Allosteric Regulation

Review: Adenylate Kinase: an "Important Metabolic Regulator"

ATP + AMP <---> 2 ADP

During intense exercise: large conversion of ATP to ADP with the muscle attempting to phosphorylate ADP back to ATP, this increase in ADP pushes the reaction to the left (mass action effect)

i.e. a 20% decrease in ATP leads to an almost 3-fold increase in ADP and ultimately a 1,400% increase in AMP

The increase in AMP is what's important here......

Because AMP is an allosteric regulator that signals a low energy state in cells --------->

rise in AMP increases metabolism through pathways to restore energy (i.e. glycolysis, citric acid cycle etc.).

Hexokinase / Glucokinase: both irreversible

Glucose + ATP ----> Glucose-6-phosphate + ADP

Hexokinase:

Catalyzes the phosphorylation of hexoses in general and is found in all cells that metabolize glucose.

Has a low Km (high affinity, strong binding) so that it is active even at low glucose concentrations.

Feedback inhibited by its product glucose-6-phosphate. Prevents build-up of glycolytic intermediates and the unnecessary hydrolysis of ATP.

Glucokinase:

Glucose specific, found in liver only. Synthesis induced by insulin.

Has a high Km (low affinity,weak binding) to ensure an appropriate response to elevation of glucose from the diet, provides kinetic control.

High Vmax (rapid phosphorylation of glucose).

Phosphofructokinase-1 (PFK-1): glycolysis, irreversible

Fructose-6-phosphate + ATP ------> Fructose-1,6-bisphosphate + ADP

Rate-Limiting, Major Regulated Step.

The concentration of AMP increases when energy is low. Excess phosphate also signals low energy (via increase in ATP use). -------> glycolysis must increase to meet energy demands by metabolizing glucose.

Allosterically (+) AMP, F-2,6-bisP, phosphate

Allosterically (-) ATP

Fructose 1,6-bisphosphatase: gluconeogenesis, irreversible

Fructose-1,6-bisphosphate ------> Fructose-6-phosphate

Can form a futile cycle with phosphofructokinase-1 (PFK-1)

The two enzymes are reciprocally regulated or ATP would be lost without energy conservation. Prevents futile cycling (like using a stationary bike).

Allosterically (+) citrate (high flux of carbons from glucose to the citric acid cycle requiring decreased glycolysis and increased gluconeogenesis).

Allosterically (-) AMP, Fructose-2,6-bisP

Fructose-2,6-bisphosphate:

Formation: Phosphofructokinase-2 (PFK-2)

Breakdown: Fructose-2,6 bisphosphatase (FBPase-2)

PFK-2 is NOT a glycolytic enzyme.

It does use some Fructose-6-P from the pathway, but Fructose-2,6-bisP is strictly an allosteric regulator of PFK-1.

High Glucose: dephosphorylated ----> PFK-2 active ----> Fructose-2,6-bisP produced ----> (+) glycolysis, (-) gluconeognesis.

Low Glucose: phosphorylation ----> FBPase-2 ----> Fructose-2,6-bisP removed ----> (+) gluconeogenesis, glycolysis (-)

Pyruvate Kinase: "Substrate Level Phosphorylation"

Phosphoenolpyruvate + ADP -------> Pyruvate + ATP

Irreversible, Highly Regulated.

Produces 2 ATP.

(-) Citrate (high flux of carbons from glucose to the citric acid cycle requiring decreased glycolysis and increased gluconeogenesis)

ATP. Both prevent the build-up of phosphorylated intermediates.

Alanine, a major precursor for gluconeogensis

(+) Fructose-1,6-bisP, PEP (ensure that intermediates between are kept at a minimal conc.)

Pyruvate Carboxylase:

Mitochondrial enzyme, requires biotin as a prosthetic group to carry CO₂.

Requires ATP.

Activated (+): by acetyl CoA (diverts some of the Pyruvate) in concert with the inhibition (-) of Pyruvate DH.

© Dr. Noel Sturm 2015