When glucose is scarce, PKA will be activated in cells responsive to glucagon , which favors FBPase2 activity, lowering levels of fructose 2,6-bisphosphate.
The loss of the PFK activation by the latter slows down glycolysis. In liver, the effect of glucagon is also to stimulate glycogen breakdown, thus making the glucose stored therein available for maintenance of blood-glucose homeostasis. The other points at which the flux through the glycolytic pathway can be controlled include the activities of hexokinase and pyruvate kinase. Hexokinase is subject to product inhibition by glucose 6-phosphate. When PFK is less active, the rise in relative concentration of fructose 6-phosphate is soon reflected in a rise in glucose 6-phosphate levels.
This also slows the rate of catalysis by hexokinase. In the liver, this mode of regulation can be bypassed as glucose 6-phosphate levels rise by the enzyme glucokinase. Glucokinase is not inhibited by G6P, but its K M for glucose is significantly higher. Regulation of pyruvate kinase occurs via allosteric effects, and through different isozymic forms that differ in their capacity for regulation by covalent modification again, phosphorylation.
In general, fructose 1,6-bisphosphate, the product of the PFK reaction, is an allosteric activator of pyruvate kinase, while ATP and alanine the latter signifies the abundance of pyruvate are allosteric inhibitors. The L isozyme of pyruvate kinase is directly regulated by phosphorylation.
The L form is expressed in the liver, and it is a substrate of PKA When blood glucose is low, glucagon stimulates a membrane associated adenylate cyclase, activating PKA, as explained above. This leads to the phosphorylation of the L form of pyruvate kinase, which inhibits its activity.
This makes metabolic sense, since when blood glucose is low, further consumption of glucose by glycolysis in the liver ought to be slowed down. The fate of glucose 6-phosphate G6P is not determined solely by the rate of glycolysis. It is also utilized in the pentose phosphate pathway , or it can be directed toward "short-term storage" in the form of glycogen. In considering the logic of metabolism in complex organisms, the specialized roles of organs must be included.
The liver, in its role as a regulator of blood glucose levels, carries out the hydrolysis of G6P to glucose for release into the bloodstream. Below is a summary of the fates of G6P in a broader overview of its relation to major metabolic pathways. Note that glucose 1-phosphate, G6P, and fructose 6-phosphate are interconvertible in reactions that are not highly exergonic or endergonic , and thus in a sense constitute a common metabolic pool.
These three points are hexokinase, phosphofructokinase, and pyruvate kinase. These three reactions are candidates to be the major points of regulation because of their high negative free energies. Once PFK converts F6P to F1,6P, the reaction will not be easily reversed because of the high amount of energy that must be overcome to go backward.
This energy barrier makes sense seeing as pyruvate kinase catalyzes the final reaction 10 and hexokinase 1 is not involved in glycolysis at all when the process is begun from glycogen. While ATP binds at the active site equally well in both R and T states, it preferentially binds the allosteric site of the T state [10] This preferential binding causes a shift from equilibrium of the two states, to a greater amount of T state [11] , which decreases the affinity for F6P.
Allosteric activator ADP also binds to the allosteric site to increase the ratio of R state phosphofructokinase. As can be seen from the graph below, the plots for the activity of PFK are sigmoidal. This further demonstrates the cooperative nature of the enzyme. The system of regulation matches well with the function of PFK. The opposite holds true as well, because high ATP concentration inhibits protein activity. And yet, this explanation cannot completely account for the regulation of PFK, because the levels of ATP do not vary greatly enough between active and resting muscles.
Another means of allosteric regulation must be found. This Kinemage exercise consists of two kinemage scenes that illustrate some of the allosterically-induced conformational changes that occur in PFK from Bacillus stearothermophilus. This kinemage shows the two subunits of the tetramer whose interface contains two active sites.
KineMage currently not supported The first view, 1: PFK dimer, shows the two subunits in their R state conformation as represented by their Ca backbones with Subunit 1 in pink tint and Subunit 2 in pink. Two side chains in each subunit are shown, those of Glu red and Arg cyan , which are part of the F6P binding site in the T and R states, srespectively see below.
In its T state, Subunit 1 is bluetint and Subunit 2 is skyblue. The side chains of Glu and Arg in both subunits are red and cyan as before only the Ca and Cb atoms of the Arg side chain in Subunit 1 are observed in the X-ray structure of the T state; those of Subunit 2 are all observed. The T state enzyme binds the inhibitor 2-phosphoglycolate gold; "PGC" , a nonphysiological analog of the glycolytic intermediate phosphoenolpyruvate PEP.
Note that the active site is located at the interface between two subunits and that the allosteric site interacts directly with the active site on the adjacent subunit. The phosphate group of PGC binds to the allosteric site in the T state in very nearly the same position that the beta phosphate group of "ADP-allo" binds to the R state allosteric site; both phosphate groups bind to the side chains of the same three residues 2 arg and 1 Lys; not shown.
Has a high Km low affinity,weak binding to ensure an appropriate response to elevation of glucose from the diet, provides kinetic control. Phosphofructokinase-1 PFK-1 : glycolysis, irreversible. The concentration of AMP increases when energy is low. Excess phosphate also signals low energy via increase in ATP use.
Fructose 1,6-bisphosphatase : gluconeogenesis, irreversible. The two enzymes are reciprocally regulated or ATP would be lost without energy conservation. Prevents futile cycling like using a stationary bike.
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