Biological Roles of Glycolysis

Liver:

Muscle:

Adipose:

Brain:

RBC's:

Other tissues:

Liver:

• energy source
• provides lipid precursors
• first glycolytic reaction is also first step in glycogen synthesis

Muscle:

• energy source
• first reaction also needed for first step in glycogen synthesis

Adipose:

• energy source
• lipid precursors

Brain:

• almost absolutely for energy 
• lipid precursors

RBC's:

• absolutely required for energy

Other tissues:

• energy source
• lipid precursors

digestion of dietary carbohydrate (glycogen or starch)

*remember that glycogen has alpha 1,4 glycosidic bonds with branch points every 10 sugars linked by alpha 1,6 glycosidic bonds.  Starch has fewer or no such branches.*

• Salivary and pancreatic (blank enzyme) degrade starch and glycogen to the disaccharides (blank) and (blank) OR larger fragments, including (blank) and (blank), and a small amount of glucose.
• other dietary disaccharides include (blank) and (blank)

• alpha-amylase enzyme 
• disaccharides --> maltose (alpha 1,4) and isomaltose (alpha 1,6)
• larger fragments --> Maltotriose and alpha-limit dextrin
• other disaccharides --> sucrose and lactose

digestion of dietary carbohydrate (cont')

Then, at small intestine brush border, the disaccharidases (blank) and (blank), and glucosidases [for maltose, maltotriose and limit dextrins] generate the mono-saccharides (blank), (blank) and (blank)

lactase and sucrase

glucose, galactose, and fructose

[image]

How are mono-saccharides absorbed? - fructose

The concentration of fructose is higher in the (blank) than in the (blank2), and higher in the (blank2) than in the (blank).

So, fructose is simply transported passively by (blank) and (blank), respectively.

lumen

enterocyte

blood stream

Glut5 (lumen -->enterocyte)

Glut2 (enterocyte--> bloodstream)

How are mono-saccharides absorbed? - glucose and galactose

present a different problem - their concentrations are (blank) in lumen than in enterocytes.  

So, they must be co-transported with (blank1) that has a high concentration outside the enterocytes.

The cotransporter protein is called (blank2)

The binding of (blank1) to (blank2) at lumenal membrane allows glucose and galactose to bind.

sugars move against conc gradient - the energy required for this is generated by the active (blank) pump at membrane, which hydrolyzes (blank) to provide the energy to move (blank1) out of the cells

once in the enterocyte, the large conc gradient allows glucose and galactose to move passively into the capillary facilitated by (blank)

lower

sodium

SGLT1

sodium/potassium

ATP

Glut2

[image]

facilitates movement of glucose, galactose, and fructose across capillary membrane only

from within enterocyte --> capillary

lactase deficiency

breaks disaccharide bond

glucose and galactose absorption 

big problems

the glycolytic pathway

under anaerobic conditions (always in RBC's), glycolysis ends in (blank)

otherwise, glycolysis ends in (blank), which is itself further oxidized to (blank), which enters the TCA cycle

lactate

(dont confuse with lactose)

pyruvate

acetyl-CoA

the reactions of glycolysis - list of enzymes (think of the reactions they catalyze)

1. hexokinase (-ATP)
2. phosphoglucose isomerase
3. 6-phosphofructo-1-kinase (-ATP)
4. fructose-bisphosphate aldolase
5. glyceraldehyde 3-phosphate dehydrogenase (+NADH, H)
6. phosphoglycerate kinase (+ATP)
7. phosphoglycerate mutase
8. enolase
9. pyruvate kinase (+ATP)

glycolysis energy yield 

under ANAEROBIC conditions, (#) ATP are used per mole of glucose, and (#) ATP are produced.

Thus, the net yield is (#) ATP/glucose.

Under AEROBIC conditions, the yield is much greater 

2 ATP used

4 ATP made

2 ATP/glucose

glucose transporters

adipose and muscle possess (blank), whose movement to the plasma membrane is stimulated by (blank), thereby increasing intracellular glucose and increasing glycolysis

Glut4

insulin

steps of glycolysis - name the enzyme

1. glucose --?--> glucose-6-phosphate
2. ""---------?--> fructose-6-phosphate
3. ""---------?--> fructose-1,6-bisphosphate
4. ""---------?--> glyceraldehyde 3-phosphate  *Dihydroxyacetone phosphate* --> ^^
5. ""---------?--> 1,3-Bisphosphoglycerate
6. ""---------?--> 3-Phosphoglycerate
7. ""---------?--> 2-Phosphoglycerate
8. ""---------?--> Phosphoenolpyruvate
9. ""---------?--> Pyruvate

1. hexokinase (-ATP)
2. phosphoglucose isomerase
3. 6-phosphofructo-1-kinase (-ATP)
4. fructose-bisphosphate aldolase         **triose phosphate isomerase**
5. glyceraldehyde 3-phosphate dehydrogenase (+NADH, H)
6. phosphoglycerate kinase (+ATP)
7. phosphoglycerate mutase
8. enolase
9. pyruvate kinase (+ATP)

regulation of glucose phosphorylation (first step in glycolysis)

HEXOKINASE 

• it is inhibited by (blank1); this is an example of typical negative feedback
• if PFK-1 and/or PK are inhibited, (blank1) will increase in cell, further inhibiting hexokinase
• this occurs under (blank) conditions (decrease glycolysis!)
• the energy needs of cell are being met by (blank) or (blank) oxidation ---- glucose must be spared for brain and RBC's

• glucose-6-phosphate
• fasting
• fatty acid or ketone body oxidation

regulation of glucose phosphorylation (first step in glycolysis)

GLUCOKINASE (GK)

• It is the glucose-phosphorylating enzyme in (blank1)
• its regulation insures that (blank1) will phosphorylate glucose when blood glucose concentration is (blank).
• when blood glucose is reduced, glucose phosphorylation in (blank1) will (blank)
• must spare glucose for RBC's and brain
• 3 features of GK insure that it is inactive when glucose concentrations decrease and active when they're high

1. GK has (blank) affinity for glucose compared to hexokinase (also in pancreatic beta cell GK); takes more glucose to start GK activity
2. Liver GK is activated by (blank).  This promotes the movment of GK from an inactive nuclear pool to an active cytosolic pool.  Thus, GK is inactive and liver glycolysis is low when glucose is low.
3. Liver GK is induced by (blank); when this is low, GK levels are low

the liver

high

decrease

GK has low affinity for glucose

activated by glucose

induced by insulin

what is rate determining step of glycolysis?

*hint = phosphofructokinase

Regulation of glycolysis via RDS 

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

PFK is inhibited allosterically by (blank) --> shows that energy needs of the cell are being met and thus RDS can slow down.

PFK is activated allosterically by (blank) --> shows that cell needs energy.

Also activated allosterically by (blank1), which is the MOST IMPORTANT allosteric regulator b/c its production is regulated by (blank)s.  --> in fasted state, glucagon action decreases production of (blank1), resulting in inhibition of (blank) in liver.

ATP, citrate, and protons

AMP

fructose 2,6-bisphosphate

(don't confuse w/ fructose 1,6-bisphosphate of glycolysis' RDS product)

regulated by hormones (specifically glucagon)

results in inhibition of glycolysis in liver

diagram of allosteric regulation of phosphofructokinase (enzyme used in RDS of glycolysis) using fructose 2,6-bisphosphate

*fructose 2,6-bisphosphate influenced by what hormone?*

glucagon

[image]

quick summary of regulation of glycolysis

Glut 4 - brings glucose into adipose and muscle cells; stimulated by insulin

Glucokinase

Insulin

AMP -- need for energy

ATP, citrate, and protons

fructose 2,6-bisphosphate (regulated hormonally)

How are levels of fructose 2,6-bisphosphate regulated?

Through a dual (blank/blank) type of enzyme called (blank).

Both activities occur on same protein molecule.

kinase/phosphatase

PFK-2/F2,6-P2ase

(phosphofructo-2-kinase/fructose 2,6-bisphosphatase)

[image]

What is the significance of both kinase and phosphatase activities being present on the same enzyme in PFK-2/F2,6-P2ase?

**When one activity is decreased, the other activity is increased**

This insures that the levels of F2,6-P2 change.

• In the case of glucagon and epinephrine, levels of F2,6-P2 (blank)
• Thus, the PFK-2 kinase activity must (blank) and the F2,6-P2 phosphatase activity must (blank).
• This is achieved by (blank) phosphorylation of the kinase/phosphatase, which (blank)s the phosphatase and (blank)s the kinase.
• This obviously decreases (blank) levels, thus decreasing (blank) activity.

• glucagon/epi DECREASE F2,6-P2
• PFK-2 activity must DECREASE (since it increases F2,6-P2)
• Then, F2,6-P2 phosphatase activity must INCREASE
• PKA phosphorylation of kinase/phosphatase
• decreases F2,6-P2 levels
• decreasing PFK-1 activity = no glycolysis

F2,6-P2 regulation of glucose -

increased glucagon and epinephrine

study mechanism of glucagon and epinephrine's effect on F2,6-P2 regulation

• G-alpha-s gets signal -->
• activates adenylate cyclase -->
• increases cAMP --> 
• activates PKA --> 
• phosphorylates PF-2-K/F2,6-P2ase --> 
• decreases F2,6-P2 by taking Pi from it

F2,6-P2 regulation of glucose -

increased insulin

• When insulin levels increase (start glycolysis), a (blank) is activated, resulting in DE-PHOSPHORYLATION of the dual enzyme [PFK-2/F2,6-P2ase] - opposite of PKA 
• Also, Insulin lowers (blank) levels, which decreases PKA activity, further contributing to dephosphorylation of dual enzyme.
• dephosphorylation of dual enzyme results in (blank) kinase activity and (blank) phosphatase activity.
• This results in (blank) levels of F2,6-P2 = glycolysis is ACTIVATED!!

• phosphoprotein phosphatase (opposite of PKA)
• cAMP levels lowered
• increase kinase activity
• decrease phosphatase activity
• increased levels of F2,6-P2

In muscle, recall that epinephrine (blanks) glycolysis.  It appears to stimulate increase of F2,6-P2 (glycolysis activator).

This occurs because the dual kinase/phosphatase is different in muscle from one in the liver.

The phosphorylation of this different dual enzyme by PKA actually (blanks) the phosphatase activity and (blanks) the kinase activity. 

increases

in reference to what the dual enzyme does in different proteins, remember these mneumonics

take De liver and you could Kill her

Pi in muscle might Kill the hustle

dual enzyme of liver - activated by dephosphorylation by phosphoprotein phosphatase (insulin)

[image]

dual enzyme of muscle - activated by phosphorylation by PKA

[image]

regulation of glycolysis with pyruvate kinase

• In the liver and muscle, PK is inhibited by (blank) and (blank).
• These are signals that energy needs are met by oxidation of other fuels such as (blanks) or (blanks), and that (blank) is underway.
• Thus, liver and muscle curtail their use of (blank) to spare it for brain and RBC's
• In the liver, but not muscle, PK is also phosphorylated and inactivated by (blank) when blood glucose is (blank).
• The PKA in liver is, of course, activated by (blank)
• When blood glucose is high, (blank) stimulates a protein phosphatase that dephosphorylates PK, thereby making it (blank). 

• PK inhibited by ATP and alanine
• phosphorylated PK = INACTIVE
• dephosphorylated PK = ACTIVE
• fatty acids or amino acids
• gluconeogenesis 
• glucose
• PKA when blood glucose is LOW
• insulin 
• ACTIVE

regulation of glycolysis with pyruvate kinase

diagram

dephosphorylated PK = ACTIVE

phosphorylated PK = INACTIVE

PK inhibited by ATP, alanine, and PKA (in liver, not muscle though)

Role of glycolysis in generating [2,3-bisphosphoglycerate] in RBC's

Recall that RBC's require 2,3-BPG in order to (blank).

RBC's posses enzymes that allow 2,3-BPG to be generated from, and converted to glycolytic intermediates.

Basically adds an extra step to glycolysis after formation of 1,3-biphosphoglycerate.

These enzymes are (blank) and (blank)

Need 2,3-BPG to regulate oxygen binding to hemoglobin.

2,3-BPG mutase 

2,3-BPG phosphatase

[image]

Entry of Fructose into Glycolysis

The liver possesses a specific fructose-phosphorlating enzyme called (blank).  

Because this enzyme generates (blank), liver fructolysis bypasses the regulated PFK-1 rate-determining step.

fructokinase

fructose 1-phosphate

[image]

Entry of Galactose into Glycolysis

Use (blank) to phosphorylate galactose, creating galactose 1-phosphate.

galactose 1-phosphate + (blank) = UDP-galactose and glucose 1-phosphate

Main point, galactose transformed into glucose 1-phosphate that is converted to glucose 6-phosphate and thrown into glycolysis

galactose --some shit--> glucose 6-phosphate

DISORDERS OF HEXOSE METABOLISM

Anemia

due to (blank1) deficiency.  

Recall that RBC's lack mitochondria and are forced to rely on glycolysis.  Thus, you would predict that (blank1) deficiency would have a negative effect on RBC's

(blank1) allows for formation of pyruvate, the last step in glycolysis that forms 2 ATP per glucose alone

Without (blank1), 2 ATP will be used and 2 made so no net gain

DISORDERS OF HEXOSE METABOLISM

Warburg Phenomenon

Tumor cells prefer (blank) for energy generation.

*First stages of tumor formation = no blood supply

Tumor cells forced to rely an anaerobic (blank)

They use a transcription factor called (blank) to increase the transcription of genes that encode the glycolytic enzymes.

Blocking this effect could inhibit tumor growth.

glucose

glycolysis

HIF (hypoxia Inducible Factor)

galactokinase

aldose reductase

galactitol

uridyl transferase

fructokinase 

aldolase B

inorganic phosphates

impairs ATP synthesis

glucokinases

toxic to liver

fat

aldolase B deficiency