Enzyme units, definition

The LCR conditions shown serve as one, specific example. For individual applications, conditions must be optimized. We have found the critical optimization factors to be probe concentration, annealing temperature, cycle number, and ligase concentration. Any commercially available thermophilic DNA ligase should be suitable with a compatible buffer. We have had good success with ligase purified from Tkermus thermophilis. Since there is no universally accepted enzyme unit definition, the optimal amount of enzyme to use must be determined experimentally. 

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. 

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. 

The term turnover number can be used in two ways. One way, which has been redefined as molecular activity or molar activity, is the number of moles of substrate transformed per minute per mole of enzyme (units per micromole of enzyme) under optimum conditions. Since many enzymes are oligomers containing n subunits, another possible turnover number is the number of moles of substrate transformed per minute per mole of active subunit or catalytic center (under optimum conditions). This latter definition of catalytic power is called... 

One convenient source of information about enzymes is the Enzyme Handbook [3] which provides for each enzyme the chemical reaction catalyzed, the kinetic constants for the enzyme, a definition of activity units, and references for the preparation and properties of the enzyme. 

Enzyme Activity (U/mg) Price ( /mg) Supplier Activity unit definition... 

Molecular Activity. Many enzymes have thus far been isolated in pure and crystallized form. In these cases, the molecular activity can be determined it is defined as the number of molecules of substrate transformed per minute per molecule of enzyme (or the number of mmoles of substrate per mole of enzyme, i.e. the number of enzyme units p er Mmole of enzyme). The term turnover number" has also been applied to this definition ( nd similar ones). For the calculation of molecular activity, the activity of the enzyme and its molecular weight must be known. A large molecular activity indicates a rapid reaction. Very large molecular activities have been found in the cases of acetylcholinesterase (18 X 10 ). The usual values range from several thousand to ten thousand molecules of substrate per enzyme molecule per minute, still a rather rapid turnover. 

The comparison of specific activities of different enzyme preparations requires the consideration of the following variables (i) the assay conditions may not be identical, (ii) the unit definition may be arbitrary, and (iii) the active conformation of an enzyme, which often depends on purification procedures and storage conditions, is usually unknown. 

Unit definition One circularization unit (or Modrich-Lehman unit) is the amount of enzyme which converts linear poly[d(AT)] (optimum length of 1000 nt) into 1 nmol of exonuclease Ill-resistant circular form in 30 min at 30°C. 

Unit definition One phosphate exchange unit (or Weiss unit) is the amount of enzyme which catalyzes the conversion of 1 nmol of PPj into a charcoal (Norit)-adsorbable form in 20 min at 37°C in an ATP—PPj exchange reaction. One Weiss unit corresponds to 0.17 Modrich-Lehman units. 

Unit definition One ligase unit is that amount of enzyme which catalyzes 50% ligation of 1 /tg of A DNA HmdIII fragments in 30 min at 16 C. One cohesive-end ligation unit is equivalent to 0.015 Weiss units or 0.0025 Modrich-Lehman units. 

Unit definition One ligase unit is the amount of enzyme that converts 1 nmol of [5 - P]rA2o to a phosphatase-resistant form in 30 min. 

Unit definition One enzyme unit converts 1 nmol of p P]PPj to a Norit-adsorbable form in 30 min at 37°C. 

Unit definition One unit is the amount of enzyme that increases the absorbance of the DNA solution at a rate of 0.001 A260 units per min per ml of reaction mixture (17). 

Unit definition One enzyme unit is defined as the amount of enzyme that causes a change of 1.0 unit of A260 per minute. 

Unit definition One Kunitz unit is defined as the amount of enzyme that produces a decrease in 4300 from 2I0 (initial) to Ai (final) in 1 min under the assay conditions. The Aq to A( decrease corresponds to the total conversion. The rate is obtained from the slope of a linear plot, log(A - Af) versus f(min). 

Unit definitions One ssDNase (or RNase) unit is usually defined as the amount of enzyme that liberates 1 /zmol of acid-soluble nucleotides per minute under the assay conditions. 

Unit definition One exonuclease unit is the amount of enzyme that releases 1 nmol of TCA-soluble nucleotides in 30 min at 37°C (2). 

Unit definition One restriction unit is usually defined as the amount of enzyme required to digest completely 1 fig of DNA (typically phage A) in 1 hr. Because the definition relies on the activity at an end point of the reaction, it is distinguished from the usual kinetic definition of enzyme units based on the initial reaction rate. 

Unit definition One restriction unit is the amount of enzyme that reduces the infectivity of unmodified fd RE to 10% of its initial value. [Note The RE of wild-type phage fd (6.6 kbp) has two sites for EcoBI restriction. Because the cleavages of any one or both sites contribute to the loss of infectivity, one enzyme unit is equivalent to 0.3 pmol of double-strand cleavages.]... 

Unit definition One kinase unit is usually defined as that amount of enzyme which catalyzes the transfer of 1 nmol of Pj in 30 min at 37°C. 

Unit definition One unit is defined as the amount of enzyme that catalyzes the incorporation of 1 nmol of pHJUMP (or [ H]AMP) into acid-insoluble products per hour. Note Some unit definitions in the literature are based on a 30-min incubation dme.)... 

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