nitrogen excretion and urea cycle Flashcards
How does use of amino acids as a fuel vary by organism? carnivores vs microorganisms vs herbivores vs plants
carnivores: 90% of energy needs can be met by amino acids after a meat meal
microorganisms: scavenge amino acids from environment for fuel when needed
herbivores: small fraction of energy needs met by amino acids, most met by polysaccharides
plants: do not use amino acids as fuel, but can degrade amino acids to form other metabolites
when does amino acid oxidation occur?
when there are leftover amino acids from normal protein turnover
when dietary amino acids exceed body’s protein synthesis needs
proteins in the body can be broken down to supply amino acids for energy when carbohydrates are scarce (starvation, diabetes mellitus)
what is the process of dietary protein degradation through digestive tract?
pepsin cuts the protein in to peptides in the stomach. trypsin and chymotrypsin cut proteins and larger peptides into smaller peptides in the small intestine. amino peptidase and carboxypeptidases A and B degrade peptides into amino acids in small intestine.
what is the first step of degradation for all amino acids?
removal of the amino group. involves a-ketoglutarate accepting the amino group, turning into glutamate. the amino group eventually is excreted as urea.
why is glutamate (a-ketoglutarate) used for removal of amino groups?
it can carry 2x the amino groups by becoming glutamine
fates of Nitrogen in different organisms. plants, aquatic vertebrates, terrestrial vertebrates (and sharks), birds, reptiles, and humans/great apes
plants: conserve almost all nitrogen
aquatic vertebrates: release ammonia into environment
terrestrial vertebrates/sharks: excrete nitrogen in urea
birds/reptiles: excrete nitrogen as uric acid
humans/great apes: excrete both urea (from amino acids) and uric acid (from purines)
how does the transfer of amino groups from amino acids work? mech
uses enzyme amino-transferase and cofactor pyridoxal phosphate PLP. enzyme is covalently linked to PLP by a Schiff base via nucleophilic attack of amino group of an active site lysine (slide 9). this forms an external aldimine, which is an electron sink and allows removal of a-hydrogen (slide 10)
what else can PLP make if applied differently?
it can alter stereochemistry and create D instead of L amino acids
it can create an amine if no water is present
what happens to the amino group after it is transferred to glutamate?
it’s removed by glutamate dehydrogenase as ammonia. uses NAD+ as electron acceptor. ammonia is processed into urea for excretion
ammonia is toxic, how is it transported through the body?
ammonia is transported in bloodstream as glutamine, not as free ammonia.
vigorously working muscles rely on glycolysis (because its anaerobic) and builds up pyruvate. it not eliminated, what happens? how is it eliminated?
lactic acid will build up if pyruvate is not eliminated. glutamate can donate ammonia to pyruvate to make alanine, which is transported into liver safely because it is uncharged
urea cycle step 1
carbamoyl phosphate synthase I captures free ammonia in mitochondrial matrix, creates carbamoyl phosphate. costs 2 ATP. mech slide 16
describe the urea cycle, just names of intermediates, no enzymes (?)
carbamoyl phosphate citrulline citrullyl AMP intermediate arginosuccinate fumarate + arginine Urea + ornithine slide 17
what links the urea cycle and the CAC?
aspartate-arginosuccinate shunt. specifically, the arginosuccinate intermediate creates arginine and fumarate. arginine continues in urea cycle while fumarate can be converted to malate and enter CAC. the creation of arginosuccinate is due to both cycles as well, aspartate comes from oxaloacetate from CAC and citrulline comes from urea cycle
slide 18
how is the urea cycle regulated?
carbamoyl phosphate synthase I is activated by N-acetylglutamate, which is formed by N-acetylglutamate synthase (glutamate + acetyl-CoA). N-acetylglutamate synthase is activated by arginine and high concentrations of glutamate and acetyl-CoA.
