Browsing Pathways

Beta cells are found in pancreatic islet cells and their main function is to release insulin. Insulin counteracts glucagon and functions to maintain glucose homeostasis when glucose levels are high. Insulin is contained in granules in the cell as a reserve ready to be released, which is dependent on extracellular glucose levels, and intracellular calcium levels and/or various proteins that activate the vesicle-associated membrane protein on the insulin granules' membranes. In the process of insulin secretion, glucose must first undergo glycolysis to increase ATP in the cell. The inside of the beta cell then becomes electrically positive due to the closure of potassium channels that were inhibited by ATP. From this closure, the potassium is no longer being shuttled out of the cell, thus depolarizing the cell due to the extra intracellular potassium. The resulting action potential from the increased membrane potential causes the voltage gate calcium channels to open, creating an influx of calcium into the cell. This triggers the vesicle-associated membrane protein on the outside of the insulin granule to tether, dock, and fuse with the beta cell membrane. Insulin is then exocytosed from the cell. However, the vesicle-associated membrane protein can be activated by other means in addition to calcium. Acetylcholine can bind to muscarinic acetylcholine receptors on the cell membrane and trigger a G protein cascade. This eventually leads to the activation of inositol trisphosphate to cause calcium release from the rough endoplasmic reticulum so that it can activate the calcium/calmodulin-dependent protein kinase to trigger the vesicle-associated membrane protein. The G protein cascade can also lead to the activation of diacylglycerol and subsequently protein kinase C to lead to the same outcome. Glucagon-like peptide can also trigger a similar G protein cascade when it binds to glucagon-like peptide receptors on the cell membrane of the beta cell. This process involves cAMP and a few other proteins in order to lead to the same eventual outcome of triggering the vesicle-associated membrane protein and the exocytosis of insulin from the beta cell.

Alpha cells are a type of islet cell found in the pancreas that release glucagon. Glucagon counteracts insulin and functions to maintain glucose homeostasis when detected glucose levels are low. Glucagon is contained in granules in the cell as a reserve ready to be released. Extracellular glucose levels and ion channels regulate the secretion of glucagon. Glucose undergoes glycolysis to increase ATP in the cell. The moderate activity of potassium ATP channels causes the membrane potential to be around -70mV. The alpha cell then becomes electrically active due to the closure of potassium channels. The cell membrane becomes depolarized due to voltage dependent sodium, potassium and calcium channels. This causes an increase in action potentials and opens voltage gate calcium channels causing an increase of calcium into the cell. This triggers the exocytosis of glucagon from the cell. Conversely, an increase in extracellular glucose leads to an increase in ATP production and inhibition of potassium ATP channels. The membrane depolarizes to a membrane potential that inactivates voltage dependent calcium channels. This results in decreased intracellular calcium and inhibits exocytosis of glucagon.

The Mevalonate Pathway is a necessary pathway that occurs in archaea, eukaryotes and select bacteria. It has mainly been studied with regard to cholesterol biosynthesis and how it relates to cardiovascular disease in humans, but has recently garnered attention for its many other essential roles within human pathology. The pathway begins in the cytoplasm with acetyl-CoA and acetoacetyl-CoA, which interact with acetyl-CoA acetyltransferase, coenzyme A and water to synthesize hydroxymethylglutaryl-CoA synthase. In turn, this synthase teams up with coenzyme A and a hydrogen ion in the endoplasmic reticulum to create 3-hydroxy-3-methylglutaryl-CoA. 3-Hydroxy-3-methylglutaryl-CoA then pairs with 2NADPH, 2 hydrogen ions and is catalyzed by 3-hydroxy-3-methylglutaryl-coenzyme A reductase to produce (R)-mevalonate, also producing byproducts CoA and NADP. Exiting the endoplasmic reticulum, and entering the peroxisome, (R)-mevalonate uses the help of ATP and mevalonate kinase to create mevalonic acid (5P). This piece is especially important to the human species as decreased activity of the enzyme mevalonate kinase has been found to be a direct link to two auto-inflammatory disorders: MVA and HIDS. Using phosphomevalonate kinase and ATP, the pathway re-enters the cytoplasm and mevalonic acid (5P) converts to (R)-mevalonic acid-5-pyrophosphate and ADP. (R)-mevalonic acid-5-pyrophosphate, ATP and diphosphomevalonate decarboxylase work together to create phosphate, carbon dioxide, ADP and isopentenyl pyrophosphate. Re-entering the peroxisome, isopentenyl diphosphate delta isomerase 1 is waiting to propel isopentenyl pyrophosphate into dimethylallylpyrophosphate. This pushes the pathway back into the cytoplasm, where another isopentenyl pyrophosphate molecule and the enzyme farnesyl pyrophosphate synthase create pyrophosphate and geranyl-PP. Yet another isopentenyl pyrophosphate molecules works with farnesyl pyrophosphate synthase to produce pyrophosphate and farnesyl pyrophosphate. Now in the endoplasmic reticulum membrane, 2 farnesyl pyrophosphate molecules with the help of NADPH and a hydrogen ion catalyze with squalene synthase and create squalene. This is an important first step in the specific hepatic cholesterol pathway. Remaining in the endoplasmic reticulum membrane, squalene, FMNH, oxygen and squalene monooxygenase synthesize (S)-2,3-epoxysqualene. This comes along with the byproducts of flavin mononucleotide, a hydrogen ion and water. In the final reaction within this pathway, lanesterol synthase converts (S)-2,3-epoxysqualene to lanosterin. Not pictured in this pathway, lanosterin will eventually be converted to cholesterol, an important part of many functions in the human body.

The Bloch pathway, named after Konrad Bloch, is the pathway following the mevalonate pathway occurring within the cell to complete cholesterol biosynthesis. Cholesterol is a necessary metabolite that helps create many essential hormones within the human body. This pathway, combined with the mevalonate pathway is one of two ways to biosynthesize cholesterol; the Kandutsch-Russell pathway is an alternative pathway that uses different compounds than the Bloch Pathway beginning after lanosterol. The first three reactions occur in the endoplasmic reticulum. Lanosterol, a compound created through the mevalonate pathway, binds with the enzyme lanosterol 14-alpha demethylase to become 4,4-dimethyl-14a-hydroxymethyl-5a-cholesta-8,24-dien-3b-ol. Moving to the next reaction, 4,4-dimethyl-14a-hydroxymethyl-5a-cholesta-8,24-dien-3b-ol utilizes the enzyme lanosterol 14-alpha demethylase to create 4,4-dimethyl-14α-formyl-5α-cholesta-8,24-dien-3β-ol. Lanosterol 14-alpha demethylase is used one last time in this pathway, converting 4,4-dimethyl-14α-formyl-5α-cholesta-8,24-dien-3β-ol into 4,4-dimethyl-5a-cholesta-8,14,24-trien-3b-ol. Entering the inner nuclear membrane, 4,4-dimethyl-5a-cholesta-8,14,24-trien-3b-ol is catalyzed by a lamin B receptor to create 4,4-dimethyl-5a-cholesta-8,24-dien-3-b-ol. Entering the endoplasmic reticulum membrane, 4,4-dimethyl-5a-cholesta-8,24-dien-3-b-ol, with the help of methyl monooxygenase 1 is converted to 4a-hydroxymethyl-4b-methyl-5a-cholesta-8,24-dien-3b-ol. The enzyme methyl monooxygenase 1 uses 4a-hydroxymethyl-4b-methyl-5a-cholesta-8,24-dien-3b-ol to produce 4a-formyl-4b-methyl-5a-cholesta-8,24-dien-3b-ol. This reaction is repeated once more, using 4a-formyl-4b-methyl-5a-cholesta-8,24-dien-3b-ol and methyl monooxygenase 1 to create 4a-carboxy-4b-methyl-5a-cholesta-8,24-dien-3b-ol. Briefly entering the endoplasmic reticulum, 4a-carboxy-4b-methyl-5a-cholesta-8,24-dien-3b-ol then uses sterol-4-alpha-carboxylate-3-dehyrogenase to catalyze into 3-keto-4-methylzymosterol. Back in the endoplasmic reticulum membrane, where the pathway will continue on for the remaining reactions, 3-keto-4-methylzymosterol combines with 3-keto-steroid reductase to create 4a-methylzymosterol. 4a-Methylzymosterol joins the enzyme methylsterol monooxgenase 1 to result in 4a-hydroxymethyl-5a-cholesta-8,24-dien-3b-ol. 4a-Hydroxymethyl-5a-cholesta-8,24-dien-3b-ol uses methylsterol monooxygenase 1 to convert to 4a-formyl-5a-cholesta-8,24-dien-3b-ol. 4a-Formyl-5a-cholesta-8,24-dien-3b-ol proceeds to use the same enzyme used in the previous reaction: methylsterol monooxygenase 1, to catalyze into 4a-carboxy-5a-cholesta-8,24-dien-3b-ol. Sterol-4-alpha-carboxylate-3-dehydrogenase is used alongside 4a-carboxy-5a-cholesta-8,24-dien-3b-ol to produce 5a-cholesta-8,24-dien-3-one (also known as zymosterone). Zymosterone (5a-cholesta-8,24-dien-3-one) teams up with 3-keto-steroid reductase to create zymosterol. Zymosterol proceeds to use the enzyme 3-beta-hydroxysteroid-delta(8),delta(7)-isomerase to catalyze into 5a-cholesta-7,24-dien-3b-ol. The compound 5a-cholesta-7,24-dien-3b-ol then joins lathosterol oxidase to convert to 7-dehydrodesmosterol. 7-Dehydrodesmosterol and the enzyme 7-dehydrocholesterol reductase come together to create desmosterol. This brings the pathway to the final reaction, where desmosterol combines with delta(24)-sterol reductase to finally convert to cholesterol.

The Kandutsch-Russell pathway is the alternative pathway stemming from the mevalonate pathway completing cholesterol biosynthesis. The Bloch pathway and the Kandutsch-Russell pathway are both key to a functioning human body as cholesterol aids in the development of many important nutrients and hormones, such as vitamin D. Starting in the endoplasmic reticulum, lanosterol is the first compound used in this pathway, and when catalyzed by delta(24)-sterol-reductase, becomes 24,25-dihydrolanosterol. 24,25-Dihydrolanosterol is quickly converted to 4,4-dimethyl-14a-hydroxymethyl-5a-cholesta-8-en-3b-ol with the help of the enzyme lanosterol 14-alpha demethylase. This same enzyme, lanosterol 14-alpha demethylase, is also responsible for the conversion of 4,4-dimethyl-14a-hydroxymethyl-5a-cholesta-8-en-3b-ol into 4,4-dimethyl-14a-formyl-5a-cholest-8-en-3b-ol. Lanosterol 14alpha demethylase is used once more here, to push the pathway into the inner nuclear membrane, converting 4,4-dimethyl-14a-formyl-5a-cholest-8-en-3b-ol into 4,4-dimethyl-5a-cholesta-8,14-dien-3b-ol. Now located in the inner nuclear membrane, 4,4-dimethyl-5a-cholesta-8,14-dien-3b-ol is converted into 4,4-dimethyl-5a-cholesta-8-en-3b-ol through the help of a lamin-b receptor. Entering the endoplasmic reticulum membrane, methylsterol monooxygenase 1 is used to convert 4,4-dimethyl-5a-cholesta-8-en-3b-ol into 4a-hydroxymethyl-4b-methyl-5a-cholesta-8-en-3b-ol. 4a-Hydroxymethyl-4b-methyl-5a-cholesta-8-en-3b-ol then uses methylsterol monooxygenase 1 to become 4a-formyl-4b-methyl-5a-cholesta-8-en-3b-ol. Once again, methylsterol monooxygenase 1 is used to convert 4a-formyl-4b-methyl-5a-cholesta-8-en-3b-ol into 4a-carboxy-4b-methyl-5a-cholesta-8-en-3b-ol. Now using sterol-4-alpha-carboxylate 3-dehydrogenase, 4a-carboxy-4b-methyl-5a-cholesta-8-en-3b-ol is turned into 4a-methyl-5a-cholesta-8-en-3-one. This puts the pathway in the cell membrane, where a 3-keto-steroid reductase is used to convert 4a-methyl-5a-cholesta-8-en-3b-one into 4a-methyl-5a-cholesta-8-en-3-ol. Moving back into the endoplasmic reticulum membrane, methylsterol monooxygenase 1 converts 4a-methyl-5a-cholesta-8-en-3-ol into 4a-hydroxymethyl-5a-cholesta-8-en-3b-ol. Methylsterol monooxygenase is used twice more in this pathway, first converting 4a-hydroxymethyl-5a-cholesta-8-en-3b-ol into 4a-formyl-5a-cholesta-8-en-3b-ol, then converting 4a-formyl-5a-cholesta-8-en-3b-ol into 4a-carboxy-5a-cholesta-8-en-3b-ol. Now using sterol-4-alpha-carboxylate 3 dehydrogenase, 4a-carboxy-5a-cholesta-8-en-3b-ol becomes 5a-cholesta-8-en-3-one and brings the pathway back to the cell membrane. 5a-Cholesta-8-en-3-one teams up with a 3-keto-steroid reductase to create 5a-cholest-8-en-3b-ol. Then, stepping back into the endoplasmic reticulum membrane, 5a-cholest-8-en-3b-ol enlists the help of 3-beta-hydroxysteroid-delta(8),delta(7)-isomerase to produce lathosterol. Lathosterol and lathosterol oxidase work together to make 7-dehydrocholesterol . Finally, 7-dehydrocholesterol partners with 7-dehydrocholesterol reductase to create cholesterol, completing the final step in cholesterol biosynthesis.

Irinotecan, branded as Camptosar, Campto, Onivyde and others, is a cancer medication used to treat colon and small cell lung cancers, alone or with other drugs. Irinotecan can be processed by the cytochrome P450 3A4 enzyme, producing both the side product glutaral, as well as a compound called NPC. the NPC can then be catalyzed by liver carboxylesterase 1 to form 7-ethyl-10-hydroxy-camptothecin, or SN-38. Alternatively, irinotecan can directly form SN-38 via catalysis by liver carboxylesterase 1. After its formation, SN-38 is converted to SN-38 glucuronide by UDP-glucuronosyltransferase 2B11. This can then be converted back to SN-38 in the lysosome by beta-glucuronidase, or can be excreted as the end product of the pathway.

Isoniazid is an antibiotic drug used to treat tunerculosis, as well as other types of mycobacteria. Through a currently unknown reaction that may be spontaneous or enzymatic, pyruvic acid or oxoglutaric acid can undergo a dehydration reaction with isoniazid, forming isoniazid pyruvate or isoniazid alpha-ketoglutaric acid. Isoniazid may also react with hydrogen peroxide in the lysosome, forming an isonicotinoyl radical catalyzed by myeloperoxidase. The isonicotinoyl radical can then have either NAD or NADP added in a non-enzymatic reaction, forming isonicotinoyl-NAD and NADP adducts. Isoniazid can have an acetyl group added to it by arylamine N-acetyltransferase 2, fvorming acetylisoniazid. This can then enter the endoplasmic reticulum and, with the addition of a water molecule, can form isonicotinic acid and acetylhydrazine. Isoniazid can also be converted to hydrazine and isonicotinic acid via the same reaction, and the hydrazine can have an acetyl group added to it by arylamine N-acetyltransferase 2 in order to form acetylhydrazine. Acetylhydrazine can have another acetyl group added to it by arylamine N-acetyltransferase 2 to form diacetylhydrazine which is then excreted. It can alternatively be processed by cytochrome P450 2E1 into hepatotoxins, which are then joined to glutatione by glutatione S-transferase omega-2 to form R-S-glutatione, which is then excreted. Finally, isonicotinic acid can react with a glycine in an unclear reaction, potentially requiring ATP and coenzyme A and forming an intermediate, producing isonicotinylglycine, which is also excreted.

Fluorouracil, sold as Adrucil, Carac, Efudex, Efudix and others, is a medication used to treat various forms of cancer. It consists of a fluorine atom on the 5th carbon of a uracil molecule, and is treated similarly to the uracil during metabolism by the body, but the fluorouracil tends to be absorbed more readily by tumor cells than healthy cells, allowing it to target cancer cells. Capecitabine is one of the prodrugs that can be metabolized into fluorouracil. First, it is converted to 5'-deoxy-5-fluorocytidine by liver carboxylesterase 1 in the endoplasmic reticulum, and then by cytidine deaminase into 5'-deoxy-5-fluorouridine. Finally, it is converted into fluorouracil by thymidine, which removes the 5'-deoxyribose-1-phosphate from it. Tegafur is another prodrug that may be converted to fluorouracil, this time by cytochrome P450 2A6 in the endoplasmic reticulum membrane. From this point, fluorouracil can be converted to 5,6-dihydro-5-fluorouracil by dihydropyrimidine dehydrogenase, which adds a hydrogen ion to it. The 5,6-dihydro-5-fluorouracil can then have a water molecule added by dihydropyrimidinase, forming alpha-fluoro-beta-ureidopropionic acid. Finally, this is converted to alpha-fluoro-beta-alanine by beta-ureidopropionase, which is an end product of the pathway, and is then excreted. Fluorouracil can also be converted to and from 5-fluorouridine by uridine phosphorylase 2, which is then converted to 5-fluorouridine monophosphate by urudine-cytidine kinase-like 1. 5-fluorouridine monophosphate is also formed from fluorouracil via catalysis by uridine 5'-monophosphate synthase. Regardless of the pathway through which it is created, 5-fluorouridine monophosphate is converted to 5-fluorouridine diphosphate by UMP-CMP kinase, which adds a phosphate group to it. Whithin the mitochondria, mucleoside diphosphate kinase 6 adds one final phosphate group to it, forming 5-fluorouridine triphosphate, another end product of the pathway. If it does not enter the mitochondria, 5-fluorouridine diphosphate can instead be converted to 5-fluorodeoxyuridine diphosphate by the ribonucleoside-diphosphate reductase complex. Finally, fluorouracil may be converted to and from floxuridine by thymidine phosphorylase, which then is converted to 5-fluorodeoxyuridine monophosphate by cytosolic thymidine kinase. This molecule also forms 5-fluorodeoxyuridine diphosphate via UMP-CMP kinase, bringing these two branches of the pathway together. Regardless of its origin, 5-fluorodeoxyuridine diphosphate can be converted to 5-fluorodeoxyuridine triphosphate by nucleoside diphosphate kinase 6 in the mitochondria. It may stop there, or be converted back to 5-fluorodeoxyuridine monophosphate by deoxyuridine 5'-triphosphate nucleotidohydrolase, also in the mitochondria.

Arsenate is a compound similar to phosphate, but containing an arsenic atom instead of the phosphorous. As such, it is treated similarly to a phosphate ion. However, if the arsenate replaces inorganic phosphates in glycolysis, it allows glycolysis to proceed, but does not generate ATP, uncoupling glycolysis. It can also bind to lipoic acid in the Krebs cycle, leading to a greater loss of ATP. Arsenate can enter into the cell via aquaporins 7 and 9, as well as facilitated glucose transporter members 1 and 4 of solute carrier family 2, and does so by diffusion. Once inside the cell, the arsenate can be converted to arsenite via the glutathione S-transferase omega-1 enzyme, or it can be converted to ribose-1-arsenate via the purine nucleoside phosphorylase. Ribose-1-arsenate then can spontaneously form arsenite through a reaction involving hydrogen and dihydrolipoate. After arsenite has been formed by either of these methods, arsenite methyltransferase catalyzes its formation into methylarsonate. From here, it forms methylarsonite via the glutathione S-transferase omega-1 enzyme again. The methylarsonite reacts with S-adenosylmethionine, catalyzed by arsenite methyltransferase, in order to become dimethylarsinate. Finally, the compound once again interacts with the glutathione S-transferase omega-1 enzyme to form dimethylarsinous acid, the final compound in this pathway.