Enzyme activity definition

Chemical Pathology. Also referred to as clinical chemistry, this monitoring procedure involves the measurement of the concentration of certain materials in the blood, or of certain enzyme activities in semm or plasma. A variety of methods exist that allow (to variable degrees of specificity) the definition of a particular organ or tissue injury, the nature of the injurious process, and the severity of the effect (76). 

In many situations, the actual molar amount of the enzyme is not known. However, its amount can be expressed in terms of the activity observed. The International Commission on Enzymes defines One International Unit of enzyme as the amount that catalyzes the formation of one micromole of product in one minute. (Because enzymes are very sensitive to factors such as pH, temperature, and ionic strength, the conditions of assay must be specified.) Another definition for units of enzyme activity is the katal. One katal is that amount of enzyme catalyzing the conversion of one mole of substrate to product in one second. Thus, one katal equals 6X10 international units. 

Thus from Equation (2.13) we see that a working definition of KM is the substrate concentration that yields a velocity equal to half of the maximum velocity. Stated another way, the Ku is that concentration of substrate leading to half saturation of the enzyme active sites under steady state conditions. 

Enzyme activity refers to the rate at which a particular enzyme catalyzes the conversion of a particular substrate (or substrates) to one or more products under a given set of conditions. Usually, activity refers to the contribution of many enzyme molecules (often expressed simply as activity per mg of protein or similar) but, in its simplest form, activity refers to the contribution of a single enzyme molecule. The turnover number of an enzyme-substrate combination refers to the number of substrate molecules metabolized in unit time (usually a period of 1 s) under a given set of conditions (see later). These definitions appear, at first glance, to be largely self-explanatory. However, many factors contribute to the final activity of an enzyme, and these must be considered during any assessment of such activity. 

While it is tempting to explain regulatory and cosolvent effects on the basis of conformational changes favorable or unfavorable to enzyme activity, it is much more difficult to demonstrate the actual involvement, amount, and structural details of such changes. Experimental evidence consists in most cases of bits and pieces provided by techniques such as absorption and fluorescence spectroscopy, circular dichroism, and magnetic circular dichroism. These tools work in solution (and, when desired, at subzero temperatures) to investigate not simply empty enzymes but enzyme—substrate intermediates. However, even with this information, the conformational basis of enzyme activity remains more postulated than demonstrated at the ball and stick level, and in spite of data about the number and sequence of intermediates, definition of their approximate nature, rate constants, and identification of the types of catalysis involved, full explanation of any particular reaction cannot be given and rests on speculative hypothesis. 

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