Glycolysis: Phases, Enzymes, and Glucose Utilization Pathways

What are the two phases of glycolysis?

Preparatory and payoff

What enzymes use up ATP in glycolysis?

hexokinase and phosphofructose kinase

What enzyme produces NADH in glycolysis?

glyceraldehyde 3-phosphate dehydrogenase

What enzymes produce ATP in glycolysis?

phosphoglycerate kinase, pyruvate kinase

What is the importance of glucose?

Used as fuel to produce ATP

Oxidative respiration

Allows for more ATP produced per glucose molecule

What is glucose a precursor for?

- amino acids

- membrane lipid

- nucleotides

- cofactors

4 major pathways of glucose utilization

1) Structural polymers: extracellular matrix and cell wall polysaccharides

2) Storage: glycogen, starch, sucrose

3) oxidation in PPP: Ribose 5-phosphate

4) oxidation in glycolysis: pyruvate

Feeder pathways for glycolysis

D-glucose, D-fructose, D-Mannose, D-galactose

Preparatory phase

First 5 steps: phosphorylation of glucose and its conversion to two high energy 3 carbon sugars

-activation via phosphorylation and primes them for phase 2

Preparatory phase - ATP

Uses up 2 ATP

Payoff Phase

Steps 6-10: Oxidative conversion of two glyceraldehyde 3-phosphates to pyruvate and the coupled formation of ATP and NADH.

Payoff phase - ATP

Produces 4 ATP and 2 NADH

Oxidoreductase

transfer of electrons

-dehydrogenase or oxidase

Transferase

group transfer reactions

-transaminase, kinase, polymerase

Hydrolases

hydrolysis reactions

-peptidase, protease, phosphatase, lipase, esterase

Lyases

cleavage of covalent bonds by elimination

-decarboxylases, aldolases, and syntases

isomerases

group transfer WITHIN a molecule

-isomerase, mutase, epimerase, racemases

Ligases

covalent bond synthesis power by ATP hydrolysis

-synthase, carboxylase, ligase

1) Phosphorylation of glucose

-traps glucose in the cell by tagging it with a charged group

-irreversible

-coupled to ATP hydrolysis so highly favored

1) Phosphorylation of glucose - R&P

Glucose to Glucose 6-phosphate

1) Phosphorylation of glucose - enzyme

hexokinase - transferase (phosphate)

Glucose -> Glucose 6-phosphate

enzyme and step

1) hexokinase - transferase

2) Phosphohexose Isomerization

-converts 6C hexagon ring to a 6C pentagon ring

-preps for symmetrical cleavage

-reversible, regulated by [F6P]

2) Phosphohexose Isomerization - R&P

Glucose 6-phosphate <-> Fructose 6-phosphate

2) Phosphohexose Isomerization - enzyme

phosphohexose isomerase

Glucose 6-phosphate <-> Fructose 6-phosphate

enzyme and step

2) phosphohexose isomerase

3) 2nd Priming Phosphorylation

-another activation via addition of phosphate

-first committed step of glycolysis (must become pyruvate)

-irreversible and favored

3) 2nd Priming Phosphorylation - R&P

Fructose-phosphate → Fructose 1,6-biphosphate

3) 2nd Priming Phosphorylation - enzyme

phosphofructokinase-1 (PFK-1) - transferase

fructose-6-phosphate -> fructose-1,6-bisphosphate

enzyme and step

3) phosphofructokinase-1 (PFK-1) - transferase

4) Aldol Cleavage of F-1, 6bp

• 6 carbon sugar to 2 high energy 3-carbon sugars

• reversible (reverse is aldol condensations) and unfavorable 

4) Aldol Cleavage of F-1, 6bp - R&P

Fructose 1,6-biphosphate ←→ Dihydroxyacetone phosphate + glyceraldehyde 3-phosphate

4) Aldol Cleavage of F-1, 6bp - enzyme

aldolase - lyase

Fructose 1,6-biphosphate ←→ Dihydroxyacetone phosphate + glyceraldehyde 3-phosphate

enzyme and step

4) aldolase - lyase

5) Triose Phosphate Interconversion

• ketone moves from C2 → C1

• matches the 3 carbon sugars so glycolysis applies to both

• completes prep phase

• reversible

5) Triose Phosphate Interconversion - R&P

Dihydroxyacetone phosphate → Glyceraldehyde 3-phosphate

Dihydroxyacetone phosphate → Glyceraldehyde 3-phosphate

enzyme and step

5) triose phosphate isomerase

5) Triose Phosphate Interconversion - enzyme

triose phosphate isomerase

6) Oxidation of GAP

• generation of high energy phosphate compound

• first step of payoff and first energy yielding step: 2 NADH per glucose

• coupled with step 7

6) Oxidation of GAP - R&P

Glyceraldehye 3-phosphate +. inorganic phosphate + NAD⁺ ←→ 1,3-Biphosphoglycerate + NADH + H⁺

Glyceraldehye 3-phosphate +. inorganic phosphate + NAD⁺ ←→ 1,3-Biphosphoglycerate + NADH + H⁺

enzyme and step

6) glyceraldehyde 3-phosphate dehydrogenase - oxidoreductase

6) Oxidation of GAP - enzyme

glyceraldehyde 3-phosphate dehydrogenase - oxidoreductase

7) 1st production of ATP

• Produces 2 ATP per glucose by transferring phosphate group to ADP

• reversible because of coupling with step 6

7) 1st production of ATP - R&P

1,3-Biphosphoglycerate ←→ 3-phosphoglycerate

1,3-Biphosphoglycerate ←→ 3-phosphoglycerate

enzyme and step

7) phosphoglycerate kinase - transferase

7) 1st production of ATP - enzyme

phosphoglycerate kinase - transferase

8) Migration of the phosphate

• swaps C3 phosphate with phosphate on His on enzyme to switch PO3^- group to C2

• generates high energy phosphate compound

• reversible

8) Migration of the phosphate - R&P

3-phosphoglycerate ←→ 2-phosphoglycerate

3-phosphoglycerate ←→ 2-phosphoglycerate

enzyme and step

8) phosphoglycerate mutase - isomerase

8) Migration of the phosphate - enzyme

phosphoglycerate mutase - isomerase

9) Dehydration of 2-PG to PEP

• forms an enol by removing a water

• generate high energy PEP to form a good phosphate donor

• reversible

9) Dehydration of 2-PG to PEP - R&P

2-phosphoglycerate ←→ H2O + phosphoenolpyruvate

2-phosphoglycerate ←→ H2O + phosphoenolpyruvate

enzyme and step

9) enolase - hydrolase

9) Dehydration of 2-PG to PEP - enzyme

enolase - hydrolase

10) 2nd production of ATP

• forms ATP from PEP

• generates 2 ATP per glucose

• keto-enol tautomer intermediate

• irreversible and favored

10) 2nd production of ATP - R&P

phosphoenolpyruvate ←→ Pyruvate

phosphoenolpyruvate ←→ Pyruvate

enzyme and step

10) pyruvate kinase

PEP

phosphoenolpyruvate

10) 2nd production of ATP - enzyme

pyruvate kinase

Substrate-level phosphorylation

When an enzyme transfers a phosphate group from a substrate molecule, often to form an ATP

What is consumed by glycolysis?

1 glucose, 2 ATP, 2 NAD+

What is produced by glycolysis?

2 pyruvate, 4 ATP, 2 NADH

glycolysis reactants

Glucose + 2 NAD⁺ + 2 ADP + P_i

glycolysis products

2 Pyruvate + 2 NADPH + 2H⁺ + 2 ATP

2 possible fates of pyruvate

fermentation & aerobic respiration

What cause fermentation of pyruvate?

no O2

What is the goal of non-oxidative pyruvate fermentation?

NADH + H⁺ → NAD+

Yeast fermentation of pyruvate

ethanol fermentation (produces CO2, NAD+, and ethanol) using

1. pyruvate decarboxylase 

2. alcohol dehydrogenase

Animal fermentation of pyruvate

lactic acid fermentation (produces NAD+ and L-Lactate) using lactate dehydrogenase

Where does glycolysis take place?

muscle and brain

Where does gluconeogensis take place?

Liver

glycolysis v. glucogenesis

different because both contain irreversible rxns. They are catalyzed with different enzymes and have different methods of regulation

“cost“ of gluconeogenesis

4 ATP, 2 GTP, 2 NADH

glucogenic amino acids

amino acids that can be used in gluconeogenesis

ketogenic amino acids

can be converted to acetyl CoA → keto bodies which can be used for energy in fasting conditions

-Both L’s

5 amino acids that are both keto- and glucogenic

PITTT

-Phe, Ile, Tyr, Trp, Thr

Pentose Phospate Pathway (PPP)

Another path for glucose 6-phosphate that produces ribose 5-phosphate, NADPH, and CO2

ribose 5-phosphate

used to make nucleotides and coenzymes

NADPH

important reductant to produce fatty acids and sterols

2 phases of PPP

oxidative and non oxidative

Oxidative phase of PPP

Glucose 6-phosphate to ribose 5-phosphate, also producing NADPH

glucose 6-phosphate dehydrogenase

type and location

oxidoreductase in step 1 of oxidative phase of PPP

glucose 6-phosphate dehydrogenase job

oxidizes alcohol to carbonyl and reduces NADP+ to NADPH + H+

lactonase

type and location

hydrolase in step 2 of oxidative phase of PPP

lactonase job

break the cyclic structure by using water to cleave the ester bond

6-phosphogluconate dehydrogenase

type and location

oxidoreductase in step 3 of oxidative phase of PPP

6-phosphogluconate dehydrogenase job

Reduction reaction to produce NADPH and oxidation of an alcohol to a ketone

-also produces CO2

phosphopentose isomerase

type and location

isomerase in step 4 of oxidative phase of PPP

Phosphopentose isomerase job

convert Ribulose 5-phosphate to Ribose 5-phosphate

Non-oxidative phase of PPP goal

produce Glucose 6-phosphate from Ribose 5-phosphate

how is xyulose 5-phosphate produced from ribose 5-phosphate in non oxidative phase of PPP?

1. isomerase to ribulose 5-phosphate

2. epimerase

how are ribulose 5-phosphate and xyulose 5-phosphate epimers?

C3 chiral center has flipped stereochemistry

general order of PPP non-oxidative phase

pentose: ribose 5- phosphate → intermediates → final hexose: Glucose 6-phosphate

What does PPP and gluconeogensis have in common?

A pentose formed by PPP can be converted to glyceraldehyde 3 and using gluconeogenesis enzymes can form fructose 6-phosphate easily converted to glucose 6-phosphate

2 enzymes specific to non-oxidative PPP

transketolase and transaldolase

transketolase

transferase that transfer a ketone from one molecule to another

-requires TPP

transaldolase

transferase

Transketolase transfers how many carbons?

2 (from ketose donor to aldose acceptor)

Transaldolase transfers how many carbons?

3