Specificity, enzymes towards

Each enzyme has a working name, a specific name in relation to the enzyme action and a code of four numbers the first indicates the type of catalysed reaction the second and third, the sub- and sub-subclass of reaction and the fourth indentifies the enzyme [18]. In all relevant studies, it is necessary to state the source of the enzyme, the physical state of drying (lyophilized or air-dried), the purity and the catalytic activity. The main parameter, from an analytical viewpoint is the catalytic activity which is expressed in the enzyme Unit (U) or in katal. One U corresponds to the amount of enzyme that catalyzes the conversion of one micromole of substrate per minute whereas one katal (SI unit) is the amount of enzyme that converts 1 mole of substrate per second. The activity of the enzyme toward a specific reaction is evaluated by the rate of the catalytic reaction using the Michaelis-Menten equation V0 = Vmax[S]/([S] + kM) where V0 is the initial rate of the reaction, defined as the activity Vmax is the maximum rate, [S] the concentration of substrate and KM the Michaelis constant which give the relative enzyme-substrate affinity. 

Biologically mediated redox reactions tend to occur as a series of sequential subreactions, each of which is catalyzed by a specific enzyme and is potentially reversible. But despite favorable thermodynamics, kinetic constraints can slow down or prevent attainment of equilibrium. Since the subreactions generally proceed at unequal rates, the net effect is to make the overall redox reaction function as a imidirectional process that does not reach equilibrium. Since no net energy is produced imder conditions of equilibrium, organisms at equilibrium are by definition dead. Thus, redox disequilibrium is an opportunity to obtain energy as a reaction proceeds toward, but ideally for the sake of the organism does not reach, equilibrium. 

Regulation of Flavonoid Synthesis in C. americanum. Biosynthesis of methylated flavonol glucosides seems to be under tight regulation, not only by the substrate specificity of the enzymes involved, but also by other factors, among which are (a) the strict position specificity of these enzymes towards their hydroxylated or partially methylated substrates (b) the apparent difference in microenvironment of the different methyl-transferases, whereby those earlier in the pathway utilized aglycones whereas later enzymes accepted only glucosides as substrates (c) the subtle characteristic differences in methyl-transferases with respect to their pH optima, pi values and requirement for Mg ions, despite their similar molecular size ... 

There is one case in which strain or induced fit could be useful in a type of specificity. These mechanisms are unimportant where competition between substrates is concerned. But given a situation in which there is no specific substrate present, these mechanisms could be of use in providing a low absolute activity of the enzyme toward, say, water. For example, induced fit could prevent hexokinase from being a rampant ATPase in the absence of glucose (although its absence is extremely unlikely). 

A meaning of specificity that is really a misuse of the term refers to the activity of an enzyme toward an alternative substrate in the absence of a specific substrate, as can happen in an experiment in vitro. In such a test tube experiment, a substrate is often described as poor because it involves either a high value of Km or a low value of cat. In biological systems both cat and KM are important. 

Many DNases are known to be activated by a divalent cation. However, only from the work of Bollum (11) did it become clear that the nature of the cation may qualitatively change the specificity of the enzyme toward adjacent bases. Quantitative changes in the requirements for the divalent cation (10) have been observed during different stages of the same reaction, e.g., micrococcal nuclease (7) where the increased Ca2+ concentration causes a decrease in the average size of the terminal product. 

In vitro Metabolism. Numerous variables simultaneously modulate the in vivo metabolism of xenobiotics therefore their relative importance cannot be studied easily. This problem is alleviated to some extent by in vitro studies of the underlying enzymatic mechanisms responsible for qualitative and quantitative species differences. Quantitative differences may be related directly to the absolute amount of active enzyme present and the affinity and specificity of the enzyme toward the substrate in question. Because many other factors alter enzymatic rates in vitro, caution must be exercised in interpreting data in terms of species variation. In particular, enzymes are often sensitive to the experimental conditions used in their preparation. Because this sensitivity varies from one enzyme to another, their relative effectiveness for a particular reaction can be sometimes miscalculated. 

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