Enzyme factors controlling activity

Lipases have also been used as initiators for the polymerization of lactones such as /3-bu tyro lac tone, <5-valerolactone, e-caprolactone, and macrolides.341,352-357 In this case, the key step is the reaction of lactone with die serine residue at the catalytically active site to form an acyl-enzyme hydroxy-terminated activated intermediate. This intermediate then reacts with the terminal hydroxyl group of a n-mer chain to produce an (n + i)-mer.325,355,358,359 Enzymatic lactone polymerization follows a conventional Michaelis-Menten enzymatic kinetics353 and presents a controlled character, without termination and chain transfer,355 although more or less controlled factors, such as water content of the enzyme, may affect polymerization rate and the nature of endgroups.360... 

Since the first report on the ferrocene mediated oxidation of glucose by GOx [69], extensive solution-phase studies have been undertaken in an attempt to elucidate the factors controlling the mediator-enzyme interaction. Although the use of solution-phase mediators is not compatible with a membraneless biocatalytic fuel cell, such studies can help elucidate the relationship between enzyme structure, mediator size, structure and mobility, and mediation thermodynamics and kinetics. For example, comprehensive studies on ferrocene and its derivatives [70] and polypy-ridyl complexes of ruthenium and osmium [71, 72] as mediators of GOx have been undertaken. Ferrocenes have come to the fore as mediators to GOx, surpassing many others, because of factors such as their mediation efficiency, stability in the reduced form, pH independent redox potentials, ease of synthesis, and substitutional versatility. Ferrocenes are also of sufficiently small size to diffuse easily to the active site of GOx. However, solution phase mediation can only be used if the future biocatalytic fuel cell... 

Figure 21.7 Control of the activity of Ras by a balance of the activities of guanine nucleotide exchange factor and GTPase. GAP is the abbreviation for GTPase-activating factor and GEF for guanine nucleotide exchange factor. Both are enzymes. Both the activities are controlled by stimuli from various cell surface receptors. Ras oncogenes are present in about 30% of all human tumours.

Phosphohpases of type Cy are activated by receptor tyrosine kinases (see Chapter 8), and thus phosphohpase Cy is involved in growth factor controlled signal transduction pathways. The receptor tyrosine kinases (see Chapter 8) phosphorylate the enzyme at specific tyrosine residues and initiate activation of the enzyme. Characteristic for the structure of phospholipase Cy is the occurrence of SH2 and SH3 domains (see Chapter 8). These represent protein modules that serve to attach further partner proteins. 

The three coefficients C, e, and R are related in a simple way the responsiveness (R) of a pathway to an outside factor that affects a certain enzyme is a function of (1) how sensitive the pathway is to changes in the activity of that enzyme (the control coefficient, C) and (2) how sensitive that specific enzyme is to changes in the outside factor (the elasticity, e) ... 

Understanding the factors controlling organic matter turnover is complicated by possible differences in population distributions of organisms responsible for enzyme production and product uptake. Although relationships between enzyme Vmax and bacterial productivity have been reported (Sinsabaugh et al., 1997), correlations between total bacterial numbers or bacterial production and enzyme activities are frequently found to be variable or weak (e.g., Mayer, 1989 Boetius et al., 1996, 2000). Vrba et al. (1992) observed that MUF-a-glucosidase activity correlated with bacterial biomass measured in a freshwater reservoir over the course of three seasons,... 

A few key points will enable us to begin addressing the factors controlling production, function, and activities of extracellular enzymes in aquatic systems ... 

Fig. 2.8. Factors controlling the production of free radicals in cells and tissues (Rice-Gvans, 1990a). Free radicals may be generated in cells and tissues through increased radical input mediated by the disruption of internal processes or by external influences, or as a consequence of decreased protective capacity. Increased radical input may arise through excessive leukocyte activation, disrupted mitochondrial electron transport or altered arachidonic acid metabolism. Delocalization or redistribution of transition metal ion complexes may also induce oxidative stress, for example, microbleeding in the brain, in the eye, in the rheumatoid joint. In addition, reduced activities or levels of protectant enzymes, destruction or suppressed production of nucleotide coenzymes, reduced levels of antioxidants, abnormal glutathione metabolism, or leakage of antioxidants through damaged membranes, can all contribute to oxidative stress.

Anaerobic reductive dechlorination of chiral PCBs confirmed that microbial reductive dechlorination in situ was possible in Lake Hartwell [163]. Microcosms with sediments from the same cores [156] spiked with racemic PCB 132 reductively meto-dechlorinated this congener nonenantioselectively to PCB 91, which in mrn was stereoselectively meta-dechlorinated to achiral PCB 51 (Figure4.12). Similarly, PCB 149wasnonstereoselectively /jflra-dechlorinated to PCB 95, in turn enantioselectively meta-dechlorinated to achiral PCB 53 [163]. The enantiomer preferences for PCB 149 dechlorination were consistent between the laboratory microcosms [163] and field observations, suggesting possible similarities in the microbial consortia in both cases. However, PCB 132 was nonracemic in the cores [156], suggesting that either the microbial consortia and/or environmental conditions affecting microbial activity were different between the laboratory and in situ. Much remains unknown about the microbial strains and enzymes involved in PCB anaerobic reductive dechlorination or the factors controlling stereospecificity. 

Cholecalciferol 25-hydroxylase is not restricted to the liver kidneys, skin, and gut microsomes also have a cytochrome P450 -dependent enzyme that catalyzes the 25-hydroxylation of cholecalciferol and la-hydroxycholecalciferol, hut not ergocalciferol. Although there is some evidence that calcitriol can reduce the activity of calciferol 25-hydroxylase, it is not known whether this is physiologically important the major factor controlling 25-hydroxylation is the rate of uptake of cholecalciferol into the liver. It is the fate of calcidiol in the kidneys that provides the most important regulation of vitamin D metabolism (Wikvall, 2001). 

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