Na+ dependent

intestinal epithelial cells

Na+ independent

RBC, blood-brain barrier

Na+ independent

pancreas, liver, kidney

Na+ independent

neurons (after crossing blood-brain barrier)

Na+ independent

muscle & adipose tissue INSULIN regulated

aerobic: pyruvate, NADH goes to oxidative phosphorylation in mitochondria to create ATP and NAD+

anaerobic: pyruvate is converted to lactate, NADH goes to LDH enzyme in cytosol to create NAD+, which goes into the G-3-P -> 1,3-BPG reaction, then 1,3-BPG -> 2-PG to create ATP

[image]

irreversible reactions regulate the rate of glycolysis

glucose -> glucose-6-phosphate

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

phosphoenolpyruvate -> pyruvate

2 of these irreversible reactions are from phosphorylation (glucose can't leave cell)

irreversible, uses ATP

glucose -> glucose-6-phosphate

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

1st phosphorylation step, irreversible, uses ATP, traps glucose in cell

D-glucose ---hexokinase/glucokinase---> glucose-6-phosphate

also ATP -> ADP

 [image]

doesn't commit to glycolysis, as G-6-P can be used in other pathways

2nd phosphorylation step, irreversible, uses ATP, commits glucose to glycolysis (G-6-P can be used by other pathways, needs conversion to F-6-P then to F-1,6-BP to COMMIT THE MOLECULE to glycolysis)

rate limiting step: regulates overall glycolysis rate

fructose-6-phosphate ---phosphofructokinase-1 (PFK-1)---> fructose-1,6-bisphosphate

also ATP -> ADP

[image]

isoenzymes (same fx): binds glucose and converts to G-6-P

hexokinase important in fasting state: Vmax doesn't have to be high because low [glucose] in fasting state. works faster at lower [glucose]

glucokinase in fed state, liver, pancreas: need higher Vmax because there is a high [glucose] in fed state. works faster at higher [glucose]

[image]

hexokinase is - regulated by G-6-P, which is a product

glucokinase is + regulated by glucose, which is a substrate. glucokinase is - regulated by F-6-P due to it binding to the GKRP (glucokinase regulatory protein). 

Wiki: Two important kinetic properties distinguish glucokinase from the other hexokinases, allowing it to function in a special role as glucose sensor.

Glucokinase has a lower affinity for glucose than the other hexokinases. Glucokinase changes conformation and/or function in parallel with rising glucose concentrations.
Glucokinase is not inhibited by its product, glucose-6-phosphate. This allows continued signal output (e.g., to trigger insulin release) amid significant amounts of its product. These two features allow it to regulate a "supply-driven" metabolic pathway. That is, the rate of reaction is driven by the supply of glucose, not by the demand for end products.

takes blood from spleen, intestines, pancreas

gets nutrient rich blood and filters it before sending it to inferior vena cava back to heart

[image]

Liver glucokinase is responsible for initial quick glucose metabolism, "bulk glucose processing," in the fed state (to prevent hyperglycemia)

Pancreatic glucokinase is responsible for stimulating the quick release of insulin in the fed state (acts as a glucose sensor)

regulated in the liver by the cell's energy state:

Activated by: high AMP (low energy)

Inhibited by: high ATP, citrate (high energy. the cell doesn't need to make more energy)

regulated by glucose levels (fed vs fasting):

Activated by: high insulin/glucagon ratio (insulin levels are high when we have high glucose. pancreas glucokinase stimulating insulin release), fructose-2,6-bisphosphate

Hormones indirectly regulate PFK-1 through the synthesis of fructose 2,6-bisphosphate (F-2,6-P, the main allosteric activator of PFK-1):

• High insulin/glucagon ratio: increased F-2,6-P levels = more active PFK-1
• Low insulin/glucagon ratio: reduced F-2,6-P levels = less active PFK-1

Note: F-2,6-P is produced by PFK-2, that is directly regulated by the hormones.

1. high insulin/glucagon ratio: decreased cAMP (which is made by glucagon signaling G protein)
2. reduced active cAMP dependent protein kinase A
3. decreased protein kinase A activity favors dephosphorylation of bifunctional enzyme phosphofructokinase-2/fructose-bisphosphotase-2 (PFK-2/FBP-2) complex
4. dephosphorylated PFK-2 is active and FBP-2 is inactive
5. active PFK-2 favors formation of fructose-2,6-bisphosphate
6. more fructose-2,6-bisphosphate activates PFK-1's catalysis of fructose-6-phosphate to fructose-1,6-bisphosphate

insulin and glucagon's relationship to

the liver's metabolism of glucose

they have opposite effects on glucose metabolism in liver

insulin: anabolic

• produced by: pancreas β cells
• fed state (i.e high glucose, USE glycolysis, add energy, build glycogen)
• Stimulates glucose use (e.g. glycolysis and glycogen synthesis)
• Downregulates glucose generation (e.g. gluconeogenesis and glycogen degradation/glycogenolysis)
• induces dephosphorylation of metabolic enzymes (insulin receptor substrates, IRC, promotes activation of kinases & phosphatases)

glucagon: catabolic

• produced by: pancreas α cells (think of it as g before i, α before 
• fasting state (i.e. low glucose, MAKE glucose, break down glycogen)
• Stimulates glucose generation (gluconeogenesis and glycogen degradation/glycogenolysis in liver)
• Downregulates glucose use (glycolysis and glycogen synthesis) in liver.
• induces phosphorylation of metabolic enzymes (cAMP activates cAMP dependent kinase A, phosphorylates intracellular proteins)

insulin receptor: tyrosine kinase receptor

kinase: phosphorylates

1. insulin binds alpha-subunit of receptor

-activates beta-subunit of receptor

2. tyrosine residues attached to the beta subunit are phosphorylated Tyr -> Tyr-P)
3. the receptor phosphorylates other proteins (e.g. insulin receptor substrates IRS -> IRS-P)
4. this activates multiple signaling pathways. IRS-P promotes activation of other protein kinases and phosphatases, leading to biologic actions of insulin

(glucose -> receptor -> activates B subunit -> tyrosine residues are auto phosphorylated -> proteins including IRS phosphorylated -> activates other enzyme kinases and phosphatases -> leads to insulin's biologic actions)

insulin's actions:

1. ^ glucose uptake into cell, glycogen synthesis, protein synthesis, fat synthesis
2. v gluconeogenesis (making more glucose), glycgenolysis (breakdown of glycogen), lipolysis (breakdown of fat)
3. changes gene expression

Generally, insulin initiates the dephosphorylation of metabolic enzymes (which changes enzyme activity)

receptor - G protein with GDP bound - inactive adenylyl cyclase

1. glucagon binds to a G-protein coupled receptor

2. glucagon-bound receptor changes conformation
-interacts with G protein
-G protein releases GDP and binds to GTP

3. alpha subunit of G protein dissociates
-activates adenylyl cyclase
-adenylyl cyclase takes ATP and creates cyclic AMP (a second messenger)

4. glucagon unbinds
-receptor in resting state
-GTP on alpha subunit G protein is hydrolyzed to GDP
-adenylyl cyclase is deactivated

[image]

1. cAMP binds to and activates cAMP-dependent protein kinase A
2. cAMP-dependent protein kinase A phosphorylates intracellular proteins.

glucagon initiates phosphorylation of metabolic enzymes, which will alter the activity of these enzymes.

(glucagon -> glucagon receptor -> changes shape & interats w/ G protein -> G protein switches binding GDP to GTP -> activates adenylyl cyclase -> uses ATP to create cAMP -> activates cAMP dependent protein kinase A -> phosphorylation of intracellular proteins)

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

x --PFK-2--> fructose-2,6-bisphosphate

fructose-2,6-bisphosphate activates/upregulates PFK-1 and fructose-1,6-bisphosphate formation

4. PFK-2 will be deactivated by phosphorylation

• if you haven't eaten (fasting), insulin/glucagon ratio is low
• glucagon signaling will create cAMP and protein kinase A
• protein kinase A will phosphorylate intracellular proteins
• PFK-2 is phosphorylated and deactivated
• no fructose-2,6-bisphosphate is created
• PFK-1 will be deactivated
• no fructose-2,6-bisphosphate is created
• glycolysis is halted

PFK-1 is deactivated by high ATP but in this case we have high AMP and low energy which activates PFK-1

fructose-1,6-bisphosphate --aldolase--> glyceraldehyde-3-phosphate (G-3-P) and dihydroxyacetone (interconvertible)

[image]

glyceraldehyde-3-phosphate (2) --glyceraldehyde-3-phosphate dehydrogenase--> 1,3-biphosphoglycerate (2)

use: Pi NAD+ cleave H

products (2): NADH + H⁺, 1,3-biphosphoglycerate

this reaction is inhibited by arsenic poisoning

[image]

1,3-bisphosphoglycerate

-> 2,3-bisphosphoglycerate
|
V
<-> 3-phosphoglycerate

1,3-bisphosphoglycerate (2)
--mutase (unimportant)--> 2,3-bisphosphoglycerate (2)

<--phosphoglycerate kinase (unimportant)--> 3-phosphoglycerate (2)

ADP (2) -> ATP (2)

2,3-bisphosphoglycerate (2) --phosphatase (unimportant) --> 3-phosphoglycerate (2)

H₂O (2) -> Pi-OH (2)

[image]

2,3-bisphosphoglycerate

high concentration in RBC, regulates O2 binding to Hb

makes O₂ binding weaker, shifts O₂ Hb dissociation curve right

• 2,3-BPG lowers Hb affinity for O2
• higher affinity for deoxygenated Hb than oxygenated Hb
• this allows Hb to release O2 at partial pressures in tissues 

[image]

3-phosphatoglycerate -> 2-phosphatoglycerate
->phosphoenolpyruvate -> pyruvate

dephosphorylating step

phosphoenolpyruvate (2) --pyruvate kinase--> pyruvate (2)

ADP -> ATP (2)

irreversible step

+ fructose-1,6-bisphosphate (allosteric activator)

- glucagon (phosphorylation): receptor -> G protein -> GDP to GTP -> alpha subunit activates adenylyl cyclase -> cAMP -> protein kinase A -> phosphorylates and deactivates pyruvate kinase (protein kinase A does the same to PFK-2 and F-2,6-BP production)

+ insulin (dephosphorylation): receptor -> B subunit activated -> tyrosine phosphorylation -> IRS-tyr-P -> phosphoprotein phosphatase activated -> dephosphorylates and reactivates pyruvate kinase

pyruvate <--lactate dehydrogenase--> lactate

reversible

anaerobic glycolysis

LDH:

• function: to provide more NAD+ to drive G-3-P -> 1,3-BPG, and 1,3-BPG -> 3-PG (ATP is a product)
• found in tissues with little vascularization
• found in cells with little/no mitochondria (not much oxidative capacity. no TCA cycle or oxidative phosphorylation)
  eg: RBC (no mitochondria), WBC, fast twitch muscles (overwork), cornea/lens of eye (little vascularization), kidney medulla
• high intensity exercise skeletal muscle
  TCA cycle runs at highest capacity (can't accept more acetyl-CoA)
  pyruvate is converted to lactate and not acetyl-CoA
  lactic acid accumulates in muscle
• collapse of circulatory system (hypoxia)
  due to MI, PE, hemorrhage, shock
  no O2, cells use anaerobic glycolysis (favors pyruvate -LDH-> lactate -> lactic acidosis)
• NAD+ favors pyruvate formation. NADH favors lactate formation. "lactate dehydrogenase" aka lactate has the H

reaction direction determined by ratio of NADH (->lactate) / NAD+ (<-pyruvate)

NAD is a coenzyme (coenzyme: a nonprotein compound that is necessary for the functioning of an enzyme)

[image]

glucose -> sorbitol -> fructose

glucose --aldose reductase (unimportant)--> sorbitol

NADPH + H⁺ --> NADP⁺

sorbitol --sorbitol dehydrogenase (unimportant)--> fructose (-> seminal fluid)

NAD+ --> NADH + H⁺

uncontrolled DM:

• hyperglycemia, glycolysis is saturated
• excess glucose -> sorbitol in eyes, nerves, kidneys
• sorbitol -X-> fructose conversion inefficient in these cells
• sorbitol accumulates and is trapped
• H₂O goes in, swelling, increased osmotic pressure, problems with these cells

glucose -HEXOKINASE/GLUCOKINASE-> glucose-6-phosphate
hexokinase: - glucose-6-phosphate

glucokinase: + glucose -fructose-6-phosphate

fructose-6-phosphate -PFK-1-> fructose-1,6-bisphosphate
+ AMP
- ATP, citrate
+ high insulin/glucagon (less protein kinase A, dephosphorylates and activates enzyme for 2,3-bisphosphoglycerate formation)

phosphoenolpyruvate -PYRUVATE KINASE-> pyruvate
+ fructose-1,6-bisphosphate
- glucagon (-> protein kinase A, phosphorylates & deactivates pyruvate kinase)

+ insulin induces phosphoprotein phosphatase to dephosphorylate and reactivate pyruvate kinase

insulin and glucagon levels regulate transcription of all 3 of these enzymes

causes hemolytic anemia because

• RBC do not have mitochondria (no TCA/oxidative phosphorylation)
• ATP depends on glycolysis
• ATP is necessary for plasma membrane pump function to maintain RBC flexibility
• w/o ATP, cells will have an abnormal shape and are destroyed by macrophages

lactate dehydrogenase (LDH)-> lactate (anaerobic glycolysis)

pyruvate dehydrogenase -> acetyl CoA (TCA cycle, irreversible)

pyruvate carboxylase -> oxaloacetate (TCA cycle, gluconeogenesis, irreversible)

alanine transaminase (ALT)-> alanine (protein synthesis, glucose/alanine cycle)