Oxidation and Production of Urea Flashcards
Which amino acids are most closely related to the citric acid cycle?
Glutamine, glutamate, alanine, and aspartate are most easily converted to citric acid cycle intermediates. Glutamine and glutamate to a-ketoglutarate, alanine to pyruvate, and aspartate to oxaloacetate.
What is the main process of urea production (general)?
Amino groups of degraded amino acids are converted to glutamate (either from transport of glutamine to liver, amino acids -or alanine and a-ketoglutarate). Glutamate is transported to mitochondria where it releases the amino group to form ammonia.
Explain the process of transamination, the enzymes involved and important aspect(s) of these
Transferral of amino groups is catalyzed by aminotransferases/transaminases. The a-amino group is transferred to the a-carbon of a-ketoglutarate, yielding the a-keto acid of the amino acid, and glutamate from a-ketoglutarate.
All transaminases have pyridoxal phosphate (PLP) as a cofactor, bound to the enzyme’s active site. PLP functions as an intermediate carrier of the amino groups.
Explain the process of deamination of glutamate
Glutamate dehydrogenase is responsible for catalyzing oxidative deamination of glutamate. This process can use either NAD+ or NADP+ as an oxidative agent (it is reduced to NAD(P)H). The reaction results in NH4+ and a-ketoglutarate, which can either enter the citric acid cycle, or be used for glucose synthesis.
Explain the process of transdeamination
Transdeamination requires the action of both transaminase and glutamate dehydrogenase. The a-amino groups of amino acids are transferred to a-ketoglutarate to yield glutamate, and the amino group of glutamate is released as NH4+ by glutamate dehydrogenase, leaving a-ketoglutarate from the carbon skeleton.
Explain the role and production of glutamine
Glutamine is an important, intercellular transporter of ammonia. This is to make toxic ammonia into a non-toxic compound for transport in the blood. Glutamate reacts with ATP and ammonia to form glutamine by the action of glutamine synthase. When glutamine is transported to the liver it is split to free ammonia and glutamate by glutaminase.
Explain the transport of ammonia from skeletal muscle
Alanine transports ammoina from muscle cells to the liver. Muscle cells produce large amounts of pyruvate from glycolysis, which can be interconverted to alanin by transamination with glutamate. Alanine aminotransferase facilitates the transfer of the amino group of glutamate to pyruvate, and results in alanine and a-ketoglutarate (from deaminated glutamate).
Explain the glucose-alanine cycle
Alanine carries ammonia and (cabon skeleton of) pyruvate to the liver. Alanine loses the amino group by transamination to a-ketoglutarate, and releases pyruvate. Pyruvate is converted to glucose through gluconeogenesis and is returned to the muscle, where it is used and again results in pyruvate which carries ammonia to the liver in the form of alanine. So on and so forth.
What is the purpose of the urea cycle?
The urea cycle is a pathway where ammonia is deposited in the mitochondria to be converted to urea so that it can enter the blood stream and be excreted in urine.
What are some key steps/intermediates of the urea cycle?
Glutamate is the main compund from which ammonia is released in the mitochindria of the liver cells.
Carbamoyl phosphate carries ammonia over to ornithine, producing citrulline.
Citrulline enters the urea cycle and ultimately releases urea and regenerates ornithine.
Explain the five enzymatic steps from ammonia to urea
Ammonia to carbamoyl phosphate: Carbamoyl phosphate synthase I catalyzes the formation of carbamoyl phosphate from ammonia and CO2 (HCO3), requires two ATP.
Carbamoyl phosphate to citrulline: carbamoyl phosphate reacts with ornithine to produce citrulline. The phosphate of carbamoyl phosphate is released. The reaction is catalyzed by ornithine transcarbamoylase.
Citrulline to arginosyccinate: Citrulline passes from the mitochindria to the cytosol. The amino group of aspartate condenses with the ureido group (H2N-CO-NH) of citrulline. The reaction occurs in two steps: first citrulline reacts with ATP to attach an AMP to the citrulline, resultin in a citrullyl-AMP intermediate. The intermediate reacts with aspartate to produce arginosuccinate. Both these steps are catalyzed by arginosuccinate synthase.
Arginosuccinate to arginine: Arginosuccinate is cleaved to fumarate and arginine by argininosuccinase. Fumarate is converted to malate to be used as citric acid cycle intermediate.
Arginine to urea (and ornithine): Arginine is cleaved to urea and ornithine by arginase. Urea is transported in the blood to be excreted. Ornithine again enters the mitochondria to initiate another round of the cycle.
Explain the interconnection between the citric acid -and urea cycle through the aspartate-arginosuccinate shunt
Fumarate, a biproduct of the convertion of argininosuccinate to arginine, can enter the citric acid cycle by being converted to malate. Aspartate, which can be produced by transamination between glutamate and oxaloacetate from the citric acid cycle can be transported into the cytosol, where it can be used to produce arginosuccinate in the urea cycle.
At what levels are the urea cycle regulated?
Regulation of the urea cycle can be accomplished by regulating the synthesis of enzymes involved in the cycle, or by allosteric regulation of carbamoyl phosphate synthase I.
Explain the regulation of the urea cycle
The synthesis of urea cycle enzymes and carbamoyl phosphate synthase I are determined by diet, starvation or a diet rich in proteins lead to an increase of synthesis of these enzymes.
Allosteric activation of carbamoyl phosphate synthase I is performed by N-acetylglutamate, a compound produced by acetyl-CoA and glutamate, catalyzed by N-acetylglutamate synthase. The amount of N-acetylglutamate is determinde by the concentration of the substrates a-CoA and glutamate. Arginine is also an activator of N-acetylglutamate synthase (and thus for the urea cycle).
What is the purpose of the citric acid -and urea cycle connection?
By using the biproducts of the urea cycle in the energy producing citric acid cycle, the energetic cost of urea production is reduced.
