Cleavage enzymes and

Sato, S-I, N Ouchiyama, T Kunura, H Nojiri, H Yamane, T Omori (1997a) Cloning of gees involved in carbazole degradation of Pseudomonas sp. strain CAIO nucleotide sequences of genes and characterization of meta cleavage enzymes and hydrolase. J Bacteriol 179 4841-4849. 

A three-substrate, three-product enzyme-catalyzed reaction scheme in which one particular substrate (A) is the only substrate that can bind to the free enzyme. After the EA binary complex has formed, the other two substrates (B and C) can bind in any order. Following the catalytic event, two of the products (P and Q) can be released in a random sequence, but the third product (R) has to be the last product released. Citrate cleavage enzyme and y-glutamylcysteine synthetase are reported to operate by this mechanism. See Multisubstrate Mechanisms... 

However, like TOL, TOM encodes the meta-fission of the resulting catechol (Figure 11.4). Consequently, it is unproductive in the assimilation of chloroaromatics like chlorobenzene and 2-chlorophenol, which are also metabolized to 3-chlorocatechol because of catechol 2,3-dioxygenase inactivation. To avoid a build-up of this product an ortho-cleavage enzyme and downstream enzymes for the complete catabolism of the product were recruited through the introduction of the 2,4-dichlorophenoxyacetic-acid-degradativeplasmid pROlOl (Kaphammer, Kukor Olsen, 1990). 

Figure 22.19. Formation of Ketone Bodies. The Ketone bodies-acetoacetate, d-3-hydroxybutyrate, and acetone from acetyl CoA are formed primarily in the liver. Enzymes catalyzing these reactions are (1) 3-ketothiolase, (2) hydroxymethylglutaryl CoA synthase, (3) hydroxymethylglutaryl CoA cleavage enzyme, and (4) d-3-hydroxybutyrate dehydrogenase. Acetoacetate spontaneously decarboxylates to form acetone.

Recent studies with bovine heart mitochondrial matrix preparations indicate that one of the major products of this pathway is octanoyl-ACP and these newly synthesized octanoyl moieties can be translocated directly to the lipoylation site of the glycine cleavage apo-H protein (S. Smith, 2007). Octanoylated mitochondrial proteins are the substrates for the enzyme lipoic acid synthase, which inserts two sulfur atoms at the C6 and C8 positions of the octanoyl moiety. These results are consistent with the hypothesis that one of the major roles of the mitochondrial FAS pathway in all eukaryotes is to ensure that an adequate supply of lipoyl moieties is always available to service the glycine cleavage enzyme and the alpha-ketoacid dehydrogenases that are essential to mitochondrial function. 

Although LOX from tomato fruits forms predominantly 9-hydroperoxides from linoleic and linolenic acids (Matthew et al., 1977), the cleavage enzyme from tomato does not attack these positional isomers but, rather, is specific for the 13-hydroperoxy isomers, producing hexanal or c/5-3-hexanal, respectively (Galliard and Matthew, 1977). Thus one can rationalize the formation of both Cg and C9 volatiles aldehydes in cucumber extracts with less specificity of LOX and cleavage enzymes and the absence of C9 volatiles in tomato with the substrate specificity of the cleavage enzyme. 

Schmidt and Katz [45] suggested an alternative cyclic process involving malate, which would bypass the citrate cleavage enzyme and transfer intramitochondrial reducing equivalents to the cytosol without producing acetyl units. Were this short-circuit of the malate transhydrogenation cycle found to play a major role in adipose tissue, it could supply more than 50% of the NADPH required by the synthetase. Another potential source of extramitochrondial NADPH is isocitrate dehydrogenase however, it does not appear to be of major importance in fatty acid synthesis, as will be discussed later. 

It has been shown that the carboxylation of acetyl-CoA is effectivelyf the rate-determining reaction of fatty acid synthesis in animal tissues [94] and therefore has regulatory potential. In the absence of tricarboxylic acid activator, acetyl-CoA carboxylase activity is lower by nearly two orders of magnitude than in the fully activated state where its catalytic capacity is nearly kinetically matched to those of the citrate cleavage enzyme and fatty acid synthetase [76,77,94,150]. Therefore, changes in the level of tricarboxylic acid effector presumably could control the rate of the carboxylase reaction, and thus regulate fatty acid synthesis. 

The enzymes discussed in the previous sections (carbamyl phosphate synthetase, ornithine transcarbamylase, argininosuccinate synthetase, cleavage enzyme, and arginase) constitute the known enzymic steps in the sequence of reactions leading to the biosynthesis of urea in ureotelic animals in accordance with the cycle originally proposed by Krebs and Hen-seleit (458). A summary scheme showing the steps in this cycle and the relationship of some of the intermediates to other systems is shown in Fig. 2. 

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