Enzyme arginase

We begin this overview of manganese biochemistry with a brief account of its role in the detoxification of free radicals, before considering the function of a dinuclear Mn(II) active site in the important eukaryotic urea cycle enzyme arginase. We then pass in review a few microbial Mn-containing enzymes involved in intermediary metabolism, and conclude with the very exciting recent results on the structure and function of the catalytic manganese cluster involved in the photosynthetic oxidation of water. 

Ornithine prodnction by the enzyme arginase is essential for maintenance of flux through the cycle, so that an increase in the concentration of ornithine shonld increase the flnx. This mechanism is analogons to regn-lation of the Krebs cycle by changes in the concentration of oxaloacetate (Chapter 9, Fignre 9.23). 

Plasma should be separated from the blood cells within a few hours. For most amino acids the levels in plasma and red cells are comparable, but glutamate, aspartate, and taurine have extremely high intracellular levels and thus tend to rise in plasma upon hemolysis. A second effect of red cell degradation is the liberation of the enzyme arginase, which will convert arginine into ornithine. 

The final step is the release of urea by the enzyme arginase, which regenerates ornithine. 

Production of urea by cestodes suggests the existence of the urea (Krebs-Henseleit) cycle, which is shown in Fig. 6.11. One of the key enzymes, arginase, has been widely reported in cestodes (796, 185-187). However, some of the other enzymes, notably carbamyl phosphate synthetase and ornithine transcarbamyl, are either absent or present in only low amounts (39) and it is doubtful if a complete cycle operates in cestodes. It is likely that the urea excreted by tapeworms comes from the activity of arginase alone. The uric acid produced and excreted by cestodes probably arises from the breakdown of purines (39). 

Fig. 5.15 demonstrates the results of the analysis of 17 protein amino acids. As Arg and His are acylated only with difficulty by this procedure, the authors recommended either decreasing the acidity by adding sodium or lithium carbonate or converting Arg into Orn with the aid of the enzyme arginase and His into aspartic acid by ozonolysis. 

Highly selective L-arginine biosensors are described where the ammonia liberated in the reaction sequence catalyzed by the enzymes arginase-urease is monitored potentiometrically. The probes exhibit linear responses in the range of arginine concentrations of 0.1 mM to 0.01 M (140) and 30 pM to 3 mM (287). 

Tropane alkaloid biosynthesis has been studied at the biochemical level, and several enzymes from the biosynthetic pathway have been isolated and cloned, although the pathway has not been elncidated completely at the genetic level (Fig. 3b) (138). L-arginine is converted to the nonproteogenic amino acid L-omithine by the nrease enzyme arginase. Ornithine decarboxylase then decarboxylates ornithine to yield the diamine pntrescine. In Hyoscyamus, Duboisia, and Atropa, putrescine serves as the common precnrsor for the tropane alkaloids. 

The enzymes that catalyze these steps are homologous to argininosuccinate synthetase and argininosuccinase, respectively. Thus, four of the five enzymes in the urea cycle were adapted from enzymes taking part in nucleotide biosynthesis. The remaining enzyme, arginase, appears to be an ancient enzyme found in all domains of life. 

Arginine from the diet or from protein breakdown is cleaved by the cytosolic enzyme arginase, generating urea and ornithine. 

Enzyme arginase, which is located primarily in the liver and is inhibited by ornithine. 

D) The enzyme arginase releases fumarate from argininosuccinate... 

Introduction Mn Chemistry and Biochemistry Photosynthetic Oxidation of Water — Oxygen Evolution Mn and Detoxification of Oxygen Free Radicals Nonredox di-Mn Enzymes — Arginase... 

One particularly key metallohydrolase that contains manganese is the enzyme arginase (4-7), an enzyme for which a crystal structure... 

Figure 13 Proposed catalytic mechanism for the dimanganese enzyme arginase.

Urea is synthesized almost exclusively in the liver and then transported to the kidneys for excretion. The process that generates urea is called the urea cycle and is depicted in Figure 20.13. Urea is synthesized almost exclusively in the liver and then transported to the kidneys for excretion. The enzyme arginase is responsible for the cyclic nature of the urea cycle and for production of urea, as follows ... 

L-Ornithine is an intermediate in the urea cycle that is H3N formed by action of the enzyme arginase on arginine. 

The specific requirement of plants for manganese is also based on the fact that this element is a component of the enzymes arginase and phosphotransferase. However,... 

The seed canavanine is rapidly metabolized on germination to canaline and urea [Eq. (1), Fig. 7] by the enzyme arginase which is able to catalyze the hydrolysis of both arginine and canavanine. Urea is further hydrolyzed to ammonia and CO2, the N is presumably utilized for protein amino acid biosynthesis (Rosenthal, 1970 Whiteside and Thurman, 1971). An analysis of a large number of canavanine-containing legumes showed that there was a correlation between canavanine content and urease activity (Rosenthal, 1974). There is limited evidence that canaline is further broken down to homoserine and ammonia. 

Arginase. Urea is formed by the hydrolysis of arginine catalyzed by the enzyme arginase, which was demonstrated in liver in 1904 by Kossel and Dakin (V). A 400-fold purification from horse liver gave a prepara tion that appears to be essentially homogeneous. Arginase requires a... 

Certain amino acids are maintained at higher concentration in blood cells compared to plasma. This is the case for taurine, glutamate, aspartate, glutathione and for argininosuccinate, when present. Thus, hemolysis often increases the levels of these amino acids in plasma or serum. Hemolysis also releases the enzyme arginase, which hydrolyzes arginine to ornithine. 

STEP 4 OF FIGURE 20.5 ARGININE HYDROLYSIS The final step to complete the urea cycle is hydrolysis of arginine to give ornithine and urea. The reaction is catalyzed by the Mn " -containing enzyme arginase and occurs by addition of H2O to the C=N" " bond, followed by proton transfer and elimination of ornithine from the tetrahedral intermediate. 

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