The TP53-inducible glycolysis and apoptosis regulator (TIGAR) also known as fructose-2,6-bisphosphatase TIGAR is an enzyme that in humans is encoded by the TIGAR gene (previously C12orf5). TIGAR is a recently discovered enzyme that primarily functions as a regulator of glucose breakdown in human cells. In addition to its role in controlling glucose degradation, TIGAR activity can allow a cell to carry out DNA repair, and the degradation of its own organelles. Finally, TIGAR can protect a cell from death. Since its discovery in 2005 by Kuang-Yu Jen and Vivian G. Cheung, TIGAR has become of particular interest to the scientific community thanks to its active role in many cancers. Normally, TIGAR manufactured by the body is activated by the p53 tumour suppressor protein after a cell has experienced a low level of DNA damage or stress. In some cancers, TIGAR has fallen under the control of other proteins. The hope is that future research into TIGAR will provide insight into new ways to treat cancer. This gene is regulated as part of the p53 tumor suppressor pathway and encodes a protein with sequence similarity to the bisphosphate domain of the glycolytic enzyme that degrades fructose-2,6-bisphosphate. The protein functions by blocking glycolysis and directing the pathway into the pentose phosphate shunt. Expression of this protein also protects cells from DNA damaging reactive oxygen species and provides some protection from DNA damage-induced apoptosis. The 12p13.32 region that includes this gene is paralogous to the 11q13.3 region.
Gene In humans the TIGAR gene, known as C12orf5, is found on chromosome 12p13-3, and consists of 6 exons. The C12orf5 mRNA is 8237 base pairs in length.
Discovery Jen and Cheung first discovered the c12orf5 gene whilst using computer based searches to find novel p53-regulated genes that were switched on in response to ionizing radiation. They published their research in Cancer Research in 2005. Later a study focused wholly on the structure and function of the c12orf5 gene was published in Cell by Karim Bensaad et al., in which c12orf5 was given the name TIGAR in honour of its apparent function.
Expression TIGAR transcription is rapidly activated by the p53 tumour suppressor protein in response to low levels of cellular stress, such as that caused by exposure to low doses of UV. However, under high levels of cellular stress TIGAR expression decreases. P53, a transcription factor, can bind two sites within the human TIGAR gene to activate expression. One site is found within the first intron, and binds p53 with high affinity. The second is found just prior to the first exon, binds p53 with low affinity, and is conserved between mice and humans. TIGAR expression can be regulated by other non-p53 mechanisms in tumour cell lines.
Structure
TIGAR is approximately 30kDa and has a tertiary structure that is similar to the histidine phosphatase fold. The core of TIGAR is made up of an α-β-α sandwich, which consists of a six-stranded β sheet surrounded by 4 α helices. Additional α helices and a long loop are built around the core to give the full enzyme. TIGAR has an active site that is structurally similar to that of PhoE (a bacterial phosphatase enzyme) and functionally similar to that of fructose-2,6-bisphosphatase. The bis-phosphatase-like active site of TIGAR is positively charged, and catalyses the removal of phosphate groups from other molecules. In contrast to Fructose-2,6-Bisphosphatase, TIGAR's active site is open and accessible like that of PhoE. The site contains 3 crucial amino acids (2 histidines and 1 glutamic acid) that are involved in the phosphatase reaction. These 3 residues are known collectively as a catalytic triad, and are found in all enzymes belonging to the phosphoglyceromutase branch of the histidine phosphatase superfamily. One of the histidine residues is electrostatically bound to a negatively charged phosphate. A second phosphate is bound elsewhere in the active site.
Function TIGAR activity can have multiple cellular effects. TIGAR acts as a direct regulator of fructose-2,6-bisphosphate levels and hexokinase 2 activity, and this can lead indirectly to many changes within the cell in a chain of biochemical events. TIGAR is a fructose bisphosphatase which activates p53, in results of inhibiting the expression of glucose transporter and also regulating the expression of hexokinase and phosphoglycerate mutase. TIGAR also inhibit the Phosphofructokinase (PFK) by lowering the level of fructose-2,6,bisphosphate, therefore, glycolysis is inhibited and pentose phosphate pathway is promoted.
Fructose-2,6-bisphosphate regulation TIGAR decreases cellular fructose-2,6-bisphosphate levels. It catalyses the removal of a phosphate group from fructose-2,6-bisphosphate (F-2,6-BP): Fructose-2,6-Bisphosphate->Fructose-6-phosphate (F-6-P) + phosphate F-2,6-BP is an allosteric regulator of cellular glucose metabolism pathways. Ordinarily F-2,6-BP binds to and increases the activity of phosphofructokinase 1. Phosphofructokinase-1 catalyses the addition of a phosphate to F-6-P to form Fructose-1,6-bisphosphate (F-1,6-BP). This is an essential step in the glycolysis pathway, which forms the first part of aerobic respiration in mammals. F-2,6-BP also binds to and decreases the activity of fructose-1,6-bisphosphatase. Fructose-1,6-bisphosphatase catalyses the removal of phosphate from F-1,6-BP to form F-6-P. This reaction is part of the gluconeogenesis pathway, which synthesizes glucose, and is the reverse of glycolysis. When TIGAR decreases F-2,6-BP levels, phosphofructokinase becomes less active whilst fructose-1,6-bisphosphatase activity increases. Fructose-6-phosphate levels build up, which has multiple effects inside the cell:
The rate of glycolysis decreases The rate of gluconeogenesis increases Excess fructose-6-phosphate is converted to glucose-6-phosphate in an isomerization reaction Excess glucose-6-phosphate enters the pentose phosphate pathway. This ultimately leads to the removal of reactive oxygen species (ROS) in the cell The removal of ROS helps to prevent apoptosis (cell suicide), and may also reduce build-up of DNA damage over time.
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