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Showing 11 - 20 of 49833 pathways
SMPDB ID Pathway Chemical Compounds Proteins


Pw000143 View Pathway

Inositol Metabolism

The carbocyclic polyol inositol (otherwise known as myo-inositol) has a significant role in physiological systems as many secondary eukaryotic messengers derive their structure from inositol. Examples of secondary messengers derived from inositol include inositol phosphates, phosphatidylinositol (PI), and phosphatidylinositol phosphate (PIP) lipids. Inositol is abundant in many commonly consumed foods such as bran-rich cereals, beans, nuts, and fruit (particularly cantaloupe, melons, and oranges). It can also be synthesized by the body through the conversion of glucose-6-phosphate into mho-inositol under the following pathway: (1) glucose-6-phosphate undergoes isomerization due to the action of inositol-3-phosphate synthase (ASYNA1) which produces myo-inositol 3-phosphate; (2) myo-inositol 3-phosphate undergoes dephosphorylation via the action of inositol monophosphatase (IMPase 1) to produce myo-inositol. From this point, myo-inositol can move through multiple different fates depending on the secondary messenger being synthesized. For phosphatidyliositol, phosphatidylinositol synthase generates it with the substrates CDP-diacylglycerol and myo-inositol. Phosphatidyliositol can be modified further to generate phosphatidylinositol phosphate lipids via the action of class I, II and III phosphoinositide 3-kinases (PI 3-kinases). Other messengers (i.e. inositol phosphates) can be produced with the phospholipase C-mediated hydrolysis of phosphatidylinositol phosphates or with the action of other enzymes that remove or add phosphate groups.


Pw000017 View Pathway

Catecholamine Biosynthesis

The Catecholamine Biosynthesis pathway depicts the synthesis of catecholamine neurotransmitters. Catecholamines are chemical hormones released from the adrenal glands as a response to stress that activate the sympathetic nervous system. They are composed of a catechol group and are derived from amino acids. The commonly found catecholamines are epinephrine (adrenaline), norepinephrine (noradrenaline) and dopamine. They are synthesized in catecholaminergic neurons by four enzymes, beginning with tyrosine hydroxylase (TH), which generates L-DOPA from tyrosine. The L-DOPA is then converted to dopamine via aromatic L-amino acid decarboxylase (AADC), which becomes norepinephrine via dopamine beta-hydroxylase (DBH); and finally is converted to epinephrine via phenylethanolamine N-methyltransferase (PNMT).


Pw000018 View Pathway

Cysteine Metabolism

The semi-essential amino aid cysteine is tightly regulated in the body to ensure proper levels for metabolism but maintaining levels below toxic thresholds. Cysteine can be obtained from diet or synthesized from O-acetyl-L-serine. Cystine is the dimeric form of cysteine. Cysteine is a precursor for protein synthesis and an antioxidant. Impaired cysteine metabolism has been linked with neurodegenerative disorders.


Pw000004 View Pathway

Glutathione Metabolism

Glutathione (GSH) is an low-molecular-weight thiol and antioxidant in various species such as plants, mammals and microbes. Glutathione plays important roles in nutrient metabolism, gene expression, etc. and sufficient protein nutrition is important for maintenance of GSH homeostasis. Glutathione is synthesized from glutamate, cysteine, and glycine sequentially by gamma-glutamylcysteine synthetase and GSH synthetase. L-Glutamic acid and cysteine are synthesized to form gamma-glutamylcysteine by glutamate-cysteine ligase that is powered by ATP. Gamma-glutamylcysteine and glycine can be synthesized to form glutathione by enzyme glutathione synthetase that is powered by ATP, too. Glutathione exists oxidized (GSSG) states and in reduced (GSH) state. Oxidation of glutathione happens due to relatively high concentration of glutathione within cells.


Pw000149 View Pathway

Propanoate Metabolism

This pathway depicts the metabolism of propionic acid. Propionic acid in mammals typically arises from the production of the acid by gut or skin microflora. Propionic acid producing bacteria (Propionibacterium sp.) are particularly common in sweat glands of mammals. After entering a cell, the propionic acid (propanoate) then enters the mitochondria where it is converted into propanol adenylate (or propionyl adenylate or propionyl-AMP) via propionyl-CoA synthetase and acetyl-CoA synthetase. The propionyl adenylate then is converted into propionyl coenzyme A (propionyl-CoA) via the same pair of enzymes. Propionyl-CoA is a relatively common compound that can also arise from the metabolic breakdown of fatty acids containing odd numbers of carbon atoms. Propionyl-CoA is also known to arise from the breakdown of some amino acids. Since propanoate has three carbons, propionyl-CoA cannot directly enter the beta-oxidation cycle (which requires two carbons from acetyl-CoA). Therefore, in most vertebrates, propionyl-CoA is carboxylated into D-methylmalonyl-CoA via propionyl-CoA carboxylase. The resulting compound is isomerized into L-methylmalonyl-CoA via methylmalonyl-CoA epimerase. A vitamin B12-dependent enzyme, called methylmalonyl CoA mutase catalyzes the rearrangement of L-methylmalonyl-CoA to succinyl-CoA, which is an intermediate of the citric acid cycle. Also depicted in this pathway is another propionic acid homolog called hydroxypropanoic acid (hydroxypropionate). This compound is also produced by bacteria and imported into cells. Hydroxypropionate can be converted into 3-hydroxypropionyl-CoA. This compound can be either enzymatically converted to acryloyl-CoA and then to propionyl-CoA or it can spontaneously convert to malonyl-CoA. Malonyl-CoA can convert into acetyl-CoA (via acetyl-CoA carboxylase in the cytoplasm or malonyl carboxylase in the mitochondria) whereupon it may enter a variety of pathways. In a rare genetic metabolic disorder called propionic acidemia, propionate acts as a metabolic toxin in liver cells by accumulating in the liver mitochondria as propionyl-CoA and its derivative methylcitrate. Both propionyl-CoA and methylcitrate are known TCA inhibitors. Glial cells are particularly susceptible to propionyl-CoA accumulation. In fact, when propionate is infused into rat brains and take up by the glial cells, it leads to behavioural changes that resemble autism (PMID: 16950524).


Pw000053 View Pathway

Vitamin B6 Metabolism

As is commonly known there are many vitamins, the vitamin B complex group being one of the most well known. An important vitamin B complex group vitamin is vitamin B6, which is water-soluble. Moreover, this vitamin comes in various forms, one of which is an active form, known by the name pyridoxal phosphate or PLP. PLP serves as cofactor in a variety of reactions including from amino acid metabolism, (in particular in reactions such as transamination, deamination, and decarboxylation). To complicate matters however, there are in fact seven alternate forms of this same vitamin. These include pyridoxine (PN), pyridoxine 5’-phosphate (PNP), pyridoxal (PL), pyridoxamine (PM), pyridoxamine 5’-phosphate (PMP), 4-pyridoxic acid (PA), and the aforementioned pyridoxal 5’-phosphate (PLP). One of these forms, PA, is in fact a catabolite whose presence is found in excreted urine. For a person to absorb some of these active forms of vitamin B6 such as PLP or PMP they must first be dephosphorylized. This done via an alkaline enzyme phosphatase. There are a wide variety of biproducts from the metabolism in question, most of which find there ways into the urine and from there are excreted. One such biproduct is 4-pyridoxic acid. In fact this last biproduct is found in such large quantities that estimates of vitamin B6 metabolism birproducts show that 4-pyridoxic acid is as much as 40-60% of all the biproducts.Of course, it is not the only product of metabolism. Others include,include pyridoxal, pyridoxamine, and pyridoxine.


Pw000006 View Pathway

Alpha Linolenic Acid and Linoleic Acid Metabolism

Linoleic acid (LNA) is a polyunsaturated fatty acid (PUFA) precursor to the longer n−6 fatty acids commonly known as omega-6 fatty acids. Omega-6 fatty acids are characterized by a carbon-carbon double bond at the sixth carbon from the methyl group. Similarly, the PUFA alpha-linoleic acid (ALA) is the precursor to n-3 fatty acids known as omega-3 fatty acids which is characterized by a carbon-carbon double bond at the third carbon from the methyl group. Both LNA and ALA are essential dietary requirements for all mammals since they cannot be synthesized natively in the body. Both undergo a series of similar conversions to reach their final fatty acid form. LNA enters the cell and is catalyzed to gamma-linolenic acid (GLA) by acyl-CoA 6-desaturase (delta-6-desaturase/fatty acid desaturase 2). GLA is then converted to dihomo-gammalinolenic acid (DGLA) by elongation of very long chain fatty acids protein 5 (ELOVL5). DGLA is then converted to arachidonic acid (AA) by acyl-CoA (8-3)-desaturase (delta-5-desaturase/fatty acid desaturase 1). Arachidonic acid is then converted to a series of short lived metabolites called eicosanoids before finally reaching it's final fatty acid form.


Pw000010 View Pathway

Arginine and Proline Metabolism

The arginine and proline metabolism pathway illustrates the biosynthesis and metabolism of several amino acids including arginine, ornithine, proline, citrulline, and glutamate in mammals. In adult mammals, the synthesis of arginine takes place primarily through the intestinal-renal axis (PMID: 19030957). In particular, the amino acid citrulline is first synthesized from several other amino acids (glutamine, glutamate, and proline) in the mitochondria of the intestinal enterocytes (PMID: 9806879). The mitochondrial synthesis of citrulline starts with the deamination of glutamine to glutamate via mitochondrial glutaminase. The resulting mitochondrial glutamate is converted into 1-pyrroline-5-carboxylate via pyrroline-5-carboxylate synthase (P5CS). Alternately, the 1-pyrroline-5-carboxylate can be generated from mitochondrial proline via proline oxidase (PO). Ornithine aminotransferase (OAT) then converts the mitochondrial 1-pyrroline-5-carboxylate into ornithine and the enzyme ornithine carbamoyltransferase (OCT -- using carbamoyl phosphate) converts the ornithine to citrulline (PMID: 19030957). After this, the mitochondrial citrulline is released from the small intestine enterocytes and into the bloodstream where it is taken up by the kidneys for arginine production. Once the citrulline enters the kidney cells, the cytosolic enzyme argininosuccinate synthetase (ASS) will combine citrulline with aspartic acid to generate argininosuccinic acid. After this step, the enzyme argininosuccinate lyase (ASL) will remove fumarate from argininosuccinic acid to generate arginine. The resulting arginine can either stay in the cytosol where it is converted to ornithine via arginase I (resulting in the production of urea) or it can be transported into the mitochondria where it is decomposed into ornithine and urea via arginase II. The resulting mitochondrial ornithine can then be acted on by the enzyme ornithine amino transferase (OAT), which combines alpha-ketoglutarate with ornithine to produce glutamate and 1-pyrroline-5-carboxylate. The mitochondrial enzyme pyrroline-5-carboxylate dehydrogenase (P5CD) acts on the resulting 1-pyrroline-5-carboxylate (using NADPH as a cofactor) to generate glutamate. Alternately, the mitochondrial 1-pyrroline-5-carboxylate can be exported into the kidney cell’s cytosol where the enzyme pyrroline-5-carboxylate reductase (P5CR) can convert it to proline. While citrulline-to-arginine production primarily occurs in the kidney, citrulline is readily converted into arginine in other cell types, including adipocytes, endothelial cells, myocytes, macrophages, and neurons. Interestingly, chickens and cats cannot produce citrulline via glutamine/glutamate due to a lack of a functional pyrroline-5-carboxylate synthase (P5CS) in their enterocytes (PMID: 19030957).


Pw000038 View Pathway

Taurine and Hypotaurine Metabolism

There is an organic acid known as Taurine, which is a derivative product of sulfhydryl amino acid (which contains sulfur), as well as cysteine. The synthesis or metabolism in mammalian systems of this acid transpires within the pancreas in such a fashion that it utilizes a pathway known as the cysteine sulfinic acid pathway. To put this process in context, its occurrence is often seen in vivo, in hepatocytes, and is fundamental in the cyclical process of recovering bile acids from the intenstine, turning them back into salts and returning them to the bile. In essence the cysteine pathway induces a sulfhydryl group to be oxidized, creating cysteine sulfinic acid, by utilizing the appropriate enzymes (ie cysteine dioxygenase). This new acid undergoes decarboxylation creating a new compound: hypotaurine. This process goes on as Taurine now is subjected to conjugation vis a vis its amino terminal group. This includes acids such as chenodeoxycholic acid and cholic acid, and in turn the formation of bile salts occurs. Moreover, this entire process can be catalyzed via bile acid and a special amino acid N-acetyltransferase.


Pw000050 View Pathway

Steroid Biosynthesis

The steroid biosynthesis (or cholesterol biosynthesis) pathway is an anabolic metabolic pathway that produces steroids from simple precursors. It starts with the mevalonate pathway, where acetyl-CoA and acetoacetyl-CoA are the first two building blocks. These compounds are joined together via the enzyme hydroxy-3-methylgutaryl (HMG)-CoA synthase to produce the compound known as hydroxy-3-methylgutaryl-CoA (HMG-CoA). This compound is then reduced to mevalonic acid via the enzyme HMG-CoA reductase. It is important to note that HMG-CoA reductase is the protein target of many cholesterol-lowering drugs called statins (PMID: 12602122). The resulting mevalonic acid (or mevalonate) is then phosphorylated by the enzyme known as mevalonate kinase to form mevalonate-5-phosphate, which is then phosphorylated again by phosphomevalonate kinase to form mevolonate-5-pyrophsophate. This pyrophosphorylated compound is subsequently decarboxylated via the enzyme mevolonate-5-pyrophsophate decarboxylase to form isopentylpyrophosphate (IPP). IPP can also be isomerized (via isopentenyl-PP-isomerase) to form dimethylallylpyrophosphate (DMAPP). IPP and DMAPP can both donate isoprene units, which can then be joined together to make farnesyl and geranylgeranyl intermediates. Specifically, three molecules of IPP condense to form farnesyl pyrophosphate through the action of the enzyme known as geranyl transferase. Two molecules of farnesyl pyrophosphate then condense to form a molecule known as squalene by the action of the enzyme known as squalene synthase in the cell’s endoplasmic reticulum. The enzyme oxidosqualene cyclase then cyclizes squalene to form lanosterol. Lanosterol is a tetracyclic triterpenoid, and serves as the framework from which all steroids are derived. 14-Demethylation of lanosterol by a cytochrome P450 enzyme known as CYP51 eventually yields cholesterol. Cholesterol is the central steroid in human biology. It can be obtained from animal fats consumed in the diet or synthesized de novo (as described above). Cholesterol is an essential constituent of lipid bilayer membranes (where it forms cholesterol esters) and is the starting point for the biosynthesis of steroid hormones, bile acids and bile salts, and vitamin D. Steroid hormones are mostly synthesized in the adrenal gland and gonads. They regulate energy metabolism and stress responses (via glucocorticoids such as cortisol), salt balance (mineralocorticoids such as aldosterone), and sexual development and function (via androgens such as testosterone and estrogens such as estradiol). Bile acids and bile salts (such as taurocholate) are mostly synthesized in the liver. They are released into the intestine and function as detergents to solubilize dietary fats. Cholesterol is the main constituent of atheromas. These are the fatty lumps found in the walls of arteries that occur in atherosclerosis and, when ruptured, can cause heart attacks.
Showing 11 - 20 of 49833 pathways