Cyclic enzyme systems

In an extensive study, Okamoto and co-workers [76-86] introduced a biochemical switching device based on a cyclic enzyme system in which two enzymes share two cofactors in a cyclic manner. Cyclic enzyme systems have been used as biochemical amplitiers to improve the sensitivity of enzymatic analysis [87-89], and subsequently, this technique was introduced into biosensors [90-93], In addition, cyclic enzyme systems were also widely employed in enzymic reactors, in cases where cofactor regeneration is required [94-107], Using computer simulations, Okamoto and associates [77,80-83] investigated the characteristics of the cyclic enzyme system as a switching device, and their main model characteristics and simulation results are detailed in Table 1.1, as is a similar cyclic enzyme system introduced by Hjelmfelt et al. [109,116], which can be used as a logic element. 

Subsequently, Okamoto and associates [84-86] investigated the connection of several cyclic enzyme systems in order to construct a network. In their models the cyclic enzyme system represents a biochemical neuron that participates in a biochemical neural network. These models are detailed in Table 1.2. Theoretical models of such networks were also proposed by Hjelmfelt and co-workers [109-111,116], and these are also presented in Table 1.2. 

Models for biochemical switches, logic gates, and information-processing devices that are also based on enzymic reactions but do not use the cyclic enzyme system were also introduced [76,115,117-122]. Examples of these models are presented in Table 1.3. It should also be mentioned that in other studies [108,112-114,116], models of chemical neurons and chemical neural networks based on nonenzymic chemical reactions were also introduced. 

Table 1.1 Models Based on the Cyclic Enzyme System... 

The dynamic behavior of the cyclic enzyme system display catastrophic behavior in response to specific changes in external input. The system can realize a neuronic model capable of storing memory. 

Coupled cyclic enzyme system Ii and I2 are inputs to the system s pools of substrate Xi and X3, respectively simple mass action kinetics irreversible reactions. 

The works presented in Tables 1.1 to 1.3 [76-86,109-122] deal only with theoretical aspects of the enzymic biochemical devices, and the biochemical devices were not carried into practice. Moreover, Okamoto [85] suggests using silicon technology instead of biomaterials for practical implementation of the device based on the cyclic enzyme system. 

This study is also based on the cyclic enzyme system, butits leading concept is to accomplish practical implementation of this system using biomaterials. In this respect, the analytical models developed here are related to several biochemical reactors in which enzymic reactions take place. This practical approach cannot be found in the models reviewed [76-86,109-122]. 

To examine the cyclic enzyme system proposed by Okamoto et al. [76-86] as an information-processing unit when this model is implemented in an experimental system and the enzymic reactions take place... 

To develop analytical models that describe the performance of a cyclic enzyme system (herein termed the basic system) and a cyclic enzyme system with an external inhibitor (termed the extended basic system) when operated in different modes as a fed-batch reactor or a continuous reactor. These models enable us to design systems and select operational conditions according to needs. 

M. Okamoto and K. Hayashi, Dynamic behaviour of cyclic enzyme systems, / neor. Biol, 104, 591-598 (1983). 

M. Okamoto, T. Sakai, and K. Hayashi, Switching mechanism of a cyclic enzyme system role as a chemical diode, BioSystems, 21, 1-11 (1987). 

Okamoto, M. Hayashi, K. Dynamic behavior of cyclic enzyme systems. J. Theor. Biol. 1983,104, 591-598. 

A cyclic enzyme system, where two enzymes share a substrate in a cyclic manner, was assumed to be the control element of a feedback system to maintain the stationary concentration of the end product at a desired level against external perturbation (Okamoto Hayashi, 1984). Switching behaviour is shown by a mass action kinetic model involving interactions among the active E and inactive ( j) form of the cyclic enzyme, the inhibitor (Tj), the activator ( 2) arid the end product ( 3). The model is ... 

A. Inositol Phosphates.—Phosphatidyl inositol (71) is hydrolysed in mammalian tissues to wyo-inositol 1,2-cyclic phosphate (72).i myoinositol 1-phosphate (73) is released simultaneously but is not converted into (72) by the enzyme system. Periodate oxidation of (73) liberates orthophosphate quantitatively, the unstable dialdehyde phosphate (74) being an intermediate. Little or no orthophosphate is released from glucose 6-phosphate under the same oxidative conditions, and this reaction has been used to assay (73). 

Relaxation of smooth mnscles is controlled by the concentration of cyclic GMP in the muscle. This is regulated by the activities of the enzyme that forms cyclic GMP (i.e. gnanyl cyclase) and the enzyme that degrades cyclic GMP, that is, cyclic GMP phosphodiesterase (see Box 12.2). This is analogons to the enzyme system that regulates the concentration of cyclic AMP, by the activities of adenyl cyclase and phosphodiesterase ... 

The same group has exploited this interesting dehydrogenase for the synthesis of cyclic amino acids from linear precursors by developing a one-pot, two-enzyme system (Scheme 2.16). t-Lysine oxidase or L- or d-AAO were initially used to... 

The critical step in the use of multiple sulfur linkages is the availability of HS" from the reduced enzyme system. The HS" could possibly attack the cyclic or the acyclic multiple sulfur linkages through a nucleophilic mechanism. The cleaved sulfide linkages undergo oxidation to sulfite or thiosulfate. This seems a plausible pathway for the oxidation of the sulfur compounds in the present study. 

Metabolism of the cyclic diesters retrorsine (166), monocrotaline (169), and crispatine (171) by a variety of mammalian liver microsome preparations and by Peptococcus heliotrinreducans has been reported. Mattocks and co-workers have studied the metabolism of retrorsine by liver microsome preparations from several sources 167-169) and have demonstrated the conversion of this alkaloid to the corresponding iV-oxide 167 and a pyrrolic metabolite formulated as 168. The formation of 168 via 167 and dehydration is mitigated against by the observation that retrorsine TV-oxide (167) does not give rise to a pyrrolic metabolite on incubation with rat liver microsomes 167), even though the enzyme system responsible for the production of 168 from retrorsine has many of the properties of the mixed-function oxygenases capable of N-oxidation 167,174). The metabolites of retrorsine... 

Adenylate cyclase is a two-component enzyme system. It ultimately catalyzes the cyclase reaction, but only when it is associated with the hormone-bound receptor and a regulatory protein called a stimulatory G-protein (guanylate nucleotide binding protein), which activates adenylate cyclase. The G-protein is the intermediate between the receptor and the synthesis of cyclic AMP. 

It is worth noting that hypothyroidism also leads to an increased lipogenesis in the adipocyte which is independent of cyclic AMP [86,87]. In other words, the activity of several enzymes of the lipogenic pathway are stimulated in hypothyroidism independently from the effects of T3 on the cyclic AMP system. 

The regulatory role of calcium ions in intermediary metabolism is well documented. Calcium has been shown to be involved in activation or inhibition of specific enzyme systems [105], For example, it activates cyclic nucleotide phosphodiesterase, phosphofructokinase, fructose 1 6 biphosphatase, glycerol phosphate dehydrogenase, pyruvate dehydrogenase phosphatase and pyruvate dehydrogenase kinase. Calcium ions inhibit pyruvate kinase, pyruvate carboxylase, Na+/K+-AT-Pase and adenylate cyclase. 

Phosphorylase kinase is one of the best characterized enzyme systems to illustrate the role of calcium ions in regulation of intermediary metabolism. Phosphorylase kinase is composed of four different subunits termed a (Mr 145000), /3 (MT 128000), y (A/r 45000) and 5 (Mr 17000) and has the structure (a/3y8)A [106]. Only one of its four subunits actually catalyses the phosphorylation reaction the other three subunits are regulatory and enable the enzyme complex to be activated both by calcium and cyclic AMP. The y subunit carries the catalytic activity the 8 subunit is the calcium binding protein calmodulin and is responsible for the calcium dependence of the enzyme. The a and /3 subunits are the targets for cyclic-AMP mediated regulation, both being phosphorylated by the cyclic-AMP dependent protein kinase. Calmodulin appears to interact with phosphorylase kinase in a different manner from other enzymes, since it is an integral component of the enzyme. Phosphorylase kinase has an absolute requirement for calcium, and is inactive in its absence. 

The cyclic AMP-dependent protein kinase system is involved in the effects of ACTH in both the short term (stimulation of conversion of cholesterol to pregnenolone) and in the long term (increased synthesis of steroidogenic enzymes). In the interaction of the ACTH/cyclic AMP system with other intracellular messengers,... 

Another cyclic pentasaccharide with more complex linkages was discovered also by the Hayashibara group (Watanabe et al, 2005). They found that the enzyme system synthesizing cyclic nigerosyl nigerose (Fig. 17.2a) synthesized a novel cyclic pentasaccharide as a minor product in the same reaction mixture. This cyclic pentasaccharide contains a-1,3-, a-1,6- and a-1.4- linkages as shown in Figure 17.2d. 

Aga, H., Higashiyama, T., Watanabe, H., Sonoda, T., Nishimoto, T., Kubota, M., Fukuda, S., Kurimoto, M., and Tsujisaka, Y. 2002a. Production of cyclic tetrasac-chaides from starch using a novel enzyme system from Bacillus globisporus Cll. J. Biosaci. Bioeng., 94,336-342. 

The molecular basis for regulation of enzymatic activity through phosphorylation and dephosphorylation has been established in many enzyme systems (29). The significance of these reactions in histones, ribosomal proteins and KNA polymerase is not known. In an attempt to establish the specificity of the cyclic AMP-dependent protein kinases, the structure of several substrates have been determined (30). The data indicate that the sequence around the phosphorylated serine residue all contain two basic amino acids separated by no more than two residues from the N-terminal of the susceptible serine (e.g. -Arg-Arg-X-Y-Ser-). 

The action of ammonia on cytidine ribitol pyrophosphate (III) gave a cyclic phosphate IX which was oxidized by periodate and bromine to yield X, the cyclic 2,3-0-phosphate of glyceronic acid. This substance was purified by paper chromatography, and hydrolyzed with acid to the 2-phos-phate (XI) and 3-phosphate (XII) of D-glyceronic acid. These substances were utilized by a multi-enzyme system from rabbit muscle which degrades carbohydrates by way of the Embden-Meyerhof pathway. The ribitol phosphate (VII) is therefore L-ribitol 1-phosphate (o-ribitol 5-phosphate). ... 

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