Browsing Pathways

The sulfur metabolism pathway starts in three possible ways. The first is the uptake of sulfate through an active transport reaction via a sulfate transport system containing an ATP-binding protein which hydrolyses ATP. Sulfate is converted by the sulfate adenylyltransferase enzymatic complex to adenosine phosphosulfate through the addition of adenine from a molecule of ATP, along with one phosphate group. Adenosine phosphosulfate is further converted to phoaphoadenosine phosphosulfate through an ATP hydrolysis and dehydrogenation reaction by the adenylyl-sulfate kinase. Phoaphoadenosine phosphosulfate is finally dehydrogenated and converted to sulfite by phosphoadenosine phosphosulfate reductase. This reaction requires magnesium, and adenosine 3',5'-diphosphate is the bi-product. A thioredoxin is also oxidized. Sulfite can also be produced from the dehydrogenation of cyanide along with the conversion of thiosulfate to thiocyanate by the thiosulfate sulfurtransferase enzymatic complex. Sulfite next undergoes a series of reactions that lead to the production of pyruvic acid, which is a precursor for pathways such as gluconeogenesis. The first reaction in this series is the conversion of sulfite to hydrogen sulfide through hygrogenation and the deoxygenation of sulfite to form a water molecule. The reaction is catalyzed by the sulfite reductase [NADPH] flavoprotein alpha and beta components. Siroheme, 4Fe-4S, flavin mononucleotide, and FAD function as cofactors or prosthetic groups. Hydrogen sulfide next undergoes dehydrogenation in a reversible reaction to form L-Cysteine and acetic acid, via the cysteine synthase complex and the coenzyme pyridoxal 5'-phosphate. L-Cysteine is dehydrogenated and converted to 2-aminoacrylic acid (a bronsted acid) and hydrogen sulfide(which may be reused) by a larger enzymatic complex composed of cysteine synthase A/B, protein malY, cystathionine-β-lyase, and tryptophanase, along with the coenzyme pyridoxal 5'-phosphate. 2-aminoacrylic acid isomerizes to 2-iminopropanoate, which along with a water molecule and a hydrogen ion is lastly converted to pyruvic acid and ammonium in a spontaneous fashion. The second possible initial starting point for sulfur metabolism is the import of taurine(an alternate sulfur source) into the cytoplasm via the taurine ABC transporter complex. Taurine, oxoglutaric acid, and oxygen are converted to sulfite by the alpha-ketoglutarate-dependent taurine dioxygenase. Carbon dioxide, succinic acid, and aminoacetaldehyde are bi-products of this reaction. Sulfite next enters pyruvic acid synthesis as already described. The third variant of sulfur metabolism starts with the import of an alkyl sulfate, in this case 1-butanesulfonate, into the cytoplasm via an aliphatic sulfonate ABC transporter complex which hydrolyses ATP. 1-butanesulfonate is dehydrogenated and along with oxygen is converted to sulfite and betaine aldehyde by the FMNH2-dependent alkanesulfonate monooxygenase enzyme. Water and flavin mononucleotide(which is used in a subsequent reaction as a prosthetic group) are also produced. Sulfite is next converted to pyruvic acid by the process already described.

The sulfur metabolism pathway starts in three possible ways. The first is the uptake of sulfate through an active transport reaction via a sulfate transport system containing an ATP-binding protein which hydrolyses ATP. Sulfate is converted by the sulfate adenylyltransferase enzymatic complex to adenosine phosphosulfate through the addition of adenine from a molecule of ATP, along with one phosphate group. Adenosine phosphosulfate is further converted to phoaphoadenosine phosphosulfate through an ATP hydrolysis and dehydrogenation reaction by the adenylyl-sulfate kinase. Phoaphoadenosine phosphosulfate is finally dehydrogenated and converted to sulfite by phosphoadenosine phosphosulfate reductase. This reaction requires magnesium, and adenosine 3',5'-diphosphate is the bi-product. A thioredoxin is also oxidized. Sulfite can also be produced from the dehydrogenation of cyanide along with the conversion of thiosulfate to thiocyanate by the thiosulfate sulfurtransferase enzymatic complex. Sulfite next undergoes a series of reactions that lead to the production of pyruvic acid, which is a precursor for pathways such as gluconeogenesis. The first reaction in this series is the conversion of sulfite to hydrogen sulfide through hygrogenation and the deoxygenation of sulfite to form a water molecule. The reaction is catalyzed by the sulfite reductase [NADPH] flavoprotein alpha and beta components. Siroheme, 4Fe-4S, flavin mononucleotide, and FAD function as cofactors or prosthetic groups. Hydrogen sulfide next undergoes dehydrogenation in a reversible reaction to form L-Cysteine and acetic acid, via the cysteine synthase complex and the coenzyme pyridoxal 5'-phosphate. L-Cysteine is dehydrogenated and converted to 2-aminoacrylic acid (a bronsted acid) and hydrogen sulfide(which may be reused) by a larger enzymatic complex composed of cysteine synthase A/B, protein malY, cystathionine-β-lyase, and tryptophanase, along with the coenzyme pyridoxal 5'-phosphate. 2-aminoacrylic acid isomerizes to 2-iminopropanoate, which along with a water molecule and a hydrogen ion is lastly converted to pyruvic acid and ammonium in a spontaneous fashion. The second possible initial starting point for sulfur metabolism is the import of taurine(an alternate sulfur source) into the cytoplasm via the taurine ABC transporter complex. Taurine, oxoglutaric acid, and oxygen are converted to sulfite by the alpha-ketoglutarate-dependent taurine dioxygenase. Carbon dioxide, succinic acid, and aminoacetaldehyde are bi-products of this reaction. Sulfite next enters pyruvic acid synthesis as already described. The third variant of sulfur metabolism starts with the import of an alkyl sulfate, in this case 1-butanesulfonate, into the cytoplasm via an aliphatic sulfonate ABC transporter complex which hydrolyses ATP. 1-butanesulfonate is dehydrogenated and along with oxygen is converted to sulfite and betaine aldehyde by the FMNH2-dependent alkanesulfonate monooxygenase enzyme. Water and flavin mononucleotide(which is used in a subsequent reaction as a prosthetic group) are also produced. Sulfite is next converted to pyruvic acid by the process already described.

The sulfur metabolism pathway starts in three possible ways. The first is the uptake of sulfate through an active transport reaction via a sulfate transport system containing an ATP-binding protein which hydrolyses ATP. Sulfate is converted by the sulfate adenylyltransferase enzymatic complex to adenosine phosphosulfate through the addition of adenine from a molecule of ATP, along with one phosphate group. Adenosine phosphosulfate is further converted to phoaphoadenosine phosphosulfate through an ATP hydrolysis and dehydrogenation reaction by the adenylyl-sulfate kinase. Phoaphoadenosine phosphosulfate is finally dehydrogenated and converted to sulfite by phosphoadenosine phosphosulfate reductase. This reaction requires magnesium, and adenosine 3',5'-diphosphate is the bi-product. A thioredoxin is also oxidized. Sulfite can also be produced from the dehydrogenation of cyanide along with the conversion of thiosulfate to thiocyanate by the thiosulfate sulfurtransferase enzymatic complex. Sulfite next undergoes a series of reactions that lead to the production of pyruvic acid, which is a precursor for pathways such as gluconeogenesis. The first reaction in this series is the conversion of sulfite to hydrogen sulfide through hygrogenation and the deoxygenation of sulfite to form a water molecule. The reaction is catalyzed by the sulfite reductase [NADPH] flavoprotein alpha and beta components. Siroheme, 4Fe-4S, flavin mononucleotide, and FAD function as cofactors or prosthetic groups. Hydrogen sulfide next undergoes dehydrogenation in a reversible reaction to form L-Cysteine and acetic acid, via the cysteine synthase complex and the coenzyme pyridoxal 5'-phosphate. L-Cysteine is dehydrogenated and converted to 2-aminoacrylic acid (a bronsted acid) and hydrogen sulfide(which may be reused) by a larger enzymatic complex composed of cysteine synthase A/B, protein malY, cystathionine-β-lyase, and tryptophanase, along with the coenzyme pyridoxal 5'-phosphate. 2-aminoacrylic acid isomerizes to 2-iminopropanoate, which along with a water molecule and a hydrogen ion is lastly converted to pyruvic acid and ammonium in a spontaneous fashion. The second possible initial starting point for sulfur metabolism is the import of taurine(an alternate sulfur source) into the cytoplasm via the taurine ABC transporter complex. Taurine, oxoglutaric acid, and oxygen are converted to sulfite by the alpha-ketoglutarate-dependent taurine dioxygenase. Carbon dioxide, succinic acid, and aminoacetaldehyde are bi-products of this reaction. Sulfite next enters pyruvic acid synthesis as already described. The third variant of sulfur metabolism starts with the import of an alkyl sulfate, in this case 1-butanesulfonate, into the cytoplasm via an aliphatic sulfonate ABC transporter complex which hydrolyses ATP. 1-butanesulfonate is dehydrogenated and along with oxygen is converted to sulfite and betaine aldehyde by the FMNH2-dependent alkanesulfonate monooxygenase enzyme. Water and flavin mononucleotide(which is used in a subsequent reaction as a prosthetic group) are also produced. Sulfite is next converted to pyruvic acid by the process already described.

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.

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.

The process of oxidative phosphorylation involves multiple interactions of ubiquinone with succinic acid, resulting in a fumaric acid and ubiquinol. Ubiquinone interacts with succinic acid through a succinate:quinone oxidoreductase resulting in a fumaric acid an ubiquinol. This enzyme has various cofactors, ferroheme b, 2FE-2S, FAD, and 3Fe-4S iron-sulfur cluster. Then 2 ubiquinol interact with oxygen and 4 hydrogen ion through a cytochrome bd-I terminal oxidase resulting in a 4 hydrogen ion transferred into the periplasmic space, 2 water returned into the cytoplasm and 2 ubiquinone, which stay in the inner membrane. The ubiquinone interacts with succinic acid through a succinate:quinone oxidoreductase resulting in a fumaric acid an ubiquinol. Then 2 ubiquinol interacts with oxygen and 4 hydrogen ion through a cytochrome bd-II terminal oxidase resulting in a 4 hydrogen ion transferred into the periplasmic space, 2 water returned into the cytoplasm and 2 ubiquinone, which stay in the inner membrane. The ubiquinone interacts with succinic acid through a succinate:quinone oxidoreductase resulting in a fumaric acid an ubiquinol. The 2 ubiquinol interact with oxygen and 8 hydrogen ion through a cytochrome bo terminal oxidase resulting in a 8 hydrogen ion transferred into the periplasmic space, 2 water returned into the cytoplasm and 2 ubiquinone, which stays in the inner membrane. The ubiquinone then interacts with 5 hydrogen ion through a NADH dependent ubiquinone oxidoreductase I resulting in NAD, hydrogen ion released into the periplasmic space and an ubiquinol. The ubiquinol is then processed reacting with oxygen, and 4 hydrogen through a ion cytochrome bd-I terminal oxidase resulting in 4 hydrogen ions released into the periplasmic space, 2 water molecules into the cytoplasm and 2 ubiquinones. The ubiquinone then interacts with 5 hydrogen ion through a NADH dependent ubiquinone oxidoreductase I resulting in NAD, hydrogen ion released into the periplasmic space and an ubiquinol. The 2 ubiquinol interact with oxygen and 8 hydrogen ion through a cytochrome bo terminal oxidase resulting in a 8 hydrogen ion transferred into the periplasmic space, 2 water returned into the cytoplasm and 2 ubiquinone, which stays in the inner membrane.

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.

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.

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.

The degradation of galactose, also known as Leloir pathway, requires 3 main enzymes once Beta-D-galactose has been converted to galactose through an Aldose-1-epimerase. These are: galactokinase , galactose-1-phosphate uridylyltransferase and UDP-glucose 4-epimerase. Beta-D-galactose can be uptaken from the environment through a galactose proton symporter. It can also be produced by lactose degradation involving a lactose permease to uptake lactose from the environment and a beta-galactosidase to turn lactose into Beta-D-galactose. Galactose is degraded through the following process: Beta-D-galactose is introduced into the cytoplasm through a galactose proton symporter, or it can be synthesized from an alpha lactose that is introduced into the cytoplasm through a lactose permease. Alpha lactose interacts with water through a beta-galactosidase resulting in a beta-D-glucose and beta-D-galactose. Beta-D-galactose is isomerized into D-galactose. D-Galactose undergoes phosphorylation through a galactokinase, hence producing galactose 1 phosphate. On the other side of the pathway, a gluose-1-phosphate (product of the interaction of alpha-D-glucose 6-phosphate with a phosphoglucomutase resulting in a alpha-D-glucose-1-phosphate, an isomer of Glucose 1-phosphate, or an isomer of Beta-D-glucose 1-phosphate) interacts with UTP and a hydrogen ion in order to produce a uridine diphosphate glucose. This is followed by the interaction of galactose-1-phosphate with an established amount of uridine diphosphate glucose through a galactose-1-phosphate uridylyltransferase, which in turn output a glucose-1-phosphate and a uridine diphosphate galactose. The glucose -1-phosphate is transformed into a uridine diphosphate glucose through UTP--glucose-1-phosphate uridylyltransferase. The product, uridine diphosphate glucose, can undergo a reversible reaction in which it can be turned into uridine diphosphategalactose through an UDP-glucose 4-epimerase, and so the cycle can keep going as long as more lactose or galactose is imported into the cell.