Phosphofructokinase 1

• CAS Number: 9001-80-3

Phosphofructokinase ( PFK1 also: fructose-6 -phosphate kinase) is the enzyme that catalyzes the rate limiting step of the glycolysis, the conversion of fructose -6-phosphate to fructose -1 ,6- bisphosphate. They determined to how much available energy has the cell ( ATP, citrate, NADH / H ). PFK1 occurs in all living things. In humans there are five isoforms, which are produced by three different genes: PFKM (muscle ), PFKL (liver), PFKP ( platelets). Mutations in PFKM are the cause of the rare Tarui disease.

Position in energy metabolism

The dual role of glycolysis is the collection of components from different degradation pathways ( collection phase ), and the production of chemical energy in the form of ATP (gain stage ). To meet these needs the glucose turnover must be regulated in such enzymes that work irreversible and are specific for glycolysis. Basic candidate for this would be the reactions of phosphofructokinase ( PFK ) and pyruvate kinase (PK ); the hexokinase reaction is ruled out, since their product, G -6-P, a multifunctional metabolite. The most important control point is now the phosphofructokinase ( PFK ), which ( in the cell irreversible) conversion of fructose -6-phosphate to fructose - accomplished the 1.6 -bisphosphate. The liver enzyme, a 340 kDa tetramer by higher concentrations of ATP inhibited ( substrate inhibition ), that is, the Michaelis constant (Km ) of the substrate F-6- P is increased ( its binding strength decreased).

These properties of the PFKI are the most important aspect of molecular explanations of the Pasteur effect, which is throttled to aerobic metabolism of the metabolite flux in glycolysis when switching to provide a constant energy status of the cell.

Regulator functions

Regulation in the cell

PFKI has its catalytic site at the N -terminus, the regulatory center at the C- terminus of a fusion protein resulting from gene duplication. Both halves show consequently sequence homologies, but subject, according to their task, separate optimization processes:

• The catalytic moiety binds the substrates fructose-6 -phosphate ( F-6- P) and ATP;
• ATP decreases at higher concentrations, a ( low-affinity ) binding site on the regulatory part and acts as an allosteric inhibitor from there ( " substrate inhibition "). An inhibitory function it shares with other endogenous energy surplus signals the cell ( NADH / H and citrate). If, however, lack of energy signals (AMP, ADP) are present, the enzyme is allosterically activated. As long as AMP and ADP prevail, they determine what is happening.
• In erythrocytes that acts in the Rapoport Luebering cycle by the enzyme Bisphosphoglyceratmutase intermediate formed 2,3- diphosphoglycerate as an inhibitor of phosphofructokinase.

Regulation in the organism

It has long been known that PFKI not only by one of its substrates ( ATP) is inhibited, but also one of its products (F-1 ,6 -BP ) can be activated in vitro ( " perverse enzyme "). In the cell, the latter effect is probably not to as F-1 ,6 -BP never achieved by Aldolasetätigkeit the required equilibrium concentration., It was found, however, that an isomeric molecule that fructose -2 ,6- bisphosphate (F-2 ,6 -BP), a physiological allosteric activator. F-2 ,6 -BP mediates hunger signals (low blood sugar), which are emitted by the organism via glucagon or epinephrine. After the manner of a " third messenger " is used for propagation along the signal transduction glucagon - cAMP - PKA (see " second messenger ").

F-2 ,6 -BP is the product of a further specialized phosphofructokinase ( PFKII ). This " PFKII ", in vertebrates, a fusion protein of phosphofructokinase and fructose -2 ,6- bisphosphatase is one of the enzymes interkonvertierbaren, i.e. their activity is determined by protein kinase A (PKA ), and thus indirectly regulated by hormonal signals: the phosphorylation of a single serine kinase activity is switched off, while the phosphatase is turned on. The light emanating from glucagon signal causes so that F- 2 ,6 -BP is no longer available. This flow of metabolites of glycolysis at the PFKI comes to a standstill. In the liver, the resulting G -6-P accumulation by conversion to glucose is broken down ( by glycolysis or gluconeogenesis vice versa), which can be discharged as a neutral molecule to the blood circulation. The Glucagonsignal " to lower blood sugar " is answered it.

The opposite (insulin ) signal " high blood sugar " is apparently realized by an extremely pH-dependent activity profile. As an antagonist of the glucagon, the insulin effect and the F-2 ,6 -BP concentration, by reducing activation of an on phosphodiesterase cAMP levels, and a phosphatase is activated. This dephosphorylated the PFKII so that their kinase activity comes to fruition and F-2 ,6 -BP is produced which has an activating effect on the PFKI and hence glycolysis. Thus, the signal triggering, excess blood glucose is broken down. The activation of PFK1 not only involves conformational changes of individual subunits, but also aggregation to higher oligomers.

In muscle cells, phosphorylation of PFKII does not inhibit glycolysis, since isoenzymes are formed, the regulation takes place in the reverse direction. This is the basis of the Cori cycle, is placed over the incompletely oxidized in muscle activity of lactate from glycolysis via the blood to the liver where it ( despite the same hormonal situation ) gluconeogenesis is supplied. In muscle cells has also held glucagon, adrenaline primarily a regulatory function.

Isozyme in the skeletal muscle does not have the PKA phosphorylation sites which allow for regulation via phosphorylation by hormones. Therefore, adrenaline acts in skeletal muscle does not inhibit glycolysis, and thus does not inhibit glucose utilization, thus the energy of the cells.

In cardiac muscle, however, in turn, phosphorylation takes place. This causes here, however, a stimulation of the kinase activity. Thus adrenaline causes an increase in the F- 2 ,6 -BP concentration, and thus stimulates glycolysis in addition.

Phosphofructokinase in photosynthesis

During photosynthesis occurs in plants by light energy ATP and NADPH / H for biosynthesis. At the same time caused by carbon dioxide fixation ( assimilation ) in C3 plants 3 -phosphoglycerate (3- PG), an intermediate in both glycolysis and glucose biosynthesis ( gluconeogenesis ). In excess energy of the latter approach is required, which eventually leads to the energy storage starch. Availability of 3 -PG regulates ( inhibits ) PFKII, which gluconeogenesis is but one - glycolysis turned off.

• Excess energy signals of the cell ( ATP, citrate, and NADH / H in animal, 3 -PG in plant tissues ) thus generally prevent the formation of excess ATP.