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From Proteopedia

Current revision

Introduction

Arginase belongs to the hydrolase family and is typically in the alpha-beta-alpha structure. Arginase has been found to have two, three and six subunits. The fold of each subunit consists of a parallel, eight stranded ß-sheet bordered on either side by multiple α-helices. The proteins associated with the hydrolase family play important roles in arginine metabolism, the urea cycle, and histidine degradation. Members of the arginase family commonly are approximately three hundred amino acids in length and have two manganese ions in the proteins active site. Arginase deficiency is hereditary and rare. If arginase is missing, arginine is not broken down properly and urea cannot be produced; thus, nitrogen accumulates in the blood in the form of ammonia ultimately causing neurological and developmental issues. Proper arginase function is critical to ensure dispersion of ammonia from the animal’s body. Arginase is the fifth and final enzyme used in the Urea Cycle to produce ornithine from arginine and exert ammonia as a biproduct. In order for arginase to to produce ornithine, the active site of arginine must be bound by a metal ion in order to facilitate hydrogen-bonding-assisted stability of the substrate. The enzyme demonstrates a highly specific active site allowing for a conserved structural arrangement allowing for the hydrolysis of arginine to form ornithine.

Structure and Functionality

Arginase is a part of the ureohydrolase enzyme family which is responsible for important roles in activities such as degradation and metabolism. It is mainly located in the liver and prostate system. This enzyme assists in transforming ammonia into urea by catalyzing the formation ofurea and ornithine from arginine.

Its structure includes a two-molecule metal cluster of . Arginase can be found in a trimeric and hexomeric form but is typically found in a monomeric form due to increased catalytic activity (seen in the green link). These forms of arginase can react with water stabilizing and orienting the molecule. Once this happens the water molecule is able to act as a nucleophile and attack arginine and transform it into urea and ornithine through hydrolysis. To be more descriptive, Arginase appears in the last reaction of the urea cycle. It catalyzes the reaction that hydrolytically cleaves the guanidinium group from the amino acid arginine. The active site containing the Manganese ions bind to a water molecule and form a nucleophilic hydroxide ion to attack the guanidinium carbon atom in arginine. Once ornithine is created it is then transported into the mitochondrion and continues through the urea cycle.

Mechanism

The active site of Arginase holds arginine in place by the guanidinium group using hydrogen bonding. (1) The guanidinium group hydrogen bonds with glutamate 227 (Glu227) setting it up for the nucleophilic attack by the hydroxide ion. (2) This results in the creation of a tetrahedral intermediate. The manganese ion stabilize the sp3 lone electron pair forming on the ammonia (NH2) group and the hydroxide ion in the tetrahedral intermediate. The formation of products is initiated by the cleavage of the C-NH2 bond to form urea and ornithine. Arginase has a specific active site. The specificity is because of the high amount of hydrogen bonds between the enzyme and substrate. Water-faciliated hydrogen bonds saturate the four acceptor positions on the three positions on the alpha amino group and the alpha-carboxylate group. Inhibitors of arginase include 2(S)-amino-6-boronhexonic acid (ABH) and N-hydroxy-L-arginine (NOHA), which is an intermediate of nitric oxide biosynthesis. Detailed mechanism:

Implications and Applications

Agricultural Biotechnology

Current strategies exist in commercial agriculture in order to prevent crop loss by applying a chemical pesticide. Recent innovative technologies have been implemented in order to present transgenic crops with intrinsic pest resistance by producing plants with increased arginase I activity. In order to do this arginase is overproduced in plants to provide the plant with an enhanced resistance by acting as an anti-nutritive defense against herbivorous insects. This would be beneficial by reducing the need for chemical pesticides. Since arginase is present in all organisms, an increased production wouldn't be considered a foreign gene in genetically modified plants.

Arginase Regulation

Immunohistochemical studies suggest an importance in understanding arginase regulation and its respective amounts in the body. Imbalances of arginase levels in the body have been found to consequently induce vascular disease, pulmonary disease, infectious disease, immune cell function, and cancer.

Over-expression of arginase

Over-expression of arginase has been found to affect proteins, nitric oxide, urea, and ornithine. If the level of arginase far exceeds that of arginine, nitric oxide synthesis could be reduced and potentially promote nitric oxide synthase. Nitric oxide synthase is a vasodilator of the body and aids in the oxidation of arginine which increases the likelihood of pulmonary hypertension. Furthermore, this could decrease the expression of particular proteins. In endothelial cells, an over-expression of either arginase I or arginase II can reduce nitric oxide synthesis. Low levels of nitric oxide have been said to inhibit the activation and aggregation of platelets; thus, decreased levels of nitric oxide could produce a narrow tubule to the stomach decreasing the effectiveness of the digestive system. An over-production of arginase can also increase amounts of urea present in the body. High concentrations of urea commonly lead to hyperammonemia, increased levels of ammonia in the body, and to often the consequences are fatal. Accompanying this increase in urea is n increase in ornithine. Ornithine acts as a precursor for formation of cells and polyamines. Increased levels of ornithine will result in an increased amount of polyamines and ultimately initiate the proliferation of cells after a vascular injury.

Under-expression of arginase

An arginase deficiency is typically present from birth and can be life-threatening. The gene which controls the amount of arginase in the body is ARG1. If this gene is absent slow growth, spastic development, and limited cognitive abilities could ensue in infants. Hypoarginemia may be controlled by the utilization of pharmacologic agents such as sodium benzoate or sodium phenylbutyrate to remove excess nitrogen from the body and reduce plasma ammonia concentrations. Additionally, diets may be fed that are high in calories from carbohydrates and fats to reduce catabolism and the amount of excess nitrogen in the body.

References

^ MeSH Ureohydrolases ^ a b Lee J, Suh SW, Kim KH, Kim D, Yoon HJ, Kwon AR, Ahn HJ, Ha JY, Lee HH (2004). "Crystal structure of agmatinase reveals structural conservation and inhibition mechanism of the ureohydrolase superfamily". J. Biol. Chem. 279 (48): -. doi:10.1074/jbc.M409246200. PMID 15355972.

^ Christianson DW, Di Costanzo L, Sabio G, Mora A, Rodriguez PC, Ochoa AC, Centeno F (2005). "Crystal structure of human arginase I at 1.29-A resolution and exploration of inhibition in the immune response". Proc. Natl. Acad. Sci. U.S.A. 102 (37): -. doi:10.1073/pnas.0504027102. PMC 1201588. PMID 16141327. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1201588.

Morris Jr., S.M. Recent Advances in Arginine Metabolism: Roles and Regulation of the Arginases. Br. J. Pharmacol. 2009, 157, 922-930.

Abumrad NN, Barbul A. The use of arginine in clinical practice. In: Cynober LA, editor. Metabolic and Therapeutic Aspects of Amino Acids in Clinical Nutrition. Boca Raton, FL: CRC Press; 2004. pp. 595–611.

Khangulov, S.V., T.M.S., D.E.A., G.C.D. L-Arginine Binding to Liver Arginase Requires Proton Transfer to Gateway Residue His141 and Coordination of the Guanidinium Group to the Dimanganese(II,II) Center. Biochemistry. 1998, 37, 8539-8550

Research Collaboratory for Structural Bioinformatics. Protein Data Bank. http://www.pdb.org/pdb/home/home.do