Glycolysis and Gluconeogenesis

Glycolysis

What is glycolysis? sequence of reactions that converts one

molecule of glucose to two molecules of pyruvate with the formation of two ATP molecules

anaerobic

Glycolysis

Why is glucose such a commonly used fuel? tends to exist in ring form, very stable,

doesn’t generally glycosylate proteins formed from formaldehyde under prebiotic

conditions

Glycolysis

What are the possible fates of glucose?

What’s the difference between a facultative anaerobe and an obligate anaerobe?

Can you give an example of habitat-dependent anaerobiosis?

What about activity-dependent anaerobiosis?

Glycolysis

Glycolysis

All the intermediates in glycolysis have either 3 or 6 carbon atoms

All of the reactions fall into one of 5 categories phosphoryl transfer phosphoryl shift isomerization dehydration aldol cleavage

Glycolysis

Entire reaction sequence may be divided into three stages glucose is trapped and destabilized six carbon molecule is split into two three

carbon molecules ATP is generated

Glycolysis – Stage 1

glucose converted to glucose-6-PO4

ATP is needed catalyzed by hexokinase or glucokinase induced fit G01= -4.0 kcal/mole

Glycolysis – Stage 1

phosphoglucoisomerase aldose is converted to ketose G01=+0.4 kcal/mole

Glycolysis – Stage 1

rate limiting enzyme – allosteric inhibited by high ATP, citric acid, long-chain fatty acids stimulated by ADP or AMP G01= - 3.4 kcal/mole

Glycolysis

Glycolysis – Stage 2

six carbon molecule split into 2- 3 carbon molecules aldose and ketose

G01=+ 5.73 kcal/mole

Glycolysis – Stage 3

At equilibrium most mixture exists as dihydroxyacetone phosphate

G01=+ 1.83 kcal/mole

Triose Phosphate Isomerase

Glycolysis – Stage 3

redox reaction energy from redox used to form acyl

phosphate G01= +1.5 kcal/mole

Glycolysis – Stage 3

Consists of two coupled processes

Glycolysis – Stage 3

formation of ATP – substrate level phosphorylation

Glycolysis – Stage 3

phosphoryl shift – uses 2,3 bisphosphoglycerate G01= +1.1 kcal/mole

dehydration G01 = +.44 kcal/mole phosphoryl transfer G01 = -7.5 kcal/mole

Glycolysis

Fate of Pyruvate

Alcoholic Fermentation

Which organisms carry out this process? yeast other microorganisms

PDC requires thiamine pyrophosphate as coenzyme NAD+ is regenerated

Lactic Acid Fermentation

Occurs in muscle cells, microorganisms Regenerates NAD+

NAD+ and Dehydrogenases

Various dehydrogenases have a similar binding domain for NAD+ showing their common origin Rossman fold

Glycolysis

How can fructose be used for energy?

Glycolysis

To use galactose it must be converted to glucose-6-PO4

Glycolysis

Glycolysis

What causes lactose intolerance?

Glycolysis

What is galactosemia? inability to metabolize galactose missing galactose 1-phosphate uridyl

transferase liver disease development of cataracts CNS malfunction

Control of Glycolysis

Of what value is glycolysis for cells? provides energy in form of ATP provides building blocks for synthetic reactions

Where are most control points found? enzymes that catalyze irreversible reactions

hexokinase phosphofructokinase pyruvate kinase

Phosphofructokinase

Most important control point in mammalian glycolytic pathway allosteric enzyme

activated by AMP and fructose 2,6 bisphosphate

inhibited by high levels of ATP, citrate, fatty acids

Phosphofructokinase

Hexokinase

Hexokinase is inhibited by its product glucose-6-PO4

glucose remains in blood

Glucokinase, an isozyme of hexokinase is not inhibited by glucose-6-PO4

found in liver has lower affinity for glucose

Pyruvate Kinase

Pyruvate kinase exists as isozymes L form – predominates in liver M form – mostly in muscle and brain

PK is an allosteric enzyme activated by fructose 1,6 bisphosphate inhibited by ATP, alanine

L form of PK influenced by covalent modification inhibited by phosphorylation

Pyruvate Kinase

Glucose Transport

What is the role of glucose transporters in animal cells? facilitate movement of glucose across cell

membrane

What kind of molecule is a transporter and where is it located? small protein embedded in plasma

membrane

Glucose Transport

mammalian glucose transporter

Glucose Transport

Glycolysis and Cancer

Why are rapidly growing tumor cells dependent upon glycolysis? insufficient oxygen supply

What is the function of HIF-1? hypoxia-inducible transcription factor

stimulates synthesis of many glycolytic enzymes and GLUT-1 and 3

also stimulates vascular endothelial growth factor

Gluconeogenesis

What is gluconeogenesis? synthesis of glucose from non-carbohydrate

precursors

Why is this an important pathway? What are some of the major precursors?

lactate, amino acids, glycerol

Where does this process occur? liver, kidney

Gluconeogenesis

If gluconeogenesis involves the conversion of pyruvate to glucose why is it not simply the reverse of glycolysis? glycolysis contains several irreversible reactions

Which reactions in glycolysis are irreversible? phosphoenolpyruvate to pyruvate fructose 6-phosphate to fructose 1,6-

bisphosphate glucose to glucose 6-phosphate

Gluconeogenesis

What is the first reaction?

CH3 CCO2-

O

CH2 CCO2-

O

CO2-

+ CO2+ ATP

+ ADP + Pi

Pyruvate

Oxaloacetate

biotin

pyruvatecarboxylase

Gluconeogenesis

Why is pyruvate carboxylase of special interest? structural properties

contains ATP-grasp domain at N-terminal end contains biotin-binding domain at C-terminal

end

Gluconeogenesis

What is the role of biotin in this reaction? prosthetic group lined to -amino group of lysine

residue carrier of activated carbon dioxide

Gluconeogenesis

Pyruvate carboxylase is an allosteric enzyme activated by acetyl

CoA needed to form

carboxybiotin

Gluconeogenesis

Carboxylation of pyruvate occurs in the mitocondria but next step in reaction sequence occurs in cytosol

Gluconeogenesis

Decarboxylation of oxaloacetate is coupled with

phosphorylation by GTP

enzyme is phosphoenolpyruvate carboxykinase

CH2 = CCO2-

OPO32 -

CH2 CCO2-

O

CO2-

+ CO2

+ GTP

Phosphoenol pyruvate

Oxaloacetate + GDP

Gluconeogenesis

Which other steps in glycolysis are irreversible? conversion of fructose 1,6-bisphosphate to

fructose 6-phosphate conversion of glucose 6-phosphate to

glucose

Gluconeogenesis

Fructose-6-phosphate

C

CH2 OP O32 -

O

HHO

OHH

OHH

CH2 OH

Fructose-1,6-bisphosphate

C

CH2 OP O32 -

O

HHO

OHH

OHH

CH2 OP O32 -

fructose-1,6-bis-phosphatase

H2 O P i

G° = -16.7 kJ mol-1

fructose-1,6-bisphosphatase is an allosteric enzyme, inhibited by AMP and activated by ATP

Gluconeogenesis

Enzyme that catalyzes last reaction not found in all tissues liver and kidney cortex

Gluconeogenesis

Is gluconeogenesis an energetically favorable reaction in the cell?

What drives this reaction?

Are glycolysis and gluconeogenesis active at the same time?

Regulation of Glycolysis and Gluconeogenesis What are some of the factors that

ensure the reciprocal regulation of these processes? allosteric regulators of key enzymes energy charge fructose 2,6-bisphosphate hormones

Regulation of Glycolysis and Gluconeogenesis

Regulation of Glycolysis and Gluconeogenesis fructose 2,6-bisphosphate stimulates

PFK and inhibits fructose 1,6-bisphosphase controlled by insulin and glucagon and

reflects the nutritional status of the cell

Regulation of Glycolysis and Gluconeogenesis How do hormones influence the

enzymes associated with these processes? influence gene expression

change transcription rate influence degradation of m-RNA

– insulin PFK, PK– glucagon PEPCK, fructose 1,6-bisphosphatase

Regulation of Glycolysis and Gluconeogenesis What are substrate

cycles and why are they important? can amplify

metabolic signals can generate heat

Regulation of Glycolysis and Gluconeogenesis What is the Cori cycle and why is it

important?

Regulation of Glycolysis and Gluconeogenesis