Phosphoglucomutase

Phosphoglucomutase (EC 5.4.2.2) is an enzyme that transfers a phosphate group on an α-D-glucose monomer from the 1' to the 6' position in the forward direction or the 6' to the 1' position in the reverse direction. Phosphoglucomutase (EC 5.4.2.2) is an enzyme that transfers a phosphate group on an α-D-glucose monomer from the 1' to the 6' position in the forward direction or the 6' to the 1' position in the reverse direction. More precisely, it facilitates the interconversion of glucose 1-phosphate and glucose 6-phosphate. After glycogen phosphorylase catalyzes the phosphorolytic cleavage of a glucosyl residue from the glycogen polymer, the freed glucose has a phosphate group on its 1-carbon. This glucose 1-phosphate molecule is not itself a useful metabolic intermediate, but phosphoglucomutase catalyzes the conversion of this glucose 1-phosphate to glucose 6-phosphate (see below for the mechanism of this reaction). Glucose 6-phosphate’s metabolic fate depends on the needs of the cell at the time it is generated. If the cell is low on energy, then glucose 6-phosphate will travel down the glycolytic pathway, eventually yielding two molecules of adenosine triphosphate. If the cell is in need of biosynthetic intermediates, glucose 6-phosphate will enter the pentose phosphate pathway, where it will undergo a series of reactions to yield riboses and/or NADPH, depending on cellular conditions. If glycogenolysis is taking place in the liver, glucose 6-phosphate can be converted to glucose by the enzyme glucose 6-phosphatase; the glucose produced in the liver is then released to the bloodstream for use in other organs. Muscle cells in contrast do not have the enzyme glucose 6-phosphatase, so they cannot share their glycogen stores with the rest of the body. Phosphoglucomutase also acts in the opposite fashion when blood glucose levels are high. In this case, phosphoglucomutase catalyzes the conversion of glucose 6-phosphate (which is easily generated from glucose by the action of hexokinase) to glucose 1-phosphate. This glucose-1-phosphate can then react with UTP to yield UDP-glucose in a reaction catalyzed by UDP-glucose-pyrophosphorylase. If activated by insulin, glycogen synthase will proceed to clip the glucose from the UDP-glucose complex onto a glycogen polymer. Phosphoglucomutase effects a phosphoryl group shift by exchanging a phosphoryl group with the substrate. Isotopic labeling experiments have confirmed that this reaction proceeds through a glucose 1,6-bisphosphate intermediate. The first step in the forward reaction is the transfer of a phosphoryl group from the enzyme to glucose 1-phosphate, forming glucose 1,6-bisphosphate and leaving a dephosphorylated form of the enzyme. The enzyme then undergoes a rapid diffusional reorientation to position the 1-phosphate of the bisphosphate intermediate properly relative to the dephosphorylated enzyme. Substrate-velocity relationships and induced transport tests have revealed that the dephosphorylated enzyme then facilitates the transfer of a phosphoryl group from the glucose-1,6-bisphosphate intermediate to the enzyme, regenerating phosphorylated phosphoglucomutase and yielding glucose 6-phosphate (in the forward direction). Later structural studies confirmed that the single site in the enzyme that becomes phosphorylated and dephosphorylated is the oxygen of the active-site serine residue (see diagram below). A bivalent metal ion, usually magnesium or cadmium, is required for enzymatic activity and has been shown to complex directly with the phosphoryl group esterified to the active-site serine.