Ornithine cycle reactions

C5. Cedrangolo, F., Della Pietra, G., De Lorenzo, F., Papa, S., and Cittadini, D., The effect of a-D,L-methylaspartate on ornithine cycle reactions and urea excretion in rats. Emymologia 25, 308-314 (1963). 

In the urea cycle, two molecules of ammonia combine with a molecule of carbon dioxide to produce a molecule of urea and water. The overall cycle involves a series of biochemical reactions dependent on enzymes and carrier molecules. During the urea cycle the amino acid ornithine (C5H12N202) is produced, so the urea cycle is also called the ornithine cycle. A number of urea cycle disorders exist. These are genetic disorders that result in deficiencies in enzymes needed in one of the steps in the urea cycle. When a urea cycle deficiency occurs, ammonia cannot be eliminated from the body and death ensues. 

The entry of activated ammonia into the urea cycle occurs by the ornithine transcarbamoylase reaction where the carbamoyl group is transferred to the side chain amino group of the non-protein amino acid, ornithine. Ornithine has five carbons its carbon chain therefore has the same length as that of arginine. The product of the ornithine transcarbamoylase reaction is the amino acid citrulline. 

Begirming and ending with ornithine, the reactions of the cycle consumes 3 equivalents of ATP and a total of 4 high-energy nucleotide phosphates. Urea is the only new compound generated by the cycle all other intermediates and reactants are recycled. 

Fig. I. A scheme presenting the biosynthesis of ornithine from glutamic acid and the ornithine cycle. The various reactions are numbered. The enzymes and the reactions they catalyze are listed in Table I. The structures are presented (without naming compounds) to enable the reader to visualize the transformations. The characteristics of the enzymatic reactions are discussed in the text.

Although the overall reaction is reversible with a measurable equilibrium constant, the reaction is probably virtually irreversible in vivo because of the ubiquitous pyrophosphatases. It is undoubtedly this step that prevents conversion of arginine to ornithine by reversal of the ornithine cycle and requires an arginase for arginine catabolism (see page 386). 

The st example concerns the intracellulEir localization of the reactions of the ornithine cycle. The enzymes catalyzing two of the reactions, carbamyl phosphate synthetase and ornithine transcarbamylase, Eire found in the mitochondrial matrix while the other three enzymes are in the cjrtosol. As a result, the mitochondrial membrane is interposed as a barrier to the operation of the cycle, necessitating two addi-... 

Recent advances in the understanding of the mechanism of the biosynthesis of urea have been concerned with (1) the elucidation of the enzymatic steps, (2) the nature of the ammonia or amino group acceptors and donors, (3) the mechanism of CO2 fixation, and (4) the nature of the energy coupling reactions. The outline of the over-all ornithine cycle as originally proposed by Krebs and Henseleit remains essentially intact and may be considered as consisting of three over-all enzymatic steps as follows ... 

The reaction a source of arginine required for the formation of guanido-acetic acid by transamidination with glycine in the kidney. The source of citruUine in the kidney is unknown. It is remarkable that the interaction of citrulline with AS or GL to form arginine apparently does not occur in liver, although conversion of citrulline to arpnine is a highly probable intermediary step in the ornithine cycle of urea formation (c/. Gomall and Hunter, 80). 

The lower homologue of lysine and ornithine, 2,4-diaminobutanoic acid, interferes (like the arginine analogue) with reactions of the ornithine cycle, thereby increasing the ammonia content in the blood and brain. 2,3-Diaminopropanoic acid acts as an analogue of glutamate in the nervous system. 

In the first reaction of the cycle the carbamoyl group is transferred from carbamoyl phosphate to ornithine to yield citrulline. Neither of these amino acids are known to occur in proteins. The remainder of the cycle reactions are cytoplasmic so citrulline is transported by a specific uniport carrier across the inner mitochondrial membrane. In the cytosol, argininosuccinate is formed by a condensation reaction which produces a covalent linkage between the carbonyl carbon atom of citrulline and the amino group of aspartate. This reaction, catalysed by argininosuccinate synthase, is readily reversible but is driven forward by the irreversible... 

Argininosuccinate lyase (AL) (Fig. 40-5 reaction 4) cleaves argininosuccinate to form fumarate, which is oxidized in the tricarboxylic acid cycle, and arginine, which is hydrolyzed to urea and ornithine via hepatic arginase. Both AL and arginase are induced by starvation, dibutyryl cyclic-AMP and corticosteroids. 

On an average Western diet, adnlt hnmans excrete around 30 g of nrea per day but this can easily triple on a protein-rich diet. The reactions and the concept of a cycle were discovered by Krebs Henseleit (1932). Snbseqnent work clari-hed the details of what has become known as the ornithine or the nrea cycle. 

Ornithine is a metabolically quite active amino acid, and the important precursor of pyrrolidine nucleus, which is found in pyrrolizidine alkaloids. Ornithine itself is a non-protein amino acid formed mainly from L-glumate in plants, and synthesized from the urea cycle in animals as a result of the reaction catalyzed by enzymes in arginine. 

FIGURE 3-8 Uncommon amino acids, (a) Some uncommon amino acids found in proteins. All are derived from common amino acids. Extra functional groups added by modification reactions are shown in red. Desmosine is formed from four Lys residues (the four carbon backbones are shaded in yellow). Note the use of either numbers or Creek letters to identify the carbon atoms in these structures, (b) Ornithine and citrulline, which are not found in proteins, are intermediates in the biosynthesis of arginine and in the urea cycle. 

The carbamoyl phosphate, which functions as an activated carbamoyl group donor, now enters the urea cycle. The cycle has four enzymatic steps. First, carbamoyl phosphate donates its carbamoyl group to ornithine to form citrulline, with the release of Pj (Fig. 18-10, step ). Ornithine plays a role resembling that of oxaloacetate in the citric acid cycle, accepting material at each turn of the cycle. The reaction is catalyzed by ornithine transcarbamoylase, and the citrulline passes from the mitochondrion to the cytosol. 

As we noted in Chapter 16, the enzymes of many metabolic pathways are clustered (p. 605), with the product of one enzyme reaction being channeled directly to the next enzyme in the pathway. In the urea cycle, the mitochondrial and cytosolic enzymes appear to be clustered in this way. The citrulline transported out of the mitochondrion is not diluted into the general pool of metabolites in the cytosol but is passed directly to the active site of argininosuccinate synthetase. This channeling between enzymes continues for argininosuccinate, arginine, and ornithine. Only urea is released into the general cytosolic pool of metabolites. 

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