function of glycolysis?
1) provide energy for cells
2) provide intermediates for other metabolic reactions - like a-glycerophosphate for triglyceride and phoshpolipid synthesis; 2,3 bis-phophoglycerate in RBC; acetyl CoA from pyruvate; amino acids such as serine, alanine, glycine
how do RBC get energy?
ONLY via glycolysis
to what organs is glycolysis the major source of energy (not the only)?
embryonic tissue, retina, adrenals, immune cells, exercising muscle
name the steps of glycolysis
* denotes irreversible step *
*1: GLUCOSE to G6P via hexokinase
2: G6P to F6P via phosphoglucose isomerase
*3: F6P to F1,6BP via PFK-1
4: F1,6BP to DAP + G3P via aldolase
5: DHAP to G3P via isomerase
(everything hereforeward is x2)
5: G3P to 1,3BPG + NADH via G3PDH
6: G3P to 1,3BPG via G3PDH
7: 1,3BPG to 3PG via PG kinase
8: 3PG to 2PG via PG mutase
9: 2PG to PEP via enolase
*10: PEP to Pyruvate via Pyruvate Kinase
what are hexokinases
constituitive enzymes w/ a low Km for glucose
subject to product inhibition by G6P and ADP
not specific for glucose
what is glucokinase?
what are its unique properties?
the hexokinase isoform in the liver and pancreas
- has a high Km for glucose, is specific for glucose
- is not subject to inhibition by G6P and ADP, so will keep working when these products are produced
- its levels are increased at transcriptional level by high carb diet and insulin
- is repressed at transcription level by glucagon/epi
- increases mRNA levels
why is glucokinase > hexokinase for liver function?
because glucokinase has a high Km for glucose, so it only works when glucose levels are high, i.e. after meals
thus liver only will respond to high glucose levels
what kind of reaction is the phosphoglucoisomerase (step 2) reaction?
G6P -> F6P is acid:bases catalysis by an enzyme
what is the rate limiting step of glycolysis?
step 3, by PFK; it is irreversible
involves input of ATP
F6P + ATP -> F1,6 bis P + ADP
what inhibits/stimulates PFK?
INHIBITORS: ATP, citrate, glucagon
ACTIVATORS: AMP, Pi, NH4+ , F1,6 bis P, insulin
is PF-1-K highly regulated?
yes, because it's the rate-limiting step of glycolysis
what is F2,6 bis P ? how is it synthesized/broken down?
powerful activator of PFK
produced from F6P + ATP -> ADP + F2,6bisP -> F6P + Pi
it is created by action of F2,6 bis phosphokinase
it is broken down by F2,6 bis phosphofructophosphatase to F6P + Pi
what is the active/inactive state of the F2,6 bis P kinase/phosphatase?
what is its effect on PFK?
what controls these states?
when F 2,6 bis P kinase is dephoshporylated it is active; when it is phosphorylated and inactive
when F 2,6 bis P phosphatase is dephoshporylated it is inactive; when it is phosphorylated and active
thus PFK is active when F2,6 bis P kinase is dephosphorylated, thus ALLOWING STEP 3 of glycolysis to proceed
this is controlled by insulin, glucagon/epi:
INSULIN promotes the DEPHOSPHORYLATED, ACTIVE state of the kinase, F 2,6 bis P
GLUCAGON/EPI promote the PHOSPHORYLATED, INACTIVE state of the kinase
what are the outcomes of the ALDOLASE reaction?
F 1,6 Bis P -> G3P + DHAP
1) DHAP can -> alpha glycero P, which is needed for triglyceride and phospholipid synthesis
2) or DHAP is converted to G3P by triose phosphate isomerase to continue glycolysis
what is the G3PDH rxn?
what inhibits the G3PDH reaction? why?
STEP 6 of glycolysis
thiol-reacting compounds and heavy metals such as lead, mercury, cadmium
they thus inhibit glycolysis
b/c G3PDH has thiol groups necessary for its activity, it is reactive w/ these compounds
what is the first ATP-producing step of glycolysis?
STEP 7: phosphoglycerate kinase reaction
produces 2 ATPs (1 for each G3P made from the 1 glucose undergoing glycolysis)
1,3 bis PGA + ADP 3 PGA + ATP
occurs b/c 1,3 bis PGA is a high-energy compound
what is substrate level phosphorylation? when in glycolysis does it occur?
production of energy in the form of ATP without mito or oxygen
occurs when 1,3 bis PGA + ADP -> 3PGA + ATP
what does 1,3 bis PGA do for energy of the glycolysis rxn?
it traps energy, allows its release as ATP
if not for 1,3 bis PGA, this energy would ahve been released as heat:
G3P + NAD+ 3PGA + NADH + heat
G3P + Pi + NAD+ NADH + 1,3 bis PGA [add ADP] 3PGA + ATP
why is arsenic toxic?
it competes w/ Pi for G3P, forming 1-arseno-3-phophoglyceric acid which is not a high energy compound like 1,3 bis PGA
this immediately decays to 3PGA + heat, w/o ATP production
what does the RBC mutase do?
moves Pi, converts small amounts of 1,3 bis PGA to 2,3 bis PGA
this is an important allosteric effector of hemoglobin:oxygen affinity
what in glycolysis can produce serine?
what is fluoride's effect on glycolysis?
inhibits enolase, STEP 9
this blocks glycolysis
what is yielded from the final step of glycolysis?
pyruvate kinase catalyzes the high energy PEP to yield 2 ATP
how many ATP are produced from each glucose -> 2 pyruates of glycolysis?
2 ATP used at the beginning in hexokinase, PFK steps
4 ATPs produced, 2 at G3P and 2 at pyruvate kinase step
NET: 2 ATP
what inhibits or activates pyruvate kinase activity?
INHIBITORS: ATP, NADH, acetyl CoA: indicate high energy
STIMULATORS: F1,6 BIS P, the PFK reaction's product
is pyruvate kinase active or inactive when phoshporylated? what causes phosphorylated or dephosphorylated state?
PK is INACTIVE when PHOSPHORYLATED, via GLUCAGON -> CAMP-PKA PHOSPHORYLATION
PK is ACTIVE when DEPHOSPHORYLATED, via INSULIN
how does K+ regulate Pyruvate Kinase activity?
PK needs high intracellular levels of K+ for its activity
this is mediated by the Na+/K+ ATPase
what amino acid can pyruvate produce?
3 irreversible steps of glycolysis?
hexokinase, PFK, PK reactions
how does insulin regulate glycolysis
STIMULATES it by:
1) INCREASING GLUCOKINASE LEVELS
2) ACTIVATING the PFK and PK reactions via DEPHOSPHRYLATIONS
how does glucagon regulate glycolysis
INHIBITS it by
1) DECREASING GLUCOKINASE levels
2) INHIBITING PFK and PK via cAMP-PKA-dependent PHOSHPORYLATIONS
what high energy signals impact PK activity?
what inhibits PK
high energy signals like ATP, NADH, acetyl CoA
how does fructose interact w/ glycolysis?
LIVER acts on F1P via liver aldolase B, producing DHAP + glyceraldehyde
DHAP enters glycolysis
glyceraldehyde is phoshporylated to G3P by triosephosphokinase, enters glycolysis
thus DHAP and G3P from F1P can enter glycolysis and become pyruvate or go opposite way and become glucose
what are liver aldolase B's actions?
1) can cleave F1P in the liver
2) can leave F1,6 bis P in other tissues, thus contribute to glycolysis
what is the cause of hereditary fructose intolerance? what happens?
aldolast B deficiency
these individuals cannot metabolize F1P
F1P accumulates in the liver; a high level of accumulation depletes ATP and is toxic to the liver
how do you treat hereditary fructose intolerance?
limit fructose and sucrose in the diet
what causes lactose intolerance?
deficiency in lactase, the enzyme that hydrolyzes lactose into glucose and galactose
what occurs in galactosemia?
deficiency of galactose-1-phosphate uridylyltransferase
thus cannot convert Galactose-1-P -> Glucose-1-P
Galactose-1-P accumulates, and this is toxic to the liver
treat by: eliminating lactose from diet, which comes from lactose
this is a problem for nursing mothers!
do tumors have rapid or slow rate of glycolysis?
because glycolysis is a rapid way to produce ATP in absence of oxygen and mitochondria which often are poorly coupled or functional in tumors