Glucose-6-phosphate dehydrogenase (G6PD, G6PDH, EC 184.108.40.206) is a rate-limiting enzyme of the pentose phosphate pathway, which controls the carbon flow of this pathway and participates the production of reduced NADPH (nicotinamide adenine dinucleotide phosphate). The NADPH catalyzed by G6PD not only provides reductive force for the biosynthesis of some biomacromolecules in cells, but also is the only reductive force for the regeneration of the reduced glutathione (GSH). Therefore, G6PD plays an important role in the process of cell resistance to oxidative stress.
Characteristics and Sources
Glucose-6-phosphate dehydrogenase was found in the red blood cells of horse in 1931. In 1956, Carson et al. found that the hemolysis was caused by the lack of red blood cell G6PD in some individuals, and researchers began to study G6PD from then on. The molecular mass of G6PD is generally 113 kDa. The isoelectric point is about 4.6, and the optimum pH is 7.8. G6PD is widely distributed in eukaryotic and prokaryotic, and provides hydrogen donors for the synthesis of purines, pyrimidines and aromatic amino acids through pentose phosphate pathway.
To date, over 150 different human G6PD mutants with a spectrum of clinical severity have been identified. Human G6PD contains about 978 amino acid residues. Human G6PD has the NADP+ sites with a `Rossmann-fold' coenzyme-binding domain and a β+α domain which forms the dimer interface (Fig. 1). The human G6PD tetramer has 222 symmetry. The tetramer interface is small and interactions are primarily electrostatic. Structural NADP+ was found to bind in the β+α domain of G6PD between the C-terminus and the β-sheet.
Figure 1. The human G6PDCanton dimer. Helices and sheet strands of the A subunit are shown in red and green,
respectively, and each of the secondary-structure elements identified in the G6PDCanton structure is labelled.
(Kotaka, et al. 2005)
Pentose Phosphate Pathway
The pentose-phosphate pathway is an important pathway of glucose metabolism organisms, especially in plants, and its main physiological function is provide NADPH, which is required for the production of reduced biosynthesis, as well as some intermediates for the synthesis of phosphopentose, amino acids and fatty acids for nucleic acid metabolism.
Especially in red blood cells, the pentose phosphate pathway is the only source of NADPH. The defense mechanism of oxidative damage is highly dependent on G6PD activity. Therefore, red blood cells of patients with G6PD deficiency are extremely vulnerable to oxidative stress. The body's antioxidant system mainly includes catalase, superoxide dismutase and glutathione. Although all antioxidant systems are important for cell survival, G6PD has a unique role to play. First, the glutathione and thiooxide-reducing systems have important antioxidant functions in cells, and NADPH is an essential substrate for both systems to convert oxidative glutathione and thiooxin into their reduction. Therefore, it plays an antioxidant role. Secondly, catalase is another important antioxidant enzyme in cells, which converts hydrogen peroxide into water and oxygen. This process does not require NADPH, but there are specific NADPH binding sites on catalase. NADPH has allosteric effect, and the binding keeps catalase in an activation state. Although there are other sources of intracellular NADPH synthesis, G6PD is found to be the most important pathway, which is of great significance for the normal function of antioxidant system and cell survival. Therefore, the decreased activity of G6PD and the decreased level of NADPH will directly affect the normal function of these antioxidant systems, which causing damage to cells and organs.
Meanwhile, it also plays a very important role in maintaining the REDOX balance of plant cells. The first irreversible oxidation reaction of G6PD in the catalytic pentose phosphate pathway, is the key regulatory enzyme (Fig. 2). In animals, G6PDH linked to the X chromosome, and its deficiency can lead to red blood cell metabolic disorder, retinal blood vessel blockage, ketosis, etc. In severe cases, it may endanger life. Its overexpression can cause abnormal lipid metabolism in human body, but can increase the lifespan of fruit flies. In plants, many researchers have studied their relationship with plant growth and development as well as various environmental stresses in order to elucidate the pentose-phosphate pathway and the possible physiological functions of this enzyme.
Figure 2. Role of G6PDH as rate-limiting step of the PPP for biosynthesis and anti-oxidant activity in higher plants.
(Yu DQ, et al. 2012)
Several Diseases Associated with G6PD
Epidemiological data suggest that G6PD deficiency may be a risk factor for diabetes. In some populations, systematic screening of G6PD activity revealed an increase in the incidence of G6PD deficiency in patients with sugar urine disease compared with the general population. Patients with G6PD deficiency have an increased risk of hemolysis in the case of diabetes, which further suggests that G6PD may play an important role in the development and progression of diabetes.
As one of the main causes of blindness today, oxidative stress and osmotic pressure are believed to be related to the pathogenesis of cataract. Chandrasena et al. found that the level of reduced glutathione (GSH) and G6PD in elder cataract was significantly lower than that in non-cataract individuals, and the risk of elderly cataract was associated with decreased G6PD and GSH levels.
Many studies have demonstrated that the islet oocytes are very sensitive to oxidative damage due to the low expression of the antioxidant enzyme of the cytokine itself. Hyperglycemic-mediated increases in ROS and low levels of the antioxidant enzymes of islet oocytes themselves lead to hypersensitivity of the cytokine to the deleterious effects of ROS accumulation.
Studies have shown that high blood glucose leads to increased ROS accumulation in various types of cells in diabetic patients. Hyperglycemia can increase cyclic adenosine phosphate by increasing the activity of adenosine cyclase, leading to increased protein kinase A activity and G6PD phosphorylation, thereby inhibiting G6PD activity and reducing NADPH, leading to excessive ROS accumulation and enhanced oxidative stress.