Phosphofructokinase 1
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Phosphofructokinase-1 (PFK-1) is one of the most important regulatory
enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...
() of
glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...
. It is an allosteric enzyme made of 4 subunits and controlled by many activators and
inhibitor Inhibitor or inhibition may refer to: Biology * Enzyme inhibitor, a substance that binds to an enzyme and decreases the enzyme's activity * Reuptake inhibitor, a substance that increases neurotransmission by blocking the reuptake of a neurotransmi ...
s. PFK-1 catalyzes the important "committed" step of glycolysis, the conversion of
fructose 6-phosphate Fructose 6-phosphate (sometimes called the Neuberg ester) is a derivative of fructose, which has been phosphorylated at the 6-hydroxy group. It is one of several possible fructosephosphates. The β-D-form of this compound is very common in cells ...
and ATP to fructose 1,6-bisphosphate and ADP. Glycolysis is the foundation for respiration, both anaerobic and aerobic. Because phosphofructokinase (PFK) catalyzes the ATP-dependent phosphorylation to convert fructose-6-phosphate into fructose 1,6-bisphosphate and ADP, it is one of the key regulatory steps of glycolysis. PFK is able to regulate glycolysis through allosteric inhibition, and in this way, the cell can increase or decrease the rate of glycolysis in response to the cell's energy requirements. For example, a high ratio of ATP to ADP will inhibit PFK and glycolysis. The key difference between the regulation of PFK in eukaryotes and prokaryotes is that in eukaryotes PFK is activated by fructose 2,6-bisphosphate. The purpose of fructose 2,6-bisphosphate is to supersede ATP inhibition, thus allowing eukaryotes to have greater sensitivity to regulation by hormones like glucagon and insulin.


Structure

Mammalian PFK1 is a 340kd tetramer composed of different combinations of three types of subunits:
muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...
(M),
liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...
(L), and
platelet Platelets or thrombocytes () are a part of blood whose function (along with the coagulation#Coagulation factors, coagulation factors) is to react to bleeding from blood vessel injury by clumping to form a thrombus, blood clot. Platelets have no ...
(P). The composition of the PFK1
tetramer A tetramer () (''tetra-'', "four" + '' -mer'', "parts") is an oligomer formed from four monomers or subunits. The associated property is called ''tetramery''. An example from inorganic chemistry is titanium methoxide with the empirical formula ...
differs according to the tissue type it is present in. For example, mature muscle expresses only the M
isozyme In biochemistry, isozymes (also known as isoenzymes or more generally as multiple forms of enzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. Isozymes usually have different kinetic parameters (e.g. di ...
, therefore, the muscle PFK1 is composed solely of homotetramers of M4. The liver and kidneys express predominantly the L isoform. In
erythrocytes Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cel ...
, both M and L subunits randomly tetramerize to form M4, L4 and the three hybrid forms of the enzyme (ML3, M2L2, M3L). As a result, the kinetic and regulatory properties of the various isoenzymes pools are dependent on subunit composition. Tissue-specific changes in PFK activity and isoenzymic content contribute significantly to the diversities of
glycolytic Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...
and gluconeogenic rates which have been observed for different tissues. PFK1 is an allosteric enzyme and has a structure similar to that of
hemoglobin Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin ...
in so far as it is a dimer of a dimer.; * One half of each dimer contains the ATP binding site whereas the other half the substrate (fructose-6-phosphate or (F6P)) binding site as well as a separate allosteric binding site. Each subunit of the tetramer is 319 amino acids and consists of two domains: one that binds the substrate ATP, and the other that binds fructose-6-phosphate. Each domain is a beta barrel, and has a cylindrical beta sheet surrounded by alpha helices. On the opposite side of the each subunit from each active site is the allosteric site, at the interface between subunits in the dimer. ATP and AMP compete for this site. The N-terminal domain has a catalytic role binding the ATP, and the C-terminal has a regulatory role


Mechanism

PFK1 is an allosteric enzyme whose activity can be described using the symmetry model of allosterism whereby there is a concerted transition from an enzymatically inactive T-state to the active R-state. F6P binds with a high affinity to the R state but not the T state enzyme. For every molecule of F6P that binds to PFK1, the enzyme progressively shifts from T state to the R state. Thus a graph plotting PFK1 activity against increasing F6P concentrations would adopt the sigmoidal curve shape traditionally associated with allosteric enzymes. PFK1 belongs to the family of phosphotransferases and it catalyzes the transfer of γ-phosphate from ATP to fructose-6-phosphate. The PFK1
active site In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate, the ''binding s ...
comprises both the ATP-Mg2+ and the F6P binding sites. Some proposed residues involved with substrate binding in ''E. coli'' PFK1 include Asp127 and Arg171. In B. stearothermophilus PFK1, the positively charged side chain of Arg162 residue forms a hydrogen-bonded salt bridge with the negatively charged phosphate group of F6P, an interaction which stabilizes the R state relative to the T state and is partly responsible for the homotropic effect of F6P binding. In the T state, enzyme conformation shifts slightly such that the space previously taken up by the Arg162 is replaced with Glu161. This swap in positions between adjacent amino acid residues inhibits the ability of F6P to bind the enzyme. Allosteric activators such as AMP and ADP bind to the allosteric site as to facilitate the formation of the R state by inducing structural changes in the enzyme. Similarly, inhibitors such as ATP and PEP bind to the same allosteric site and facilitate the formation of the T state, thereby inhibiting enzyme activity. The hydroxyl oxygen of carbon 1 does a nucleophilic attack on the beta phosphate of ATP. These electrons are pushed to the anhydride oxygen between the beta and gamma phosphates of ATP.


Regulation

PFK1 is the most important control site in the mammalian glycolytic pathway. This step is subject to extensive regulation since it is not only highly
exergonic An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process. Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs ...
under physiological conditions, but also because it is a committed step – the first irreversible reaction unique to the glycolytic pathway. This leads to a precise control of glucose and the other
monosaccharide Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. Chemically, monosaccharides are polyhy ...
s
galactose Galactose (, ''wikt:galacto-, galacto-'' + ''wikt:-ose#Suffix 2, -ose'', ), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweetness, sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epime ...
and
fructose Fructose (), or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and gal ...
going down the glycolytic pathway. Before this enzyme's reaction,
glucose-6-phosphate Glucose 6-phosphate (G6P, sometimes called the Robison ester) is a glucose sugar phosphorylated at the hydroxy group on carbon 6. This dianion is very common in cells as the majority of glucose entering a cell will become phosphorylated in this wa ...
can potentially travel down the
pentose phosphate pathway The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt or HMP shunt) is a metabolic pathway parallel to glycolysis. It generates NADPH and pentoses (five-carbon sugars) as well as ribose 5-ph ...
, or be converted to glucose-1-phosphate for
glycogenesis Glycogenesis is the process of glycogen synthesis or the process of converting glucose into glycogen in which glucose molecules are added to chains of glycogen for storage. This process is activated during rest periods following the Cori cycl ...
. PFK1 is allosterically inhibited by high levels of ATP but AMP reverses the inhibitory action of ATP. Therefore, the activity of the enzyme increases when the cellular ATP/AMP ratio is lowered. Glycolysis is thus stimulated when energy charge falls. PFK1 has two sites with different affinities for ATP which is both a substrate and an inhibitor. PFK1 is also inhibited by low pH levels which augment the inhibitory effect of ATP. The pH falls when muscle is functioning anaerobically and producing excessive quantities of
lactic acid Lactic acid is an organic acid. It has the molecular formula C3H6O3. It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as wel ...
(although lactic acid is not itself the cause of the decrease in pH). This inhibitory effect serves to protect the muscle from damage that would result from the accumulation of too much acid. Finally, PFK1 is allosterically inhibited by PEP, citrate, and ATP. Phosphoenolpyruvic acid is a product further downstream the
glycolytic Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...
pathway. Although citrate does build up when the Krebs Cycle enzymes approach their maximum velocity, it is questionable whether citrate accumulates to a sufficient concentration to inhibit PFK-1 under normal physiological conditions. ATP concentration build up indicates an excess of energy and does have an allosteric modulation site on PFK1 where it decreases the affinity of PFK1 for its substrate. PFK1 is allosterically activated by a high concentration of AMP, but the most potent activator is
fructose 2,6-bisphosphate Fructose 2,6-bisphosphate, abbreviated Fru-2,6-''P''2, is a metabolite that allosterically affects the activity of the enzymes phosphofructokinase 1 (PFK-1) and fructose 1,6-bisphosphatase (FBPase-1) to regulate glycolysis and gluconeogenesis. ...
, which is also produced from fructose-6-phosphate by PFK2. Hence, an abundance of F6P results in a higher concentration of
fructose 2,6-bisphosphate Fructose 2,6-bisphosphate, abbreviated Fru-2,6-''P''2, is a metabolite that allosterically affects the activity of the enzymes phosphofructokinase 1 (PFK-1) and fructose 1,6-bisphosphatase (FBPase-1) to regulate glycolysis and gluconeogenesis. ...
(F-2,6-BP). The binding of F-2,6-BP increases the affinity of PFK1 for F6P and diminishes the inhibitory effect of ATP. This is an example of feedforward stimulation as glycolysis is accelerated when glucose is abundant. PFK activity is reduced through repression of synthesis by
glucagon Glucagon is a peptide hormone, produced by alpha cells of the pancreas. It raises the concentration of glucose and fatty acids in the bloodstream and is considered to be the main catabolic hormone of the body. It is also used as a Glucagon (medic ...
. Glucagon activates
protein kinase A In cell biology, protein kinase A (PKA) is a family of serine-threonine kinases whose activity is dependent on cellular levels of cyclic AMP (cAMP). PKA is also known as cAMP-dependent protein kinase (). PKA has several functions in the cell, in ...
which, in turn, shuts off the kinase activity of PFK2. This reverses any synthesis of F-2,6-BP from F6P and thus de-activates PFK1. The precise regulation of PFK1 prevents
glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...
and
gluconeogenesis Gluconeogenesis (GNG) is a metabolic pathway that results in the biosynthesis of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In verte ...
from occurring simultaneously. However, there is substrate cycling between F6P and F-1,6-BP. Fructose-1,6-bisphosphatase (FBPase) catalyzes the hydrolysis of F-1,6-BP back to F6P, the reverse reaction catalyzed by PFK1. There is a small amount of FBPase activity during glycolysis and some PFK1 activity during gluconeogenesis. This cycle allows for the amplification of metabolic signals as well as the generation of heat by ATP hydrolysis. Serotonin (5-HT) increases PFK by binding to the 5-HT(2A) receptor, causing the tyrosine residue of PFK to be phosphorylated via phospholipase C. This in turn redistributes PFK within the skeletal muscle cells. Because PFK regulates glycolytic flux, serotonin plays a regulatory role in glycolysis


Genes

There are three phosphofructokinase genes in humans: * PFKL
liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...
* PFKM
muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...
* PFKP
platelet Platelets or thrombocytes () are a part of blood whose function (along with the coagulation#Coagulation factors, coagulation factors) is to react to bleeding from blood vessel injury by clumping to form a thrombus, blood clot. Platelets have no ...


Clinical significance

A genetic mutation in the PFKM gene results in Tarui's disease, which is a glycogen storage disease where the ability of certain cell types to utilize
carbohydrates A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ma ...
as a source of energy is impaired. Tarui disease is a glycogen storage disease with symptoms including muscle weakness (myopathy) and exercise induced cramping and spasms, myoglobinuria (presence of myoglobin in urine, indicating muscle destruction) and compensated hemolysis. ATP is a natural allosteric inhibitor of PFK, in order to prevent unnecessary production of ATP through glycolysis. However, a mutation in Asp(543)Ala can result in ATP having a stronger inhibitory effect (due to increased binding to PFK's inhibitory allosteric binding site). Phosphofructokinase mutation and cancer: In order for cancer cells to meet their energy requirements due to their rapid cell growth and division, they survive more effectively when they have a hyperactive phosphofructokinase 1 enzyme. When cancer cells grow and divide quickly, they initially do not have as much blood supply, and can thus have hypoxia (oxygen deprivation), and this triggers O-GlcNAcylation at serine 529 of PFK. This modification inhibits PFK1 activity and supports cancer proliferation, in contrast with the view that high PFK1 activity is necessary for cancer. This may be due to redirecting glucose flux towards the pentose phosphate pathway to generate NADPH to detoxify reactive oxygen species. Herpes simplex type 1 and phosphofructokinase: Some viruses, including HIV, HCMV and Mayaro affect cellular metabolic pathways such as glycolysis by a MOI-dependent increase in the activity of PFK. The mechanism that Herpes increases PFK activity is by phosphorylating the enzyme at the serine residues. The HSV-1 induced glycolysis increases ATP content, which is critical for the virus's replication.


See also

* PFK2 (converts fructose 6-phosphate to fructose 2,6-bisphosphate through on site, or the opposite, on another site) * PFP (reversibly interconverts fructose 6-phosphate and fructose 1,6-bisphosphate using inorganic
pyrophosphate In chemistry, pyrophosphates are phosphorus oxyanions that contain two phosphorus atoms in a linkage. A number of pyrophosphate salts exist, such as disodium pyrophosphate () and tetrasodium pyrophosphate (), among others. Often pyrophosphates a ...
rather than ATP) * Fructose bisphosphatase (hydrolyses fructose 1,6-bisphosphate to fructose 6-phosphate)


References


External links

*
Proteopedia.org Phosphofructokinase
* * {{Portal bar, Biology, border=no EC 2.7.1 Glycolysis