Glucose 6-phosphatase (EC 3.1.3.9, G6Pase) is an enzyme that hydrolyzes glucose-6-phosphate, resulting in the creation of a phosphate group and free glucose. Glucose is then exported from the cell via glucose transporter membrane proteins. This catalysis completes the final step in gluconeogenesis and glycogenolysis and therefore plays a key role in the homeostatic regulation of blood glucose levels.

Glucose-6-phosphatase is a complex of multiple component proteins, including transporters for G6P, glucose, and phosphate. The main phosphatase function is performed by the glucose-6-phosphatase catalytic subunit. In humans, there are three isozymes of the catalytic subunit: glucose-6-phosphatase-α, encoded by G6PC; IGRP, encoded by G6PC2; and glucose-6-phosphatase-β, encoded by G6PC3.

Glucose-6-phosphatase-α and glucose-6-phosphatase-β are both functional phosphohydrolases, and have similar active site structure, topology, mechanism of action, and kinetic properties with respect to G6P hydrolysis. In contrast, IGRP has almost no hydrolase activity, and may play a different role in stimulating pancreatic insulin secretion.

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 helicies. 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.

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