Nitrogen fixation Nitrogen fixation is the conversion of N 2 gas into ammonia, a process carried out by some soil bacteria, cyanobacteria 蓝细 菌, and the symbiotic 共生的 bacteria Rhizobium 根瘤菌 that invade the root nodules of leguminous 豆类 plants.
Nitrogen fixation This process is carried out by the nitrogenase 固氮酶 complex, which consists of a reductase and an iron-molybdenum 钼 - containing nitrogenase. At least 16ATP molecules are hydrolyzed to form two molecules of ammonia. Leghemoglobin 豆血 红蛋白 is used to protect the nitrogenase in the Rhizobium from inactivation by O2.
Nitrogen assimilation Ammonia is assimilated by all organisms into organic nitrogen-containing compounds(amino acids,nucleotides,etc.) by the action of glutamate dehydrogenase (to form glutamate) and glutamine synthetase (to form glutamine).
Amino acid degradation Amino acids are degraded by the removal of the α-amino group and the conversion of the resulting carbon skeleton into one or more metabolic intermediates.
Amino acid degradation Amino acids are termed glucogenic if their carbon skeletons can give rise to the net synthesis of glucose,and ketogenic of they can give rise to ketone bodies. Some amino acids give rise to more than one intermediate and these lead to the synthesis of glucose as will as ketone bodies. Thus these amino acids are both glucogenic and ketogenic.
Transamination The α-amino group of most amino acids is transferred to α-ketoglutarate to form glutamate and the corresponding α-keto acid α-amino acid + α-ketoglutarate α-keto acid + glutamate Enzyme: transaminases
The urea cycle In the urea cycle ammonia is first combined with CO 2 to form carbamoyl phosphate. This then combines with ornithine to form citrulline. Citrulline then condenses with aspartate, the source of the second nitrogen atom in urea,to form argininosuccinate. This compound is in turn split to arginine and fumarate, and the arginine then splits to form urea and regenerate ornithine.
NH 4 + + HCO 3 - + H 2 O + 3ATP +aspartate urea + 2ADP + AMP + 2P i +PP i + fumarate
Oxaloacetate has several possible fates Transamination to aspartate which can then feed back into the urea cycle; Condensation with acetyl CoA to form citrate which then continues on round the citric acid cycle ; Conversion into glucose via gluconeogenesis ; Conversion into pyruvate
Hyperammonemia A block in any of the urea cycle enzymes leads to an increase in the amount of ammonia in the blood, so-called hyperammonemia
The reason of brain damage in hyperammonemia. 1. Excess ammonia leads to the formation of glutamate and glutamine. 2. It may compromise energy production. 3. It also leads to increase [H + ]
Formation of creatine phosphate The urea cycle is also the starting point for the synthesis of another important metabolite creatine phosphate. This phosphate provides a reservoir of high- energy phosphate in muscle cells.
ATP As the energy released upon is hydrolysis is greater than that released upon the hydrolysis of ATP (ΔG for creatine phosphate hydrolysis=-10.3kal mol -1 compared with –7.3 kcal mol -1 for ATP hydrolysis ).
Creatine phosphate The first step in the formation of creatine phosphate is the condensation of arginine and glycine to form guanidinoacetate 胍基乙酸. Ornithine is released in this reaction and can then be re-utilized by the urea cycle. The guanidinoacetate is then methylated by the methyl group donor S-adenosyl methionine to form creatine, which is in turn phosphory- lated, by creatine kinase to form creatine phosphate.
The activated methyl cycle S-Adenosyl methionine serves as donor of methyl groups in numerous biological reactions [e.g.in the for- mation of creatine phosphate and in the synthesis of nucleic acids].It is formed through the action of the activated methyl cycle.
The activated methyl cycle During donation of its Methylgroup to another compound, S-adenosyl methionine is converted into S-adenosyl homocysteine. To regenerate S-adenosyl methionine, the adenosyl group is removed from the S- adenosyl homocysteine to form homo- cysteine 高半胱氨酸.
The activated methyl cycle This is then methylated by the enzyme homocysteine methyltransferase, one of only two vitamin B12 containing enzymes found in eukaryotes, to form methionine. The resulting methionine is then activated to S- adenosyl methionine. With the release of all three of the phosphate from ATP.
Uric acid Uric acid is the main nitrogenous waste product of uricotelic organisms (reptiles, birds and insects), but is also formed in ureotelic organisms from the breakdown of the purine bases from DNA and RNA. Gout 痛风
HEMES AND CHLOROPHYLLS 1. Tetrapyrrol 2. Biosynthesis of hemes and chorophylls 3. Heme degradation
Tetrapyrroles The tetrapyrroles are a family of pigments based on a common chemical structure that includes the hemes and chlorophylls. Hemes are cyclic tetrapyrroles that contain iron and are commonly found as the prosthetic group of hemoglobin, myoglobin and he cytochromes.
Heme is as the prosthetic roup Globin 珠蛋白 Protein Heme Cytochomes 细胞色素 Catalases 过氧化氢酶 Enzyme Peroxidase 过氧化物酶
chlorophylls The chlorophylls are modified tetrapyrroles containing magnesium that occur as light-harvesting and reaction center pigments of photosynthesis in plants, algae and photosynthetic bacteria.
Biosynthesis of hemes and chlorophylls The staring point for heme and chlorophyll synthesis is aminolaevulinic acid 氨基乙酰丙酸 (ALA) which is made in animals from glycine and succinyl CoA by the enzyme ALA synthase.
Biosynthesis of hemes and chlorophylls This pyridoxal 吡哆醛 pyosphate- requiring enzyme is feedback regulated by heme. Two molecules of ALA then condense to form porphobilinogen 胆色 素原 in a reaction catalyzed by ALA dehydratase 脱水酶.
Biosynthesis of hemes and chlorophylls Porphobilinogen deaminase catalyzed the condensation of four porphobilinogen to form a linear tetrapyrrole. This compound then cyclizes to form uroporphyhrinogen 尿卟啉原 III, the precursou of hemes, chlorophylls and vitamin B12.
Biosynthesis of hemes and chlorophylls Further modifications take place to form protoporphyrin 原卟啉 IX. The biosynthetic pathway then branches, and either iron is inserted to form heme, or magnesium is inserted to begin a series of conversions to form chlorophyII.
Heme degradation Heme is broken down by heme oxygenase to the linear tetrapyrrole biliverdin 胆绿素. This green pigment is them converted to the red-orange bilirubin 胆红素 by biliverdin reductase. The lipophilic bilirubin is carried in the blood bound to serum albumin, and is then converted into a more water-soluble compound in the liver by conjugation to glucuronic acid 葡萄糖醛酸.
Heme The resulting bilirubin Diglucuronide 胆红素二葡糖苷酸 is secreted into the bile, and finally excreted in the feces. Jaundice is due to a build up of insoluble bilirubin in the skin and whites of the eyes.
In higher plants heme is broken down to the phycobiliprotein 藻胆蛋白 phytochrome which is involved in coordinating light responses, while in algae it is metabolized to the light- harvesting pigments phycocyanin and phycoerythrin.