1.
- low intracellular free [glucose] is low compared with extracellular [glucose]
- glucose enters cells via carrier-mediated facilitated diffusion (GLUT-4 transporter) that is
enhanced in adipose and muscle cells by insulin
- Vmax increased  increase in number of transporters; not a Km change (increased binding
affinity)
- glucose transporters associated with membrane fragments stored in Golgi
- transporters translocated to plasma membrane  fusion; increase in GLUT-4 transporters
- insulin receptor inactivated  excess GLUT-4 returns to Golgi
- Liver doesn’t promote transport of glucose into hepatocytes (no transporters stored in Golgi)
- Liver uses GLUT-2 transporter  high capacity to handle increased blood [glucose] after a
meal
- Insulin indirectly enhanced net inward flux of glucose by converting intracellular glucose to
glucose-6-P through induction of glucokinase
2. Regulation of glucose phosphorylation
- allosteric control of hexokinase (low Km, high affinity); feedback inhibition but its product
glucose-6-P  prevents accumulation of phosphorylated glycolytic intermediates that would
trap phosphate needed for ATP synthesis (excess glucose-g-6-P promotes the storage of
glycogen for the same reason)
- hormonal control of glucokinase (high Km, low affinity); liver and pancreas use glucokinase
ensuring that when glucose is in excess it continues to be metabolized in these tissues
- in liver  insulin induces synthesis of glucokinase in the fed state; involves increased
synthesis of a specific protein by signaling DNA to generate mRNA from the region that encodes
for that protein; induction important for the prevention of hyperglycemia after a large meal
containing many carbs
3. Regulation of phosphofructokinase-1 (PFK-1)
Reaction that opposes PFK-1
- PFK-1 is rate-determining step of glycolysis
- fructose-1,6-BPase catalyzes conversion of fructose-1,6-BP to fructose-6-P; gluconeogenic
pathway in liver; also present in muscle to permit conversion of lactate  glycogen
- no regulation of opposing reaction  energy consumed because fructose-1,6-BPase does not
regenerate ATP consumed by PFK-1
Activation of PFK-1 by AMP/inhibition of PFK-1 by ATP
- energy low in muscle  glycolysis increases
- AMP regulates PFK-1; it signal this need for energy in the muscle cell
- ATP declines  [AMP] increases 100-fold  signal that energy availability is low
- muscle in resting state  [AMP] is low
- AMP also activates glycogenolysis (glycogen  glucose-6-P)
- AMP is an allosteric activator of PFK-1
- when [ATP] is high, [AMP] is low; ATP inhibits PFK-1
- AMP inactivates fructose-1,6-BPase to prevent wasteful use of ATP
Inhibition by acidic conditions
- during anaerobic carbohydrate metabolism in muscle, lactic acid is a product
- excess production of acid  lower intracellular pH; to prevent further decline of cell pH
through production of more acid via metabolism  elevated [H+] concentration inhibits PFK1 to slow acid production
Inhibition by citrate
- citrate is a feedback inhibitor of glycolysis by allosterically inactivating PFK-1
- fed state  citrate derived from glucose carbons for fat production; excess accumulation of
citrate in cytoplasm  glycolysis inhibited to reduce this supply of carbons
- excess citrate serves as a signal for carbon accumulation; activates F-1,6-BPase
Activation by fructose-2,6-BP
- most important regulator of glycolysis in liver is fructose-2,6-BP
- intracellular [fructose-2,6-BP] related to amount of glucose in blood
- blood glucose levels increase after a meal  amount of fructose-2,6-BP increases in parallel
- hepatic glycolysis generates pyruvate that is converted to acetyl CoA (can be used for fatty
acid synthesis  provides a means of storing excess dietary carbohydrates that isn’t glycogen)
- excess glucose in blood after a meal  intracellular liver [fructose-2,6-BP] rises to activate
PFK-1  glycolysis
- fructose-2,6-BP inhibits fructose-1,6-BPase (FBPase-1)  gluconeogenesis
4. Metabolism and regulation fructose-2,6-BP pathway
Pathway
- formation of fructose-2,6-BP catalyzed by PFK-2
- breakdown of fructose-2,6-BP catalyzed by fructose-2,6-BPase (FBPase-2)
- PFK-2 uses fructose-6-P from glycolytic pathway  product of kinase reaction (fructose-2,6BP) is only a regulator of PFK-1 and fructose-1,6-BPase
Regulation under high glucose conditions
- PFK-2 and FBPase-2 involved in fructose-2,6-BP metabolism for a single protein complex
called a bifunctional enzyme
- high glucose in blood after meal  circulating insulin increases  signal in cell that causes
PFK-2 activity to be expressed and FBPase-2 to be suppressed
- fructose-6-P (increased during metabolism of dietary glucose) activates PFK-2; allosteric
effects on bifunctional enzyme
- fructose-6-P inhibits FBPase-2
- glucose and insulin increase in blood  fructose-2,6-BP produced in liver in response to both
hormonal and allosteric regulation  increase in glycolytic rate by allosterically activating
PFK-1 to maximally process dietary glucose in liver
Regulation under low glucose conditions
- when blood glucose falls  increased glucagon in circulation  phosphatase activity
(FBPase-2) is expressed, kinase activity (PFK-2) is suppressed
- active fructose-2,6-BPase (FBPase-2) degrades fructose-2,6-BP so that stimulation of
glycolysis is lost  hepatic use of glucose ceases under conditions when blood glucose is low
5. Regulation of pyruvate kinase
Inhibition by citrate and ATP
- liver glycolysis is regulated by pyruvate kinase; regulation of this enzyme coordinates with
control of PFK-1
- citrate and ATP are allosteric inhibitors of both enzymes
- prevent accumulation of phosphorylated intermediates
Allosteric activation
- pyruvate kinase activated by fructose-1,6-BP and phosphoenolpyruvate (PEP) ensuring
that glycolytic intermediates between PFK-1 and pyruvate kinase are kept at minimal
concentration
- flux through PFK-1 increases  [fructose-1,6-BP] increases  pyruvate kinase activated
Inhibition by alanine
- alanine is the primary amino acid precursor for glucose synthesis
- essential that glycolysis be shut off when liver is synthesizing glucose
- alanine allosterically inhibits pyruvate kinase
6. Regulation of glucagon and insulin of pyruvate kinase via covalent modification
Hormonal regulation
- liver pyruvate kinase activity decreases following its phosphorylation in response to
glucagon
- pyruvate kinase activity increases under high blood glucose conditions because insulin
promotes dephosphorylation of the enzyme
- insulin/high blood glucose  increased activity of liver glycolysis by causing activation of
both PFK-1 (by raising fructose-2,6-BP levels by activating PFK-2) and pyruvate kinase (by
dephosphorylation)
- starvation/low blood glucose  glucagon reverses effects on both enzymes: decreases PFK-1
activity (by decreasing fructose-2,6-BP via activating FBPase-2) and decreases pyruvate
kinase activity (phosphorylation)