Reciprocal Regulation of Glycolysis and Gluconeogensis Prevent Futile Cycle

Similar to most metabolic pathways, glucose synthesis and breakdown is regulated by three different mechanisms:
a) Allosteric regulators
b) Covalent modification
c) Changes in gene expression

Regulation of glycolytic pathway:
As described on the previous page and in figure 1, glycolysis is regulated by three irreversible enzymes namely: Hexokinase/glucokinase, Phosphofructokinase, and Pyruvate kinase.

Figure 1: Allosteric Regulators of Glycolysis and Gluconeogenesis 

Hexokinase/glucokinase:
Hexokinase is a ubiquitously expressed enzyme that sets the pace of glycolysis. Hexokinase has a high affinity for glucose and transfers a negatively charged phosphate group to a glucose molecule. This step traps the glucose inside the cells and funnels it into various metabolic pathways. The high concentration of glucose-6-phosphate signals that the cell no longer requires energy or other biosynthetic pathways, and inhibits enzyme hexokinase.

Glucokinase is expressed in tissues(e.g. liver, pancreas) that play an important role in maintaining glucose concentration in blood. Glucokinase differs from hexokinase in two aspects: 

i) Glucokinase has a low affinity for glucose with Km above normal blood glucose concentration 

ii) Glucose-6-phosphate has no inhibitory effect on glucokinase. 

The low affinity and high catalytic activity of glucokinase are suited for its function in the liver. When the blood glucose is high, glucokinase rapidly binds and converts glucose to glucose-6-phosphate. In contrast, when blood glucose is limited, glucokinase has low activity and allows glucose to be distributed into tissues such as the brain and red blood cells.


Figure 2: Glucokinase and Hexokinase differ in the affinity of the substrate. 

This is an example of how isoform regulates the metabolic pathway in different tissue

Phosphofructokinase:

• Phosphofructokinase is the second irreversible step of glycolysis which is regulated by various allosteric effector molecules. 
• When ATP concentration is high in cells, ATP binds to the allosteric site and inhibits the enzyme activity of PFK. In contrast, AMP reverses the inhibitory action of ATP. 
• Citrate and hydrogen ions also inhibit phosphofructokinase. Citrate is a metabolic intermediate formed in the TCA cycle. The abundance of citrate signals the abundance of the metabolic intermediates and energy equivalents in the cell. 
• The high hydrogen concentration in muscle tissues inhibits glycolysis by inhibiting phosphofructokinase.
• In the liver, fructose-2,6-bisphosphate acts as an allosteric activator of PFK. Fructose-1,6-bisphosphate binds and activates PFK by decreasing the inhibitory effect of ATP. 
• When fructose-6-phosphate is high, phosphofructokinase-2 (an isoform of PFK) phosphorylates it to form fructose -2,6-bisphosphate. PFK-2 is a bifunctional enzyme that has both the kinase domain and the phosphatase domain. 
• When fructose-6-phosphate is low, the phosphatase domain of PFK-2 catalyzes the hydrolysis of fructose-2,6-bisphosphate to form fructose-6-phosphate. The kinase and phosphatase domain of PFK-2 is regulated by covalent modification.
• In response to glucagon, increased cAMP signals the activation of protein kinase A, phosphorylation of PFK2, and activation of the phosphatase domain. In contrast, insulin activates protein phosphatase that in turn dephosphorylates PFK-2 thereby activating the PFK domain.

Pyruvate Kinase:
The different isoforms of pyruvate kinase are expressed in different tissues and are regulated differently. The L-type isoform of pyruvate kinase that is expressed in the liver is regulated by covalent modification. The phosphorylated state is inactive and dephosphorylated enzyme is active. 

High blood glucose leads to activation of pyruvate kinase whereas low blood glucose inactivates pyruvate kinase. 

In contrast, the M-type pyruvate kinase isoform expressed in muscle and brain is not sensitive to regulation by covalent modification. T Both L and M-type isoforms are regulated by allosteric modifiers such as ATP, citrate, and Fructose-1,6-bisphosphate as shown in figure 1:.