Glucose 6-phosphatase

Although a clear consensus has not been reached, a large number of scientists adhere to a substrate-transport model to account for the catalytic properties of glucose 6-phosphatase. In this model, glucose 6-phosphatase has a low degree of selectivity. The transfer of the glucose 6-phosphate is carried out by a transporter protein (T1) and the endoplasmic reticulum (ER) contains structures allowing the exit of the phosphate group (T2) and glucose (T3). Glucose 6-phosphatase consists of 357 amino acids, and is anchored to the endoplasmic reticulum (ER) by nine transmembrane helices. Its N-terminal and active site are found on the lumen side of the ER and its C-terminus projects into the cytoplasm. Due to its tight association to the ER, the exact structure of glucose 6-phosphatase remains unknown. However, sequence alignment has shown that glucose 6-phosphatase is structurally similar to the active site of the vanadium-containing chloroperoxidase found in Curvularia inaequalis. Based on pH kinetic studies of glucose 6-phosphatase-α catalysis, it was proposed that the hydrolysis of glucose 6-phosphate was completed via a covalent phosphohistidine glucose 6-phosphate intermediate. The active site of glucose 6-phosphatase-α was initially identified by the presence of a conserved phosphate signature motif usually found in lipid phosphatases, acid phosphatases, and vanadium haloperoxidases. Within the Vanadium-containing chloroperoxidase, Lys353 was found to stabilize the phosphate in the transition state. However, the corresponding residue in glucose 6-phosphatase-α (Lys76) resides within the ER membrane and its function, if any, is currently undetermined. With the exception of Lys76, these residues are all located on the luminal side of the ER membrane. Glucose 6-phosphatase-β is a ubiquitously expressed, 346-amino acid membrane protein that shares 36% sequence identity with glucose 6-phosphatase-α. Within the glucose 6-phosphatase-β enzyme, sequence alignments predict that its active site contains His167, His114, and Arg79. Similar to that of the glucose 6-phosphatase-α active site, His167 is the residue that provides the nucleophilic attack, and His114, and Arg79 are the hydrogen donors. Glucose 6-phosphatase-β is also localized in the ER membrane, although its orientation is unknown. ==Mechanism==

The hydrolysis of glucose 6-phosphate begins with a nucleophilic attack on the sugar-bound phosphate by His176 resulting in the formation of a phosphohistidine bond and the degradation of a carbonyl. A Negatively charged oxygen then transfers its electrons reforming a carbonyl and breaking its bond with glucose. The negatively charged glucose-bound oxygen is then protonated by His119 forming a free glucose. The phospho-intermediate produced by the reaction between His176 and the phosphate group is then broken by a hydrophilic attack; after the addition of another hydroxide and the decomposition of a carbonyl, the carbonyl is reformed kicking off the electrons originally donated by the His176 residue thereby creating a free phosphate group and completing the hydrolysis. ==Expression==

Genes coding for the enzyme are primarily expressed in the liver, in the kidney cortex and (to a lesser extent) in the β-cells of the pancreatic islets and intestinal mucosa (especially during times of starvation). Thus, the glycogen that muscles store is not usually available for the rest of the body's cells because glucose 6-phosphate cannot cross the sarcolemma unless it is dephosphorylated. The enzyme plays an important role during periods of fasting and when glucose levels are low. It has been shown that starvation and diabetes induces a two- to threefold increase in glucose 6-phosphatase activity in the liver. ==Clinical significance==

Mutations of the glucose 6-phosphatase system, to be specific the glucose 6-phosphatase-α subunit (glucose 6-phosphatase-α), glucose 6-transporter (G6PT), and glucose 6-phosphatase-β (glucose 6-phosphatase-β or G6PC3) subunits lead to deficiencies in the maintenance of interprandial glucose homeostasis and neutrophil function and homeostasis. Mutations in both glucose 6-phosphatase-α and G6PT lead to glycogen storage disease type I (GSD 1, von Gierke's disease). To be specific, mutations in the glucose-6-phosphatase-α lead to Glycogen Storage Disease Type-1a, which is characterized by accumulation of glycogen and fat in the liver and kidneys, resulting in hepatomegaly and renomegaly. GSD-1a constitutes approximately 80% of GSD-1 cases that present clinically. Absence of G6PT leads to GSD-1b (GSD-1b), which is characterized by the lack of a G6PT and represents 20% of the cases that present clinically. The specific cause of the GSD-1a stems from nonsense mutations, insertions/deletions with or without a shift in the reading frame, or splice site mutations that occur at the genetic level. It can also lead to cardiac and urogenital malformations. This third class of deficiency is also affected by a G6PT deficiency as glucose-6-phosphatase-β also lies within the ER lumen and thus can lead to similar symptoms of glucose-6-phosphatase-β deficiency be associated with GSD-1b. The neutrophil glycosylation has a profound effect on neutrophil activity and thus may also be classified as a congenital glycosylation disorder as well. == See also ==