Fructokinase
Encyclopedia
Fructokinase also known as D-fructokinase or D-fructose (D-mannose) kinase, is an enzyme of the liver
, intestine
, and kidney cortex. Fructokinase is in a family of enzymes called transferase
s, meaning that this enzyme transfers functional groups; it is also considered a phosphotransferase
(or, frequently, a kinase
) since it specifically transfers a phosphate group. Fructokinase specifically catalyzes the transfer of a phosphate group from ATP (the substrate) to fructose as the initial step in its utilization. The main role of fructokinase is in carbohydrate metabolism, more specifically, sucrose and fructose metabolism. The reaction equation is as follows: ATP + D-fructose = ADP + D-fructose 1-phosphate. This is notable because in most tissues this reaction is catalyzed by hexokinase, forming instead fructose 6-phosphate.
(Persera americana) fruit, and maize
(Zea mays) kernels, and many more.
Specifically, fructokinase may also regulate starch synthesis in conjunction with SS, sucrose synthase, which first metabolizes sink tissue in plant tissues such as in potatoes. There are also two divergent fructokinase genes that are differentially expressed and which also have different enzymatic properties such as those found in tomatoes. In tomatoes, fructokinase 1 (Frk 1) mRNA is expressed at a constant level during fruit development. However, fructokinase 2 (Frk 2) mRNA has a high expression level in young tomato fruit but then decreases during the later stages of fruit development. Frk 2 has a higher affinity for fructose than Frk 1 but Frk 2 activity is inhibited by high levels of fructose, whereas Frk 1 activity is not.
In Sinorhizobium meliloti
, a common gram-soil bacterium, fructokinase is also used in the metabolism of mannitol and sorbitol, in addition to the metabolism of fructose.
In rat liver cells (hepatocytes), GTP is also a substrate of fructokinase. It can be used at a substantial rate by fructokinase. In these isolated hepatocytes, in vivo, when the concentration of ATP falls to about 1 millimole in a short time interval, GTP becomes an important substrate under these specific conditions.
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
, intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
, and kidney cortex. Fructokinase is in a family of enzymes called transferase
Transferase
In biochemistry, a transferase is an enzyme that catalyzes the transfer of a functional group from one molecule to another . For example, an enzyme that catalyzed this reaction would be a transferase:In this example, A would be the donor, and B would be the acceptor...
s, meaning that this enzyme transfers functional groups; it is also considered a phosphotransferase
Phosphotransferase
Phosphotransferases are a category of enzymes that catalyze phosphorylation reactions. The general form of the reactions they catalyze is: A—P + B ⇔ B—P + A...
(or, frequently, a kinase
Kinase
In chemistry and biochemistry, a kinase is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates, a process referred to as phosphorylation. Kinases are part of the larger family of phosphotransferases...
) since it specifically transfers a phosphate group. Fructokinase specifically catalyzes the transfer of a phosphate group from ATP (the substrate) to fructose as the initial step in its utilization. The main role of fructokinase is in carbohydrate metabolism, more specifically, sucrose and fructose metabolism. The reaction equation is as follows: ATP + D-fructose = ADP + D-fructose 1-phosphate. This is notable because in most tissues this reaction is catalyzed by hexokinase, forming instead fructose 6-phosphate.
Role in Plants and Bacteria
Fructokinase has been characterized from various organisms such as pea (Pisum sativum) seeds, avocadoAvocado
The avocado is a tree native to Central Mexico, classified in the flowering plant family Lauraceae along with cinnamon, camphor and bay laurel...
(Persera americana) fruit, and maize
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
(Zea mays) kernels, and many more.
Specifically, fructokinase may also regulate starch synthesis in conjunction with SS, sucrose synthase, which first metabolizes sink tissue in plant tissues such as in potatoes. There are also two divergent fructokinase genes that are differentially expressed and which also have different enzymatic properties such as those found in tomatoes. In tomatoes, fructokinase 1 (Frk 1) mRNA is expressed at a constant level during fruit development. However, fructokinase 2 (Frk 2) mRNA has a high expression level in young tomato fruit but then decreases during the later stages of fruit development. Frk 2 has a higher affinity for fructose than Frk 1 but Frk 2 activity is inhibited by high levels of fructose, whereas Frk 1 activity is not.
In Sinorhizobium meliloti
Sinorhizobium meliloti
Sinorhizobium meliloti is a Gram-negative nitrogen-fixing bacterium . It forms a symbiotic relationship with legumes from the genera Medicago, Melilotus and Trigonella, including the model legume Medicago truncatula. This symbiosis results in a new plant organ termed a root nodule. The S...
, a common gram-soil bacterium, fructokinase is also used in the metabolism of mannitol and sorbitol, in addition to the metabolism of fructose.
Role in Animals and Humans
In human liver, purified fructokinase, when coupled with aldolase, has been discovered to contribute to an alternative mechanism to produce oxalate from xylitol. In coupled sequence, fructokinase and aldolase produce glycolaldehyde, a precursor to oxalate, from D-xylulose via D-xylulose 1-phosphate.In rat liver cells (hepatocytes), GTP is also a substrate of fructokinase. It can be used at a substantial rate by fructokinase. In these isolated hepatocytes, in vivo, when the concentration of ATP falls to about 1 millimole in a short time interval, GTP becomes an important substrate under these specific conditions.
Diseases
Fructosuria or hepatic fructokinase deficiency is a rare but benign inherited metabolic disorder. This condition is caused by a deficiency of fructokinase in the liver. Affected individuals usually display a large blood fructose concentration after the ingestion of fructose, sucrose or sorbitol. The disease is mainly characterized by the detection of the abnormal excretion of fructose in the urine through a urinalysis. Fructokinase is needed for the synthesis of glycogen, the body's form of stored energy, from fructose. The presence of fructose in the blood and urine may lead to an incorrect diagnosis of diabetes mellitus. Symptoms that may lead to the eventual diagnosis of fructosuria are hepatic fructokinase deficiency, levulosuria and ketohexokinase deficiency.See also
- Hepatic fructokinaseHepatic fructokinaseHepatic fructokinase is an enzyme that catalyzes the phosphorylation of fructose to produce fructose-1-phosphate....
- PFKLPFKL6-phosphofructokinase, liver type is an enzyme that in humans is encoded by the PFKL gene.-Further reading:...
- PFKMPFKM6-phosphofructokinase, muscle type is an enzyme that in humans is encoded by the PFKM gene.-Further reading:...
- PFKPPFKPPhosphofructokinase, platelet, also known as PFKP is an enzyme which in humans is encoded by the PFKP gene.- Function :The PFKP gene encodes the platelet isoform of phosphofructokinase...
- GlucokinaseGlucokinaseGlucokinase is an enzyme that facilitates phosphorylation of glucose to glucose-6-phosphate. Glucokinase occurs in cells in the liver, pancreas, gut, and brain of humans and most other vertebrates...
- HexokinaseHexokinaseA hexokinase is an enzyme that phosphorylates a six-carbon sugar, a hexose, to a hexose phosphate. In most tissues and organisms, glucose is the most important substrate of hexokinases, and glucose-6-phosphate the most important product....