJ ·mol−1

second of final 3 glyconeogenesis reactions

glycolytic enzyme phosphoglucose isomerase catalyzes the reverse of step 2 of glycolysis to produce glucose-6-phosphate(reversible reaction).

third/final of final 3 glyconeogenesis reactions

gluconeogenic enzyme glucose-6-phosphatase catalyzes a hydrolytic reaction that yields glucose and Pi

hydrolytic reactions catalyzed by fructose bisphosphatase and glucose-6-phosphatase undo the work of ___

two kinases in glycolysis (phosphofructokinase and hexokinase).

Gluconeogenesis is regulated at the ___ step

fructose bisphosphatase step (reaction3 of glycolysis)

regulatory cycle between fructose bisphosphatase (FBPase) and phosphofructokinase (PFK)

when the concentration of fructose-2,6-bisphosphate is high, glycolysis is

___ and gluconeogenesis is ___ and vice versa.

stimulated; inhibited

futile cycle, in relation to glycolysis/glyconeogenesis

When both fructose bisphosphatase (FBPase) and phosphofructokinase (PFK) are active, the net result is the hydrolysis of ATP, so many cells that do not carry out gluconeogenesis do contain the gluconeogenic enzyme fructose bisphosphatase.

dual regulatory effects of glycolysis/glyconeogenesis with PFK/FBPase results in…

greater possible range of net flux than if the regulator merely activated or inhibited a single enzyme (~car’s speed easier to regulate if it has an accelerator and brake)