Nitrogen Metabolism II
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45 terms
Terms | Definitions |
|---|---|
 protein turnover | The cellular concentration of each type of protein is a consequence of a balance between its synthesis and its degradation |
| ubiquitin proteasomal system | Mediated by a covalent modification called ubiquitination; Most cellular proteins are degraded by the autophagy lysosomal system |
| N-terminal residues | Feature of protein marked for destruction; often basic or bulky |
| peptide motifs | Feature of protein marked for destruction; PEST sequence indicates short half-lives |
| proteasome | A giant protein complex that recognizes and destroys proteins tagged for elimination by the small protein ubiquitin. |
| ubiquitination | The attachment of the small, highly conserved 76-residue protein ubiquitin to a molecule, in this case to mark it for degradation |
| autophagy | cellular degradation pathway in which cellular components are degraded by hydrolytic enzymes in the lysosome; Involved in nutrient recycling, regulation of development, and invading microorganism destruction |
| amino acid catabolism | usually begins with the removal of the amino group (used in urea synthesis) |
| metabolic products | Acetyl-CoA, acetoacteyl-CoA, pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate; can be used to synthesize fatty acids or glucose or to generate energy |
| deamination | Removal of the α-amino group from amino acids involves two types of reactions: transamination and oxidative deamination; reversible reactions |
| ammonia | most is generated in amino acid degradation is produced by the oxidative deamination of glutamate |
| enzymes in ammonia production | L-amino oxidases, serine and threonine dehydrases, bacterial urease (intestinal bacteria), adenosine deaminase |
| urea synthesis | formed from ammonia, CO2, and aspartate in a cyclic cycle: 1) begins with the formation of carbamoyl phosphate in the mitochondrial matrix; 2) Carbamoyl phosphate reacts with ornithine to form citrulline; 3) Citrulline is transported to the cytoplasm |
| urea cycle | disposes of approximately 90% of surplus nitrogen, in ureotelic organisms; CO2 + NH4+ + Aspartate + 3 ATP + 2H2O --> Urea + Fumarate + 2 ADP + 2 Pi + AMP + PPi + 5H+ |
| ornithine | generated in urea synthesis; goes back into the cycle to make more citrulline |
| fumarate | produced by the urea cycle; Oxaloacetate gives an amino group to aspartate |
| metabolic intermediates | 1) Acetyl-CoA (Lys, Trp, Tyr, Phe, Leu); 2) Pyruvate (Ala, Cys, Thr, Gly, Ser); 3) α-Ketoglutarate (Gln, Arg, Pro, His, Glu); 4) Succinyl-CoA (Met, Ile, Val, Thr); 5) Oxaloacetate (Asp, Asn); 6) Fumarate (Tyr) |
| phenylalanine | hydroxylated to tyrosine, which is degraded to acetoacetate and fumarate |
| albinism | caused by a deficient tyrosinase (melanine no longer produced; albinos are very sensitive to sunlight) |
| PKU | caused by a deficiency of Phe hydroxylase [Phe 4-monooxygenase] (mental retardation) |
| acetylcholine | inactivated by acetylcholinesterase |
| catecholamines | (dopamine, epinephrine); inactivated by monoamine oxidase (MAO), inactivated by oxidation reactions |
| serotonin | degraded after reuptake by a two-step pathway inactivated by MAO; The first reaction involves oxidation by MAO; The product 5-hydroxyindole-3-acetylaldehyde is then further oxidized to form 5-hydroxyindole-3-acetate |
| purine nucleotides | de novo formation begins with 5-phospho-α-D-ribosyl-pyrophosphate (PRPP) synthesis; PRPP is converted to inosine monophosphate (IMP); IMP conversion to AMP or GMP requires two reactions |
| purine salvage pathway | purine bases obtained from normal turnover of cellular nucleic acids or diet are reconverted to nucleotides |
| Hypoxanthine-guaninephosphoribosyltransferase (HGPRT) | catalyzes nucleotide synthesis using PRPP and hypoxanthine or guanine |
| GMP | conversion from IMP begins with a dehydrogenation utilizing NAD+; The product is then converted by the donation of an amino nitrogen from glutamine in an ATP-requiring reaction |
| pyrimidine nucleotides | ring is synthesized first and then linked to ribose phosphate; The carbon and nitrogen atoms in the ring are derived from bicarbonate, aspartate, and glutamine; Synthesis begins with the formation of carbamoyl phosphate in an ATP-requiring reaction |
| deoxyribonucleotides | required for DNA synthesis; Catalyzed by ribonucleotide reductase using NADPH and thioredoxin |
| nucleotide degradation | During digestion, nucleases are hydrolyzing nucleic acids into oligonucleotides (e.g., DNases and RNases); Oligonucleotides are further degraded to free bases and ribose or deoxyribose |
| oligonucleotide | short (<50 bp) nucleic acid segments |
| nucleases | Nucleic acids to oligonucleotides |
| phosphodiesterases | Oligonucleotides to Nucleotides |
| nucleotidases | Nucleotides to Nucleosides |
| nucleosidases | Nucleoside to Base and Ribose-1-P |
| purine catabolism | In most tissues, AMP is hydrolyzed to form adenosine; Adenosine is then deaminated to form inosine; Purine nucleoside phosphorylase converts inosine, guanosine, and xanthosine to hypoxanthine, guanine, and xanthine, respectively |
| hypoxanthine | oxidized to xanthine |
| guanine | deaminated to xanthine |
| xanthine | molecules are further oxidized to uric acid |
| purines | degraded to uric acid |
| pyrimidines | degraded to: NH4+, CO2 and either: β-alanine or β-aminoisobutyrate |
| gout | results from high blood levels of uric acid and recurrent attacks of arthritis |
| adenosine deaminase deficiency | results in increased levels of deoxyadenosine, which is toxic, especially in T and B cells; Causes immunodeficiency due to its effect on T and B lymphocytes |
| pyrimidine catabolism | Cytidine and deoxycytidine are converted to uridine and deoxyuridine by deamination; These are further degraded to form uracil; Uracil is converted to β-alanine and eventually acetyl-CoA |
| heme biotransformation | Porphyrin heme degraded to form bilirubin with heme oxygenase; The heme oxygenase gene is inducible by cellular stressors (free heme, ROS); Increased CO, biliverdin, and bilirubin protects against oxidative stress |
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