Browsing Pathways

Cardiolipin (CL) is an important component of the inner mitochondrial membrane where it constitutes about 20% of the total lipid composition. It is essential for the optimal function of numerous enzymes that are involved in mitochondrial energy metabolism (Wikipedia). Cardiolipin biosynthesis occurs mainly in the mitochondria, but there also exists an alternative synthesis route for CDP-diacylglycerol that takes place in the endoplasmic reticulum. This second route may supplement this pathway. All membrane-localized enzymes are coloured dark green in the image. First, dihydroxyacetone phosphate (or glycerone phosphate) from glycolysis is used by the cytosolic enzyme glycerol-3-phosphate dehydrogenase [NAD(+)] to synthesize sn-glycerol 3-phosphate. Second, the mitochondrial outer membrane enzyme glycerol-3-phosphate acyltransferase esterifies an acyl-group to the sn-1 position of sn-glycerol 3-phosphate to form 1-acyl-sn-glycerol 3-phosphate (lysophosphatidic acid or LPA). Third, the enzyme 1-acyl-sn-glycerol-3-phosphate acyltransferase converts LPA into phosphatidic acid (PA or 1,2-diacyl-sn-glycerol 3-phosphate) by esterifying an acyl-group to the sn-2 position of the glycerol backbone. PA is then transferred to the inner mitochondrial membrane to continue cardiolipin synthesis. Fourth, magnesium-dependent phosphatidate cytidylyltransferase catalyzes the conversion of PA into CDP-diacylglycerol. Fifth, CDP-diacylglycerol--glycerol-3-phosphate 3-phosphatidyltransferase synthesizes phosphatidylglycerophosphate (PGP). Sixth, phosphatidylglycerophosphatase and protein-tyrosine phosphatase dephosphorylates PGP to form phosphatidylglycerol (PG). Last, cardiolipin synthase catalyzes the synthesis of cardiolipin by transferring a phosphatidyl group from a second CDP-diacylglycerol to PG. It requires a divalent metal cation cofactor.

Fructose metabolism begins with the transport of beta-D-glucose 6-phosphate through a glucose PTS permease. This compound is isomerized by a glucose-6-phosphate isomerase resulting in fructose 6-phosphate. This compound can be phosphorylated by two different enzymes: a pyridoxal phosphatase/fructose 1,6-bisphosphatase or an ATP-driven 6-phosphofructokinase-1, resulting in fructose 1,6-biphosphate. This compound can either react with a fructose bisphosphate aldolase class 1 resulting in D-glyceraldehyde 3-phosphate and dihydroxyacetone phosphate or through a fructose biphosphate aldolase class 2 resulting in D-glyceraldehyde 3-phosphate. This compound can then either react in a reversible triosephosphate isomerase resulting in dihydroxyacetone phosphate or react with a phosphate through an NAD-dependent glyceraldehyde 3-phosphate dehydrogenase resulting in glyceric acid 1,3-biphosphate. This compound is dephosphorylated by a phosphoglycerate kinase resulting in 3-phosphoglyceric acid. This compound, in turn, can either react with a 2,3-bisphosphoglycerate-independent phosphoglycerate mutase or a 2,3-bisphosphoglycerate-independent phosphoglycerate mutase resulting in 2-phospho-D-glyceric acid. This compound interacts with an enolase resulting in a phosphoenolpyruvic acid and water. Phosphoenolpyruvic acid can react either through an AMP-driven phosphoenoylpyruvate synthase or an ADP-driven pyruvate kinase protein complex resulting in pyruvic acid. The pyruvic acid reacts with CoA through an NAD-driven pyruvate dehydrogenase complex resulting in carbon dioxide and an acetyl-CoA which gets incorporated into the TCA cycle pathway.

Biotin (vitamin H or vitamin B7) is the essential cofactor of biotin-dependent carboxylases, such as pyruvate carboxylase and acetyl-CoA carboxylase. In E. coli and many organisms, pimelate thioester is derived from malonyl-ACP. The pathway starts with a malonyl-[acp] interacting with S-adenosylmethionine through a biotin synthesis protein BioC resulting in an S-adenosylhomocysteine and a malonyl-[acp] methyl ester. The latter compound is then involved in the synthesis of a 3-ketoglutaryl-[acp] methyl ester through a 3-oxoacyl-[acyl-carrier-protein] synthase. The compound 3-ketoglutaryl-[acp] methyl ester is reduced by a NADPH-mediated 3-oxoacyl-[acyl-carrier-protein] reductase resulting in a 3R-hydroxyglutaryl-[acp] methyl ester. It is then dehydrated through a (3R)-hydroxymyristoyl-[acp] dehydratase producing an enoylglutaryl-[acp] methyl ester. enoylglutaryl-[acp] methyl ester is then reduced through an NADPH mediated enoyl-acp-reductase [NADH] resulting in a glutaryl-[acp] methyl ester. Continuing, glutaryl-[acp] methyl ester interacts with a malonyl-[acp] through a 3-oxoacyl-[acp] synthase 2 resulting in a 3-ketopimeloyl [acp] methyl ester then is further reduced through an NADPH 3-oxoacyl [acp] reductase producing a 3-hydroxypimeloyl-[acp] methyl ester and then dehydrated by (3R)-hydroxymyristoyl-[acp] dehydratase to produce an enoylpimeloyl-[acp] methyl ester. The product is then reduced by an NADPH-dependent enoyl-[acp]reductase resulting in a pimeloyl-[acp] methyl ester. Reacting with water through a carboxylesterase, pimeloyl-[acp] methyl ester is converted into a pimeloyl-[acp] and a methanol. The pimeloyl-acp reacts with L-alanine through an 8-amino-7-oxononanoate synthase resulting in 8-amino-7-oxononanoate which in turn reacts with S-adenosylmethionine through a 7,8-diaminonanoate transaminase resulting in an S-adenosyl-4-methylthio-2-oxobutanoate and 7,8-diaminononanoate. The latter compound is then dephosphorylated through a dethiobiotin synthetase resulting in a dethiobiotin. This compound interacts with a sulfurated[sulfur carrier), a hydrogen ion, and an S-adenosylmethionine through a biotin synthase to produce biotin and releasing L-methionine and a 5-deoxyadenosine. Finally, biotin is then metabolized by a bifunctional protein resulting in pyrophosphate and biotinyl-5-AMP which in turn reacts with the same protein (bifunctional protein birA resulting in a biotin carboxyl carrying protein. This product then enters fatty acid biosynthesis.

Histidine biosynthesis starts with a product of PRPP biosynthesis pathway, phosphoribosyl pyrophosphate which interacts with a hydrogen ion through an ATP phosphoribosyltransferase resulting in an pyrophosphate and a phosphoribosyl-ATP. The phosphoribosyl-ATP interacts with water through a phosphoribosyl-AMP cyclohydrolase / phosphoribosyl-ATP pyrophosphatase resulting in the release of pyrophosphate, hydrogen ion and a phosphoribosyl-AMP. The same enzyme proceeds to interact with phosphoribosyl-AMP and water resulting in a 1-(5'-Phosphoribosyl)-5-amino-4-imidazolecarboxamide. The product is then isomerized by a N-(5'-phospho-L-ribosyl-formimino)-5-amino-1-(5'-phosphoribosyl)-4-imidazolecarboxamide isomerase resulting in a PhosphoribosylformiminoAICAR-phosphate, which reacts with L-glutamine through an imidazole glycerol phosphate synthase resulting in a L-glutamic acid, hydrogen ion, 5-aminoimidazole-4-carboxamide and a D-erythro-imidazole-glycerol-phosphate. D-erythro-imidazole-glycerol-phosphate reacts with a imidazoleglycerol-phosphate dehydratase / histidinol-phosphatase, dehydrating the compound and resulting in a imidazole acetol-phosphate. Next, imidazole acetol-phosphate reacts with L-glutamic acid through a histidinol-phosphate aminotransferase, releasing oxoglutaric acid and L-histidinol-phosphate. The latter compound interacts with water and a imidazoleglycerol-phosphate dehydratase / histidinol-phosphatase resulting in L-histidinol and phosphate. L-histidinol interacts with a NAD-driven histidinol dehydrogenase resulting in a Histidinal. Histidinal in turn reacts with water in a NAD driven histidinal dehydrogenase resulting in L-Histidine. L-Histidine then represses ATP phosphoribosyltransferase, regulation its own biosynthesis.

Glycine biosynthesis is dependent on L-serine. L-serine is enters the cell through transporters (serine / threonine:H+ symporter TdcC, serine/threonine: Na symporter , serine:H+ symporter SdaC) and then proceeds through reversible reaction with a tetrahydrofolic acid through a serine hydroxymethyltransferase enzyme in order to produce glycine, 5,10-methylene tetrahydrofolate and water. 5,10-methylene tetrahydrofolate is a major source of one-carbon units used in other metabolic pathways.

Cardiolipin (CL) is an important component of the inner mitochondrial membrane where it constitutes about 20% of the total lipid composition. It is essential for the optimal function of numerous enzymes that are involved in mitochondrial energy metabolism (Wikipedia). Cardiolipin biosynthesis occurs mainly in the mitochondria, but there also exists an alternative synthesis route for CDP-diacylglycerol that takes place in the endoplasmic reticulum. This second route may supplement this pathway. All membrane-localized enzymes are coloured dark green in the image. First, dihydroxyacetone phosphate (or glycerone phosphate) from glycolysis is used by the cytosolic enzyme glycerol-3-phosphate dehydrogenase [NAD(+)] to synthesize sn-glycerol 3-phosphate. Second, the mitochondrial outer membrane enzyme glycerol-3-phosphate acyltransferase esterifies an acyl-group to the sn-1 position of sn-glycerol 3-phosphate to form 1-acyl-sn-glycerol 3-phosphate (lysophosphatidic acid or LPA). Third, the enzyme 1-acyl-sn-glycerol-3-phosphate acyltransferase converts LPA into phosphatidic acid (PA or 1,2-diacyl-sn-glycerol 3-phosphate) by esterifying an acyl-group to the sn-2 position of the glycerol backbone. PA is then transferred to the inner mitochondrial membrane to continue cardiolipin synthesis. Fourth, magnesium-dependent phosphatidate cytidylyltransferase catalyzes the conversion of PA into CDP-diacylglycerol. Fifth, CDP-diacylglycerol--glycerol-3-phosphate 3-phosphatidyltransferase synthesizes phosphatidylglycerophosphate (PGP). Sixth, phosphatidylglycerophosphatase and protein-tyrosine phosphatase dephosphorylates PGP to form phosphatidylglycerol (PG). Last, cardiolipin synthase catalyzes the synthesis of cardiolipin by transferring a phosphatidyl group from a second CDP-diacylglycerol to PG. It requires a divalent metal cation cofactor.

Fatty acid oxidation is also known as beta-oxidation. The fatty acid oxidation pathway converts fatty acids into acetyl-CoA under both anaerobic and aerobic conditions. Enzymes in this pathway can process short and long chain fatty acids, both starting the conversion of acyl-CoA to enoyl-CoA. The pathway continues in a cycle, each turn removing two carbon atoms from the input acyl-CoA to produce acetyl-CoA and NADH. Fatty acids are an important source of energy because they can be reduced due to their anhydrous properties. In addition, fatty acid oxidation yields more energy when compared to carbohydrates.

Tryptophan biosynthesis is a critical anabolic pathway in bacteria that starts from chorismate, an intermediate derived from the shikimate pathway. In this process, chorismate is first converted into anthranilate through a reaction with glutamine, catalyzed by anthranilate synthase, which also releases ammonia as a byproduct. Anthranilate is then activated and condensed with phosphoribosyl pyrophosphate (PRPP) to form N-(5'-phosphoribosyl)-anthranilate, which undergoes several enzyme-catalyzed rearrangements and cyclizations to produce indole-3-glycerol phosphate. This intermediate is subsequently cleaved to generate indole, which reacts with serine in a reaction catalyzed by tryptophan synthase to form tryptophan. The pathway highlights the importance of chorismate as a branching point for the biosynthesis of aromatic amino acids, and tryptophan itself serves as a key building block for protein synthesis and a precursor for bioactive molecules like auxins and indole derivatives.

The Calvin-Benson cycle, also known as the reductive pentose phosphate cycle, is a central pathway for carbon fixation in bacteria, particularly in photoautotrophs and chemoautotrophs. This cycle enables bacteria to convert inorganic carbon dioxide (CO₂) into organic molecules, primarily glyceraldehyde-3-phosphate (G3P), which serves as a precursor for glucose and other essential cellular components. The cycle begins with the carboxylation of ribulose-1,5-bisphosphate (RuBP) by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), producing two molecules of 3-phosphoglycerate (3-PGA). These molecules are then phosphorylated and reduced using ATP and NADPH, respectively, to generate G3P. Part of the G3P is used to regenerate RuBP through a series of reactions involving sugar phosphate intermediates, while the remainder can be directed toward biosynthesis pathways. The Calvin-Benson cycle is energetically demanding, requiring significant input of ATP and NADPH, often supplied by photosynthesis in phototrophic bacteria or oxidation of inorganic compounds in chemolithoautotrophs. This pathway is essential for autotrophic bacterial growth and plays a key role in global carbon cycling by converting atmospheric CO₂ into biomass, contributing to primary productivity in diverse ecosystems.

Sulfameter, also known as sulfamethoxydiazine, is a long-acting antibacterial from the sulfonamide drug class. This drug is indicated in the treatment of leprosy, and urinary, and respiratory tract infections. It shows bacteriostatic effects against gram-positive and gram-negative bacteria. Sulfameter is a competitive inhibitor of the enzyme dihydropteroate synthase. This enzyme does the condensation of para-aminobenzoic acid (PABA) to produce folic acids. Without the synthesis of folate, the bacteria can not grow (bacteriostatic). Sulfameter is given orally as a tablet.