Interconversion cycle

Figure 1.21 Interconversion cycle oftetravalent (U4+) and hexavalent (U6+) uranium in nature.

Since the change in rate of glycogen degradation, catalysed by glycogen phosphorylase, depends on a balance between the two activities, which constimte a cycle, the relationship is also known as an interconversion cycle. By convention, the active form is the a form and the inactive form is the b form (Figure 3.12). 

Figure 3.12 The regulation of phosphorylase activity by reversible phosphoiylation. A reversible phosphorylation process is also known as an interconversion cycle the latter term is preferred in this text, since the individual reactions must be irreversible, which can be confusing if the term reversible is used to describe the overall process. In resting muscle, almost all phosphorylase is in the b form.

The topic of interconversion cycles in providing inhibition or activation of a target enzyme, the activity of which regulates the flux through a pathway, is discussed above. In brief, an enzyme exists in two forms, conventionally designated a and b, one being a covalent modification of the other. This is brought about, for example, by phosphorylation with ATP, so that one form is a phos-... 

Pyruvate dehydrogenase, the activity of which is affected by a number of factors the ATP/ADP, the NADH/NAD+ and the acetyl-CoA/CoASH concentration ratios. It is also activated by Ca ions. These factors change the enzyme activity via an interconversion cycle (Fignre 6.16(b)). 

Since the enzyme glycogen synthase catalyses the rate-limiting step in glycogen synthesis, it is the activity of this enzyme that must be increased as the blood glucose concentration increases. This is achieved via an interconversion cycle (i.e. reversible phosphorylation). A protein kinase phosphorylates it, which inactivates the enzyme, whereas a protein phosphatase dephosphorylates it, which... 

There are only five key biochemical factors that provide for the regnlation (i) snbstrate cycles (ii) regulation of phosphofrncto-2-kinase (PFK-2) (iii) phosphorylation/ dephosphorylation interconversion cycles (i.e. reversible phosphorylation) (iv) gene expression of gluconeogenic enzymes (v) concentrations of precnrsors in the blood. 

Two key enzymes in the pathway are regnlated by interconversion cycles they are the regnlatory enzyme PFK-2, and the glycolytic enzyme pyruvate kinase. There are two separate protein kinases that phosphorylate these enzymes and they both resnlt in activation of these enzymes. Dephosphorylation inactivates them. 

HMG-CoA synthase is not snccinylated, i.e. the interconversion cycle involving snccinylation does not occur (Appendix 7.5). 

As indicated above, the flux-generating step for fatty acid synthesis is the conversion of acetyl-CoA to malonyl-CoA, catalysed by acetyl-CoA carboxylase. Consequently, regulation of the rate of synthesis is achieved via changes in the activity of this enzyme. The properties of the carboxylase identify three mechanisms for regulation allosteric regulation, reversible phosphorylation (an interconversion cycle) and changes in the concentration of the enzyme. (The principles underlying the first two mechanisms are discussed in Chapter 3.)... 

The principle underlying the changes in activity of a G-protein is similar to that of an interconversion cycle (Chapter 3). The classic example of an interconversion cycle is that between the two forms of the enzyme phos-phorylase phosphorylase a and b. The interconversions between b and a are catalysed by a protein kinase and a protein phosphatase. The similarities are as follows. 

Figure 12.23 Comparison of the G-protein cycle and the phosphorylase interconversion cycle. The comparison should help to explain the principle underlying the concept of the interconversion cycle and to understand more readilly the G-protein system.

It is likely that the advantage of regulation of ion channel activity via the interconversion cycle is an improvement in sensitivity of the change in activity of the ion channel to the change in concentration of the neurotransmitter in the... 

As might be expected from other mechanisms of regulation described in this text, phosphorylation and dephosphorylation of key proteins is the main mechanism for regulating the cycle, i.e. reversible phosphorylation, also known as interconversion cycles (discussed in Chapter 3). In the cell cycle, several of these interconversion cycles play a role in control at the checkpoints. Two important terms must be appreciated to help understand the mechanism of regulation of the cycle the phosphorylation of proteins is catalysed by specific protein kinases, known as cell-division kinases (cdck) or cell cycle kinases (cck) and these enzymes are activated by specific proteins, known as cyclins. 

Figure 20.31 The principle of interconversion cycles in regulation of protein activity or changes in protein concentration as exemplified by translation/proteolysis or protein kinase/protein phosphatase. They result in very marked relative changes in regulator concentration or enzyme activity. The significance of the relative changes (or sensitivity in regulation) is discussed in Chapter 3. The principle of regulation by covalent modihcation is also described in Chapter 3. The modifications in cyclin concentration are achieved via translation and proteolysis, which, in effect, is an interconversion cycle. For the enzyme, they are achieved via phosphorylation and dephosphorylation reactions. In both cases, the relative change in concentration/activity by the covalent modification is enormous. This ensures, for example, that a sufficient increase in cyclin can occur so that an inactive cell cycle kinase can be converted to an active cell cycle kinase, or that a cell cycle kinase can be completely inactivated. Appreciation of the common principles in biochemistry helps in the understanding of what otherwise can appear to be complex phenomena.

The simultaneous activities of the kinase and phosphatase are yet another example of regnlation by reversible protein phosphorylation (i.e. an interconversion cycle -Chapter 3). An increased force of contraction could be caused either by inhibition of the phosphatase or by activation of the kinase. However, physiologically relevant inhibitors of the phosphatase have not yet been discovered. 

Different structures and various equilibria have been suggested to account for the observed nmr spectra. The scheme in (89) shows the relation and principal interconversion cycle of cations [124]-[I26] around a tricyclo-butonium ion structure [135] which is energetically disfavoured according to the Jahn-Teller theorem. 

It should be noted that if a response involves an interconversion cycle (e.g., the eflFect of pyruvate on pyruvate dehydrogenase via inhibition of pyruvate dehydrogenase kinase), its eflFect is incorporated into the catalystic (rj component (see Appendix E). 

If a response involves an interconversion cycle its eflFect must be included in the catalystic component of the intrinsic sensitivity of the pathway enzyme to X, i.e., in r pc) (Section III,B,2). Moreover, since it represents an independent route of communication from X to pathway enzyme, a function such as must be added to any other catalystic components of the response (Section... 

Olefins interconversion cycle olefin carbon pool ... 

So, the ethylene production does correlate with coke presence, in particular with aromatics formation as far as the diffusion limitations are not significant. However, it seems that the majority of ethylene is not always formed directly from MeOH [115]. The aromatics and other coke species could be the products of the conversion of primary carbenium ions, which are mobile and could equilibrate each other [28]. This may explain the isotopic distribution in products and retained coke molecules and the coexistence of aromatics and carbenium ions [28], In addition to the coproduction of ethylene with aromatics in olefins interconversion cycle, formation of ethylene via alkylation-dealkylation of methyl aromatics with heavy olefins or with the equivalent carbenium ions like ethyP, propyP, and butyP could be an option. The alkyl aromatics with the side chain length of two carbons or longer are not stable in the pore and dealkylates on the acid sites due to too long residence time and steric hindrances. This may lead to formation of ethylene, other olefins, and alkylaromatics with different structure, namely PMBs [129]. In other words, the ethylene is formed via interaction of the carbenium ions like ethyP, propyP, and butyP formed from MeOH or heavy olefins with aromatics and other coke precursors followed by cracking and in a less extent by a direct alkylation of PMBs with methanol. The speculation is based properly on analysis of the prior arts and is not contradictory with the concept of the aromatic cycle for ethylene formation. 

The fact that ZSM-22 does not produce a lot of ethylene and propylene is explained by a suppression of both aromatic/coke and olefins interconversion cycles on this catalyst. SAPO-34 favors aromatic/coke cycle and limits the olefins cycle. On the contrary,... 

---