Enzyme unit

The lUBMB Commission on Nomenclature has issued a number of recommendations dealing with areas of a more biochemical nature (72), such as peptide hormones (86), conformation of polypeptide chains (87), abbreviations for nucleic acids and polynucleotides (88), iron—sulfur proteins (89), enzyme units (90), etc. The Commission has also produced rules and recommendations for naming enzymes (91,92). 

Different enzymes exhibit different specific activities and turnover numbers. The specific activity is a measure of enzyme purity and is defined as the number of enzyme units per milligram of protein. During the purification of an enzyme, the specific activity increases, and it reaches its maximum when the enzyme is in the pure state. The turnover number of an enzyme is the maximal number of moles of substrate hydrolyzed per mole of enzyme per unit time [63], For example, carbonic anhydrase, found in red blood cells, is a very active enzyme with a turnover number of 36 X 106/min per enzyme molecule. It catalyzes a very important reaction of reversible hydration of dissolved carbon dioxide in blood to form carbonic acid [57, p. 220],... 

Lederberg, J. (1956), Comments on the gene-enzyme relationship , in Gaebler, O. H. (Ed.), Enzymes Units of Biological Structure and Function, Academic Press, New York, pp. 161 -169. 

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. 

The asymmetric synthesis explains why most asymmetric compounds obtained from natural sources are optically active. In nature, the syntheses are carried out under the influence of optically active enzymes. The enzymes unite with the substance and when the synthesis is complete, they separate from the product and are again free to combine with fresh molecules of the original substance. 

Know ing the volume ot sample used in the assay, the number of enzyme units per litre or millilitre... 

PDH is a multi-enzyme complex consisting of three separate enzyme units pyruvate decarboxylase, transacetylase and dihydrolipoyl dehydrogenase. Serine residues within the decarboxylase subunit are the target for a kinase which causes inhibition of the PDH the inhibition can be rescued by a phosphatase. The PDH kinase (PDH-K) is itself activated, and the phosphatase reciprocally inhibited, by NADH and acetyl-CoA. Figure 3.12(a and b) show the role and control of PDH. 

Fig. 8. Variation of the skin wound half-life with degradation rate R (in collagenase) of collagen-GAG matrix. The half-life is the time required for a wound to contract to 50% of the original area. The degradation is in empirical units, which are defined in terms of an in vitro assay. A somewhat arbitrary broken vertical line is drawn near R = 140 enzyme units. This line shows the level of degradation rate above which the half-life of matrices rapidly drops to the level of the ungrafted wound. The horizontal scale is logarithmic [79]...

Lyophilized [200 unit single-dose vials 400 unit singledose vial (an enzyme unit (U) is defined as the amount of enzyme that catalyzes the hydrolysis of 1 mM of the synthetic substrate para-nitrophenyl-(beta)-D-glucopyranoside (pNP-Glc) per min at 37° C)]... 

It is usually difficult to express the enzyme concentration in molar unit because of difficulties in determining enzyme purity. Thus, the concentration is sometimes expressed as a unit, which is proportional to the catalytic activity of an enzyme. The definition of an enzyme unit is arbitrary, but one unit is generally defined as the amount of enzyme that produces 1 pmol of the product in 1 min at the optimal temperature, pH and substrate concentration. 

By international agreement, 1.0 unit of enzyme activity is defined as the amount of enzyme causing transformation of 1.0 gmol of substrate per minute at 25 °C under optimal conditions of measurement. The term activity refers to the total units of enzyme in a solution. The specific activity is the number of enzyme units per milligram of total protein (Fig. 3-23). The specific activity is a measure of enzyme purity it increases during purification of an enzyme and becomes maximal and constant when the enzyme is pure (Table 3-5). 

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. 

Use the plot of AiAQ vs. time to calculate AH/min over the linear portion of the curve. Convert the rate in absorbance terms to activity units. One enzyme unit is the amount of malate dehydrogenase that catalyzes the reduction of 1 micromole of oxaloacetate to L-malate in 1 minute under the described assay conditions. The reduction of 1 micromole of oxaloacetate leads to the oxidation of 1 micromole of NADH therefore, Equation E10.4 may be used to calculate the specific activity of malate dehydrogenase. 

The main data associated with the purification are summarized in table 6.4. This table indicates the total protein obtained in each step, the number of enzyme units3 for each enzyme, and the ratio of enzyme units to total protein, called the specific activity. In the absence of enzyme inactivation, the specific activity should be directly proportional to the enrichment. The percent recovery refers to the amount of enzyme activity in the indicated fraction, as compared with the amount present in fraction 1. This number is usually less than 100%. The apparent losses may reflect actual losses of enzyme during purification, or they may reflect inactivation (usually due to unknown causes) of the enzyme during purification. 

Enzyme units are proportional to the amount of enzyme activity. The relationship between enzyme units and absolute amount of enzyme need not concern us here. 

At the start of optimization of the reaction system, suitable values for pH and temperature have to be chosen as a function of the properties of the reactants and enzymes. Fortunately, most enzyme reactions operate in a narrow band with respect to pH value (7-10) and temperature (30-50 °C). The initial substrate concentration and, in the case of two-substrate reactions, the stoichiometric ratio of the two reactants, have to be selected. The selected enzyme concentration influences both the achievable space-time-yield as well as the selectivity in the case of undesired parallel or consecutive side reactions. In the case of multi-enzyme systems, the optimal activity ratio has to be found. The activity and stability of all the enzymes involved have to be known as a function of the reaction conditions, before the kinetic measurements are made. Enzyme stability is an important aspect of biocatalytic processes and should be expressed preferably as an enzyme unit consumption number, with the dimension unit of activity per mass of product (such as mole, lb, or kg). In multi-enzyme systems the stability of all the enzymes has to be optimized so that an optimal reaction rate and space-time-yield result. 

Enzyme activity is commonly expressed by the initial rate (l/0) of the reaction being catalyzed. The units of V0 are pmol min-1, which can also be represented by the enzyme unit (U) or the katal (kat), where 1 pmol min-1 = 1 U = 16.67 nanokat. The term activity (or total activity) refers to the total units of enzyme in a sample, whereas specific activity is the number of units per milligram of protein (units mg-1). 

Enzyme activity may be expressed in a number of ways. The commonest is by the initial rate (V0) of the reaction being catalyzed (e.g. pmol of substrate transformed per minute gmol min ). There are also two standard units of enzyme activity, the enzyme unit (U) and the katal (kat). An enzyme unit is that amount... 

Enzyme units are expressed as munits/mg protein. 1Values are means Se for 6 animals. 

The reaction rale that is measured depends on a number of experimental conditions such as temperature, pH, ionic strength, and the presence or absence of inhibitors or activators. In actual assays one usually chooses conditions ensuring iwwiwti iwriMB iste, It is only under the conditions specified in the- prescribed assay procedure that enzyme units are defined. Whenever different conditions are used, the rate measured does not have a well-defined relationship to the potency... 

The question of the enzyme units was reexamined by the commission of clinical chemistry of the I.U.P.A.C. The recommendations made appeared in a monograph of R. Dybkaer and EL Jtttgensen, "Quantities and Units in Clinical Chemistry. Cooenhaaen 1967 " Some formal imnerffictions of the international... 

The concept of enzyme unit as a physically undefined "amount of enzyme be abandoned. 

It was assumed that the katal, being related to the second as unit of time, fits better with rate constants of reaction used in chemical kinetics than does the Conner enzyme unit, since reaction rates am commonly expressed in moles (or appropriate submultiple thereof) per second. 

The most commonly used unit of enzyme activity has been defined as the amount of activity that catalyzes the transformation of 1 (xmol of substrate per minute under specified assay conditions. Specific activity is the number of enzyme units per milligram of protein ((imol/min per milligram of protein). The turnover number or catalytic constant is equal to the units of enzyme activity per nmole of enzyme ((xmol/min per nmol of enzyme). 

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