Introduction
Glucose 6-Phosphate Dehydrogenase (G6PDH) is a cytosolic enzyme that participates in the pentose phosphate pathway. G6PDH is present among many species, ranging from bacteria to humans where it plays a vital role in maintaining the proper level of nicotinamide adenine dinucleotide phosphate (NADPH) as reducing power for the cell. [1] In humans, two isoforms exist from a single gene coding for G6PDH. [2] It is thought by some scientists that the genetic variation of G6PDH present in humans is due to generational adaption to malarial infections. [3] Present in approximately 400 million people around the world, G6PDH deficiency is the most common enzyme defect in human beings. [4]
Structure
Generally found as a dimer, G6PDH consists of two identical monomers. [5] In the correct conditions, it is possible for these dimers to dimerize with one another to form tetrameric units. [6] Within the dimer, each monomer has a binding site for the substrate, glucose 6-phosphate (G6P), as well as a catalytic binding site for the coenzyme, NADP+/NADPH. [7] It is estimated that each monomeric unit is approximately 500 amino acids in length, with this number increasing to 514 amino acids for G6PDH in humans. [6] An additional binding site, deemed the NADP+ structural site, is present in some higher species such as in humans. The purpose of this additional site is not currently known, and it does not appear to participate in the reaction catalyzed by G6PDH. [1]
Figure 1: The active site with important labeled amino acid residues is displayed from Altaetran (2017). [8]
Figure 2: The 3D structure of G6PDH is shown from Christaras and Jfdwolff (2017). [9]
Function and General Role in Metabolism
G6PDH catalyzes the following reaction:
D-glucose 6-phosphate + NADP+ ⇌6-phospho-D-glucono-1,5-lactone + NADPH + H+
G6PDH is a cytosolic enzyme that is encoded by a housekeeping gene. This X-linked gene functions mainly to produce NADPH, a necessary electron donor, for the cell. These NADPH molecules help to protect the cell against oxidative stress and provide reduction power for biosynthetic reactions. [10] As a part of the pentose phosphate pathway, G6PDH contributes to a series of reactions that convert glucose to ribose 5-phosphate which can then be used for the production of nucleotides, DNA, and RNA. NADPH, a product of the oxidative pentose phosphate pathway, plays a key role in protecting cells from a group of molecules called reactive oxygen species, such as peroxides and superoxides. This is something essential in red blood cells, which are incapable of producing NADPH. [11] Production of these reactive oxygen species is a part of natural cell metabolism, and red blood cells are at a much greater risk for damage from these oxidizing free radicals because they serve as oxygen carriers. NADPH is important for keeping adequate levels of reduced glutathione in the cell, if this level goes down, the cell will become more susceptible to hemolysis.
Figure 3: An overview of the pentose phosphate pathway is displayed in a flowchart. Key products are highlighted in pink from Lehninger Prinsiples of Biochemistry (2008). [12]
Regulation
G6PDH is responsible for the rate-limiting step of the pentose phosphate pathway and any changes to its regulation can have consequences for the activity of the entire pathway. The substrate, glucose 6-phosphate (G6P) acts as an activator for the enzyme. Because G6PDH helps to produce reducing power for the cell, it is also influenced by the ratio of NADPH to NADP+ in the cell. This ratio is usually around 100:1 for cells engaged in biosynthetic reactions. If, for example, increased fatty acid synthesis causes a decrease in the levels of NADPH in the cell, G6PDH will be stimulated in order to produce more.
G6PDH also has negative modes of regulation. Within the enzyme, there exists an amino acid residue, Lysine 403, that has been evolutionarily conserved among species. If this residue is acetylated, G6PDH suffers a complete loss of activity due to it being unable to form active dimers. Another mechanism of inhibition includes acetylation of a different Lysine reside, Lysine 304, which causes steric hindrance preventing the binding of NADP+ into the structural site, causing a reduction in stability of the enzyme. [13]
Pathologies
Deficiency in G6PDH is a very common issue around the world and can lead to issues with hemolytic anemia, the spontaneous rupturing of red blood cells, especially when the individual has been exposed to an oxidative stress such as infection, new medications, antimalarial, antibiotics, or ingestion of fava beans. [14] This condition is an X-linked recessive missense mutation that is most common among those from Mediterranean descent, such as Spaniards, Italians, Greeks, Armenians, etc., or those of African descent. The mutations that cause a G6PDH deficiency are typically found “on the long arm of the X chromosome, on band Xq28. The G6PDH gene spans some 18.5 kilobases.” [15] In 2013, G6PDH deficiency resulted in 4,100 deaths globally. [5] Those who are carriers of the G6PDH allele are thought to have increased protection against malarial infections, in fact, some display complete immunity against the disease. This selective advantage is thought to account for the persistence of this allele in certain populations around the world and seems to correlate with areas endemic for malaria infections from Plasmodium falciparum and Plasmodium vivax. [16] However, certain antimalarial drugs, such as primaquine, can actually cause hemolytic anemia in patients with a G6PDH deficiency. [17] Primaquine, an 8-aminoquinolone, is one of the only readily available medicines effective against the hypnozoite stage of Plasmodium vivax. Because of this, it is important to identify those with this deficiency in areas where malaria treatment is common in order to avoid unnecessary complications in the affected patients. [18] The occurrence of symptoms from G6PDH deficiency also appears to act as evidence that some atypical drug reactions may be influenced by the genetics of the patient. [19] It is also unknown how certain non-steroidal anti-inflammatory drugs, such as aspirin when used for antiplatelet therapy, will affect those with a G6PDH deficiency. Proper protocol has not yet been established, and further research will be needed in order to determine the safety of these drugs for certain patients. [20]
Figure 4: National allele frequency of G6PD deficiency from Howes (2013). [21]
Most of those affected by a deficiency in G6PDH actually present as asymptomatic. Those that do display symptoms are disproportionally in the male population due to the X-linked nature of inheritance for the deficiency. Because women carry two X chromosomes, those with one affected chromosome will display a deficiency in only half of her red blood cells. Hemolytic anemia can present in a number of ways in affected individuals and can include symptoms such as prolonged neonatal jaundice, diabetic ketoacidosis, hemolytic crisis in response to illness or certain drugs, and in very severe cases, acute kidney failure.
If a hemolytic response is invoked due to the consumption of a specific type of broad bean, the fava bean, the condition is referred to as favism. Favism is thought to be more prevalent in younger children and infants. Individuals with favism also have a deficiency in G6PDH but not all with a G6PDH deficiency will suffer from favism. The specific details of the relationship between favism and G6PDH deficiency is not currently well understood. A deficiency in 6-Phosphogluconate Dehydrogenase can present with similar symptoms and may often be mistaken for a G6PDH deficiency because both enzymes are involved in the pentose phosphate pathway. [22]
Diagnosis
A deficiency in G6PDH is often suspected when those from certain ethnic groups suffer from anemia, jaundice, or hemolysis after exposure to one of the known triggers. Certain lab tests such as a complete blood count and reticulocyte count, liver enzyme panel, lactate dehydrogenase, haptoglobin, and a direct antiglobin test (Coomb’s test) can be done to aid diagnosis. Direct DNA testing and sequencing of the G6PDH gene can also aid in diagnosis. The World Health Organization (WHO) classifies deficiencies in G6PDH into Classes I to V, with Class I being the most severe. [23] The prognosis of those deficient in G6PDH is that typically hemolytic episodes will clear on their own and these patients do not appear to be any more susceptible to illness than the general population. In fact, it is thought that they may actually have a decreased risk for certain diseases such as cerebrovascular disease and ischemic heart disease. [24]
Treatment
At the current time, specific treatment options are not readily available. Affected individuals are encouraged to avoid any known triggers of the condition such as the certain foods and medications that can provoke hemolysis. [25] Vaccination is also encouraged as that may help to protect against any infection-induced oxidative stress. If the patient is suffering from acute hemolysis, it may be necessary to do a blood transfusion because the newly transfused blood cells will not be G6PDH deficient and will live a normal lifespan in the recipient. Folic acid is recommended as a supplement, and in some cases, patients have benefited from a splenectomy as the spleen is a main site of the destruction of erythrocyte destruction. [26]
Conclusion
In conclusion, Glucose 6-Phosphate Dehydrogenase is a very important enzyme that is ubiquitous in many different species, ranging from humans to bacteria. G6PDH functions in the pentose phosphate pathway and carries out a vital role for the cell by producing adequate levels of NADPH to be used as reducing power as well as ribose 5-phosphate to be used in the production of nucleotides, DNA, and RNA. The levels of NADPH produced by the pentose phosphate pathway play a key role in providing adequate levels of reduced glutathione to combat oxidative stress in red blood cells, which are incapable of producing NADPH for themselves. A deficiency in G6PDH is one of the most common enzyme deficiencies around the world and is most commonly seen in those of African or Mediterranean descent. Due to the X-linked nature of the mutation, this condition is most commonly seen in males, although there are occurrences of the deficiency in females as well. A deficiency in G6PDH is thought to provide either partial or complete protection against malarial infections from Plasmodium falciparum and a coevolution is thought to have occurred with this genetic mutation in areas where malaria is endemic. Although treatment options are not currently available for those suffering from a deficiency in G6PDH, avoiding triggers and blood transfusions in severe cases of hemolytic episodes have proved to be effective.