Browsing Pathways

Beta-ureidopropionase deficiency (Beta Alanine-Synthase Deficiency, UPB1, BUP1) is an autosomal recessive disease caused by mutations in the UPB1 gene which codes for beta-ureidopropionase. A deficiency in this enzyme results in accumulation of N-carbamyl-beta-amino acids. Symptoms include hypotonia, dystonic movements, scoliosis, microcephaly, and severe developmental delay.

D-Glyceric aciduria is an extremely rare inherited inborn error of metabolism (IEM) of serine and fructose metabolism. It is an autosomal recessive disorder that is caused by a defect in the D-glycerate kinase (GLYCTK) gene. GLYCTK codes for D-glycerate kinase, an enzyme that is responsible for phosphorylating D-glyceric acid into phosphoglycerate. D-Glycerate kinase is an enzyme that participates in 3 metabolic pathways: (1) serine/glycine/threonine metabolism, (2) glycerolipid metabolism, and (3) glyoxylate-dicarboxylate metabolism (which is a minor pathway in fructose metabolism). Defects in the enzyme will lead to accumulations of D-glyceric acid in tissues and biofluids. D-Glyceric aciduria was first described in 1974 and is characterized by elevated levels of D-glyceric acid in the urine. Clinical symptoms of D-glyceric aciduria are highly variable. Some patients have neurological symptoms, with severe mental retardation, seizures, microcephaly, and sometimes early death, whereas others have a mild phenotype with only mild speech delay or even normal development.

Fanconi-Bickel Syndrome is a syndrome first described in 1949, by Falconi and Bickel, where the body is missing a glucose transport, causing glycogen stores to accumulate and cause symptoms such as swelling of the liver and spleen, rickets, and failure to thrive. Hypoglycemia may occur in between meals for patients with this condition. This condition happens through a mutation on the SLC2A2 gene, which has the instructions on how to create a protein which is the glucose-transporter protein 2 (GLUT2). GLUT2 does not work properly when there is an SLC2A2 gene mutation, which makes glycogen build up in the liver and kidneys, causing the symptoms of this condition.

Apparent mineralocorticoid excess (AME), also known as cortisol 11-beta-ketoreductase deficiency, is an extremely rare inborn error of metabolism (IEM) and autosomal recessive disorder of the steroidogenesis pathway. It is caused by a mutation in the HSD11B2 gene which encodes for corticosteroid 11-beta-dehydrogenase isozyme 2, and enzyme that converts cortisol to cortisone in the cell. Without this enzyme being functional, an accumulation of tetrahydrocortisol builds up, while tetrahydrocortisone levels dissipate. AME is characterized excessive thirst and urination, and along with this, symptoms include low levels of aldosterone, failure to thrive and hypertension. Treatment with corticoids that suppress the secretion of cortisol within the body can affect blood pressure and aldosterone levels. Antihypertensive agents are also effective. It is estimated that AME affects less than 1 in 1,000,000 individuals, with less than 100 reported cases as of 2019.

Sarcosinemia (SAR), also known as hypersarcosinemia, sarcosine dehydrogenase complex deficiency, SARDH deficiency, SARDHD or SARD deficiency, is an autosomal recessive metabolic disorder leading to increased levels of the amino acid sarcosine in blood plasma, as well as increased levels of sarcosine excreted in urine. SAR can be caused by a mutation, either homozygous or compound heterozygous, in the SARDH gene which codes for the sarcosine dehydrogenase enzyme. This enzyme converts sarcosine to glycine, and its absence leads to an increase in the amount of sarcosine in the body. It can also potentially be caused by a lack of folate, as folate is used in the sarcosine dehydrogenase reaction, and even with a working enzyme, the lack of substrates can prevent the conversion from occurring, leading to the same effects. The condition has been associated with mental and motor retardation, visual impairment, however other cases have been detected with no mental or physical abnormalities other than increased sarcosine levels, so it is possible that the defect is benign, or that there exist some phenotypes that are more severe than others, or unknown disorders present in the cases showing symptoms. Sarcosine can be formed from a series of reactions starting with trimethylglycine. This, along with homocysteine, react using betaine-homocysteine S-methyltransferase to form L-methionine, as well as dimethylglycine. The dimethylglycine then enters the mitochondrial matrix, and interacts with dimethylglycine dehydrogenase along with a water molecule, forming formadehyde and sarcosine. Sarcosine can also be formed in a reversible reaction from S-adenosylmethionine and glycine, using glycine N-methyltransferase as the enzyme, and forming S-adenosylhomocysteine as another product. Normally, sarcosine can interact with sarcosine dehydrogenase in the mitochondria, forming both formaldehyde and glycine. However, in this disorder, the gene encoding sarcosine dehydrogenase has been mutated and the protein is not produced, preventing this reaction from occurring. This leads to an increased concentration of sarcosine, which leads to the effects of the condition.

Gemcitabine is a cytidine analogue used in the treatment of certain cancers. Gemcitabine enters the cell via sodium nucleoside co-transporters (SLC29A1, SLC28A1, and SLC28A3), where it acts through multiple mechanisms to produce a cytotoxic effect. Gemcitabine is phosphorylated into gemcitabine monophosphate by deoxycytidine kinase, which is then subsequently phosphorylated into the diphosphate and triphosphate nucleotides by UMP-CMP kinase and nucleoside diphosphate kinase respectively. Gemcitabine diphosphate inhibits ribonucleoside-diphosphate reductase, a crucial enzyme in the conversion of ribonucleotides into deoxyribonucleotides for DNA synthesis. Gemcitabine triphosphate on the other hand can be incorporated into DNA, causing chain termination. Furthermore, gemcitabine monophosphate can be deaminated into difluoro-deoxyuridine monophosphate, which inhibits thymidylate synthase, an enzyme involved in the production of dTTP for DNA synthesis.

Nonketotic hyperglycinemia (GCE) is a rare inborn error of metabolism (IEM) and autosomal recessive disorder caused by a defective GLDC gene. GLDC encodes for the enzymes involved in the conversion of glycine to CO2, NH3 and hydroxymethyltetrahydrofolic acid. Most patients have abnormally low oxalate excretion in the urine. Other symptoms start presenting in the first few days of life and include lethargy, hypotonia, and myoclonic jerks, and progressing to apnea. GCE often leads to death, and those who regain spontaneous respiration develop intractable seizures and profound mental retardation. Currently there is no cure for Nonketotic hyperglycinemia therefore treatment involves managing symptoms. Frequency for Nonketotic hyperglycinemia has not been documented worldwide.

Esomeprazole, sold as Nexium, is a proton pump inhibitor (PPI) class drug that suppresses the final step in gastric acid production. In this pathway, esomeprazole is taken orally and is oxidized in the stomach to form the active metabolite of esomeprazole. This active metabolite then binds covalently to the potassium-transporting ATPase protein subunits, found at the secretory surface of the gastric parietal cell, preventing any stimulus. Because the drug binds covalently, its effects are dose-dependent and last much longer than similar drugs that bind to the protein non-covalently. This is because additional ATPase enzymes must be created to replace the ones covalently bound by pantoprazole. Esomeprazole is used to manage gastroesophageal reflux disease, to prevent stomach ulcers, and can be used to help treat the effects of a H. pylori infection.

5-Oxoprolinuria (5-Oxoprolinase deficiency) is a result of a defect in the gamma-glutamyl cycle due to either 5-oxoprolinase or glutathione synthetase deficiency. In the case of glutathione synthetase deficiency, the glycine is not incorporated into gamma-glutamylcysteine. In the case of 5-oxoprolinase, however, pyroglutamic acid accumulates. Symptoms include anemia, mental retardation, metabolic acidosis, respiratory distress and urolithiasis.

Triamterene is a diuretic that belongs to the potassium-sparing class of drugs which are commonly used to manage hypertension and edema. It acts by blocking epithelial sodium channels in the late distal convoluted tubule of the nephron. Specifically, triamterene inhibits amiloride-sensitive sodium channels which are responsible for the reabsorption of sodium in the late distal convoluted tubule in the nephron. This primarily contributes to an increase in sodium excretion and consequentially, fluid excretion which decreases blood volume and blood pressure. Potassium secretion is indirectly affected by the inhibition of sodium reabsorption due to the elimination of the electrochemical gradient that drives potassium loss. This leads to an increase in serum potassium concentration -- a common action for potassium-sparing drugs -- and has the potential to induce hyperkalemia which can potentially lead to severe heart arrhythmias.