Definition of enzymes

Figure 1. Conceptual product definitions of enzyme-based biofuel cells as they are compared in their specific energy and energy density to the existing primary battery technology. Based on Figure 2 of ref 15. Reproduced with permission. Copyright 1999 The Electrochemical Society, Inc.

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. 

Introduction. Our aim is to obtain the physicochemical background in order to be able to set up an absolute viscosimetric method for the assay of endocellulases in enzyme mixtures, using a buffered HEC substrate and giving the definition of enzymic activities in katals, according to the recommendations of the International Union of Biochemistry. 

C. Implications of Superfamily Analysis for Current Definitions of Enzyme Function... 

There are no internationally standard assay methods for industrial enzymes and the definition of enzyme activity unit is also different for each enzyme. The activity of industrial enzymes is shown by various methods depending on manufacturers. For instance, commercial lipase activities are measured by the hydrolysis of olive oil under the various conditions and these figures are not comparable with each other. When customers apply these biocatalysts for chemical synthesis in organic solvents, these figure are sometimes reliable, and sometimes not. Users should not judge commercial enzymes based only on price and the activity shown in the table the manufacturer provides. Enzymes should be evaluated based on their practical performance under the conditions used. Most users of biotransformation are not experts in measuring enzyme activity, so the establishment of an assay method and practice are essential if one is to optimize the performance of enzymes. 

Present the definition of enzyme and discuss the characteristic features of these compounds in living organisms. 

Notice that the definition of enzyme activity implies that all of the conditions, including the identity of the substrate, are specified. Thus a change of rate observed when a different substrate is used in the assay does not imply a change in enzyme activity (though if the change of substrate is adopted as part of the definition of the standard assay, it will imply a change in the definition of enz5nne activity). 

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. 

C and T E Klein 1986. Molecular Graphics and QSAR in the Study of Enzyme-Ligand ractions. On the Definition of Bioreceptors. A ccounts of Chemical Research 19 392-400,... 

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). 

For biochemical reactions in which hydrogen ions (H ) are consumed or produced, the usual definition of the standard state is awkward. Standard state for the ion is 1 M, which corresponds to pH 0. At this pH, nearly all enzymes would be denatured, and biological reactions could not occur. It makes more sense to use free energies and equilibrium constants determined at pH 7. Biochemists have thus adopted a modified standard state, designated with prime ( ) symbols, as in AG°, AH°, and so on. For values determined... 

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. 

Viewed in this way, the best definition of rate enhancement depends upon the relationship between enzyme and substrate concentrations and the enzyme s kinetic parameters. 

Further progress may derive from a more accurate definition of the chemical and physical properties of the humic substances present at the rhizosphere and how they interact with the root-cell apoplast and the plasma membrane. An interaction with the plasma membrane H -ATPase has already been observed however this master enzyme may not be the sole molecular target of humic compounds. Both lipids and proteins (e.g., carriers) could be involved in the regulation of ion uptake. It therefore seems necessary to investigate the action of humic compounds with molecular approaches in order to understand the regulatory aspects of the process and therefore estimate the importance of these molecules as modulators of the root-soil interaction. 

Pratt, R. F. On the definition of mechanism-based enzyme inhibitors. Bioorg. Med. Chem. Lett. 1992, 2, 1323-1326. 

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. 

Hence, for any irreversible enzyme inactivator, we can quantify the effectiveness of inactivation using the second-order rate constant kanJKi. This constant thus becomes the key metric that the medicinal chemist can use in exploring the SAR of enzyme inactivation by a series of compounds. In terms of individual rate constants, the definitions of both nact and A) depend on the details of the mechanisms of inactivation, as will be described below. 

The turnover number of an enzyme is defined as the maximum number of moles of substrate reacted per mole of enzyme (or molecules per molecule) per minute under optimum conditions (i.e., saturating substrate concentration, optimum pH, etc). If 2 mg/cm3 of a pure enzyme (50,000 molecular weight, Michaelis constant Km = 0.03 mole/m3) catalyzes a reaction at a rate of 2.5 jumoles/nUksec when the substrate concentration is 5 x 10 3 moles/m3, determine the turnover number corresponding to this definition and the actual number of moles of substrate reacting per minute per mole of enzyme. 

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