Specific activity of an enzyme

For purposes of dosage, the specific activity of an enzyme is usually expressed as International Units (lU) rather than in terms of weight. However, unit measurements do not provide information on the absolute purity of a given product. Moreover, purity is not as critical an attribute for oral enzymes, as opposed to those adrninistered parenteraHy, inasmuch as the gastrointestinal tract is capable of disposing of most inert contaminants. 

The specific activity of an enzyme preparation is expressed as the catalytic activity per milligram of protein (units mg-1 protein) and is a convenient... 

Figure 9.4 Typical dependence of the specific activity of an enzyme on the water activity.

Another useful quantitative definition of enzyme efficiency is specific activity. The specific activity of an enzyme is the number of enzyme units or katals per milligram of protein. This is a measure of the purity of an enzyme. If a solution contains 20 mg of protein that express 2 units of activity (33 nkatals), the specific activity of the enzyme is 2 units/20 mg = 0.1 units/mg or 33 nkatals/20 mg = 1.65 nkatals/mg. As an enzyme is purified, its specific activity increases. That is, during purification, the enzyme concentration increases relative to the total protein concentration until a limit is reached. The maximum specific activity is attained when the enzyme is homogeneous or in a pure form. 

Describe the difference between the activity and the specific activity of an enzyme. How is it possible that the total activity of an enzyme in a particular fraction could decrease while the specific activity of that same fraction increases ... 

The routine unit of enzyme activity has been the international unit (I.U.), namely xmoles P formed (or S consumed) per minute. The specific activity of an enzyme preparation is the number of xmoles P formed (or S consumed) per minute per milligram of protein (clearly this will be very low in a crude cell extract and have a maximal value for a pure preparation of the enzyme). If the molecular mass is known, the specific activity of a pure enzyme measured in saturating (Fmax conditions) can be used to calculate the turnover number (or molecular activity ) of an enzyme, namely the number of P molecules formed (or S molecules transformed) per molecule of enzyme per second (units sec- ). If we recall that the maximal velocity (Fmax) equals k+2 (sec " ) [ET], we can see that the molecular activity equals k+2 (sec -1), that is, fal (sec-1). The katal is the S.I. unit of enzyme activity (moles substrate transformed sec -I) from whence come the corresponding units for specific activity (katal kilogram-1) and molar activity (katal per mole of enzyme). 

Using the value of a determined above, the results of the standard assay made initially to check the enzyme activity, the assay in part C, and the given concentration of the enzyme stock solution in g L , calculate the specific activity of the enzyme— that is, the number of micromoles of sucrose hydrolyzed per minute per gram of enzyme present. (The specific activity of an enzyme preparation is of course a function of the purity of the enzyme. As inactive protein is removed from the preparation, the specific activity will rise. When the specific activity can no longer be increased by any purification method, a homogeneous enzyme preparation may have been achieved but proof of this depends on other criteria.) The exact chemical composition of invertase is still unknown, but its molar mass has been estimated at 100,000 g mol Combining this datum with your calculated specific activity, estimate the turnover number for the enzyme. 

The specific activity of an enzyme almost always decreases on immobilization. The active sites are less accessible to substrate, and the diffusion of substrates and products across the stagnant layer of solution at the particle surface, and within polymer networks, lowers apparent values of Vmax and raises apparent Km values. The activity of an immobilized enzyme should be expressed as specific activity... 

The specific activity of an enzyme would be reported in which of the following units of measure ... 

The use of values for the specific activity of an enzyme is only significant if the conditions of measurement are specified, especially temperature, substrate concentration and pH value. Additionally, enzyme activities are always measured at initial reaction rate conditions (see above). 

Enzymes play a vital role in the majority of biochemical reactions. Not only do they allow for life as we know it, they demonstrate remarkable specificity in many cases. How do enzymes work on only one chemical process, whereas hundreds of perhaps similar processes occur in the same general vicinity Protein structure in general provides the answers. The primary, secondary, and tertiary structnres of a protein all serve to direct the specific activity of an enzyme, but the region where the specific reaction is catalyzed is referred to as the active site. 

The specific activity of an enzyme is its activity per milligram of enzyme. There is also a molar specific activity, which is the amount of substrate transformed per unit time per mole of enzyme. 

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