Structure and Function of Ornithine Carbamoyltransferases

Abstract

The reaction catalyzed by ornithine carbamoyltransferase can participate in either the anabolism or the catabolism of arginine. The carbamoylation of ornithine, yielding citrulline, is involved in the biosynthetic sequence; the reverse reaction, the phosphorolysis of citrulline, is the second step of the arginine deiminase pathway.The ornithine carbamoyltransferases of a number of microorganisms which can fulfil both of these functions have been studied in this work. This group of organisms was found to possess two distinct ornithine carbamoyltransferases. The functions of these enzymes were surmised by determining the type of genetic regulation to which they were subjected.The kinetic properties of these various enzymes have been determined. All of them, regardless of the role they play in the cell, catalyze both the synthesis and arsenolysis of citrulline. The anabolic transferase of Pseudomonas is the only enzyme which displays functional irreversibility. A comparison of the quaternary structure of these transferases was performed and reveals interesting features in relation to the metabolic function of these enzymes. All well-characterized anabolic enzymes have low molecular weights (from 150000-105000) and are likely to be trimers. Catabolic enzymes, with the exception of those of Bacillus licheniformis and Halobacterium salinarium, display much higher molecular weights and more elaborate quaternary structure.The properties of these two groups of transferases are discussed in relation to their metabolic role in the cells.Ornithine carbamoyltransferase is an enzyme of particular metabolic interest, since it can fulfil two different functions : the carbamoylation of ornithine participating in the anabolism of arginine, and the thermodynamically less favored phosphorolysis of citrulline, which is involved in the catabolic arginine deiminase pathway.We have shown previously that these two functions exist in Pseudomonas and are fulfilled by distinct enzymes, which differ in their kinetic, regulatory and structural properties. They also differ in their ability to catalyze the two directions of the reaction [l-51. Starting from such observations we have undertaken a comparative study of the ornithine carbamoyltransferases of various organisms. We expected such investigations to provide information concerning the origin and the evolution of these enzymes.The microorganisms were chosen so as to present the widest range of distinct situations with respect to

Lmo0036, an ornithine and putrescine carbamoyltransferase in Listeria monocytogenes, participates in arginine deiminase and agmatine deiminase pathways and mediates acid tolerance

“…4). It exhibited higher in vitro specific activities of aOTC and aPTC than those of cOTC and cPTC (Table 3), consistent with previous reports that in vitro OTC or PTC reactions lie towards the anabolic direction (Legrain & Stalon, 1976;Legrain et al, 1977;Sainz et al, 1998).…”

“…Generation of ornithine or putrescine is determined based on the production of a red colour (catabolic), while consumption of citrulline or carbamoylputrescine is linked to the colour turning lighter pink (anabolic). Kinetic studies revealed that the equilibrium of in vitro ornithine or putrescine carbamoyltransfer reactions lies overwhelmingly towards the formation of citrulline or carbamoyl (anabolic) (Legrain & Stalon, 1976;Legrain et al, 1977;Sainz et al, 1998).…”

Mechanism of Inactivation of Ornithine Transcarbamoylase by N δ-(N′-Sulfodiaminophosphinyl)-l-ornithine, a True Transition State Analogue?

“…Although not thermodynamically favored, the reverse reaction is efficiently catalyzed by a specialized catabolic OTCase (1). This activity is found in microbes that possess the arginine deiminase pathway, which enables the generation of ATP under anaerobic conditions (2).…”

“…The basic quaternary structure is a trimer, with active sites located at the interface between the protein monomers (4,5). Anabolic OTCase is the simplest form of the enzyme, comprising a single homotrimeric unit (1), whereas the catabolic OTCase is a dodecamer of four trimers in a tetrahedral arrangement (5). ATCase is also a dodecamer with two trimers and three regulatory dimers (4).…”

Two Asparagine Synthetases in Saccharomyces cerevisiae

“…Molecular weight markers are : bovine liver catalase (240000) and Eschericl~ia coli alkaline phosphatase (78 000) from Worthington Biochemical Corporation (Freehold, NJ, U.S.A.); Pseudornonas putida catabolic ornithine carbamoyltransferase (420000) from this laboratory (B. Wargnies) and Sacclzarom,yces cerevisiae ornithine carbamoyltransferase (125 000), a normal enzyme from the sample. These enzymes are assayed following [6].…”