Integrated Regulation of Aerobic Glucose Metabolism

• PFK is the Main regulatory enzyme (flux generating step, first committed step)

  • ATP is a strong negative effector of PFK.

  • Inhibition by "product" of pathway rather than enzyme product, as expected for flux generating step.

    • AMP is a strong positive effector for PFK, overcoming ATP effects, used as a "proxy" for ATP energy levels because [ ] changes much greater, thus "amplifying" changes in [ATP].

  • Citrate is a negative effector - citrate is a measure of carbon and flux through TCA Cycle, when [ ] is hi then not all carbon is being burned up in cycle, rather it is building up and available for biosynthesis, so slow down production.

  • F-2,6-bisP is the most potent positive effector (activator) of PFK in the Liver:

    • F-2,6-bisP is synthesized by PFK-2 which in turn is activated by a hormone dependent phosphorylation by Protein Kinase A,

    • Dephosphorylation activates an independent F-2,6-bisPase active site (on a different domain of the same enzyme);

    • The enzyme activities are also allosterically regulated by various metabolites.

• PFK/BisPase "Futile cycle" enhances control in Muscle

  • The flux thru glycolysis can change by over 100x

  • But [AMP] can only account for about 10x

  • PFK rate of 100x and FbisPase 10x

    • Low [AMP] flux = 10-9 = 1 (PFK-BisPase)

    • Hi [AMP] flux = 90-1 = 89

• Hexokinase is regulated indirectly by PFK.

  • Regulated by product inhibition by G-6-P.

    • G-6-P indicator of glucose use by the various pathways leading from it (Glycolysis, Glycogen synthesis/breakdown, Pentose Phosphate Pathway)

  • If PFK shuts down, then G-6-P builds up (if other uses also shut down), turning off HK.

  • If PFK active, then [G-6-P] decreases, allowing HK to resume activity.

• Pyruvate Kinase is also regulated indirectly by PFK. (Note there are other isozymes with differing regulation.)

  • PK shows homotropic allosterism towards its substrate, PEP (sigmoidal kinetics).

  • PFK product, F-1,6-bisP, is a positive feed-forward activator of PK.

    • If PFK is active F-1,6-bisP builds up (free energies of glycolysis between F-6P and PEP favor).

Data from E. A. Newsholme and A. R. Leach Biochemistry for the Medical Sciences. John Wiley & Sons, New York (1983) pp 101 & 110.

Enzyme	Substrate	Substrate Conc. (mM)	Km (mM)	G (kJ/mol)	Effectors
Citrate synthase	acetyl-CoA	100-600	5-10	-53.9	Succinyl CoA (-), ATP (-), NADH (-)
	oxaloacetate	1-10	5-10
Isocitrate DH (NAD+)	isocitrate	150-700	50-200	-17.5	Ca2+ (+), ATP (-), ADP(+), NADH (-)
2-Oxoglutarate DH	2-oxoglutarate	600-5900	60-200	-43.9	Ca2+ (+), Succinyl CoA (-), NADH (-)

© R. A. Paselk 2010; Last modified 3 April 2013