Enzymes, defined

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. 

Horseradish peroxidase (HRP) is a member of the large class of peroxidases, which are enzymes defined as oxidoreductases using hydroperoxide as electron acceptor. HRP has been widely used for the construction of amperometric biosensor for the determination of H202 and small organic and inorganic substrates. 

Why are there so many DUBs and how do they achieve specificity The numerous DUBs identified to date suggest that DUBs have specifically evolved to act on distinct cellular substrates rather than to have general deubiquitinating activity (see Figure 8.1). We can ask what common features of these enzymes define them as DUBs and what differences allow specific DUBs to act on mono- vs. polyubiquitin How have they evolved to cleave only ISG15 or SUMO-modified substrates, for instance A body of data has been accumulated that at least partially answers these questions. 

Derived kind of quantity, molar catalytic amount, be defined as the catalytic amount of the enzyme defined by its amount of substrate (derived unit = kai/mol). Ordinarily molar activity is expressed ptf micromole (or nanomole) of preparation. 

Ui The enzymatic charge offered for immobilization in U/g of support, U t the enzymatic charge theoretically immobilized in U/g of support, R1 the immobilization yield, defined as (C/rr/Ui)x 100, U i the measured immobilized enzyme activity, in U/g of biocatalyst, RA the activity recovered in the immobilized enzyme, defined as (Uei/[/it)=< 100, Note I gelation/encapsulation=45 °C, ethanolic/acid medium, 155 min, aging=18 h at 4 °C, drying=suck dried by vacuum, followed by a 24-h resting period in a desiccator All the results of this immobilization are the averages of duplicates. 

The model was applied to estimate the theoretical relationship between the fixed bed initial flow velocity resulting in 45% of glucose being isomerized to fructose and the operating temperature for an immobilized enzyme defined by the characteristics given in table I. The results of this evaluation, in terms of the initial flow velocity relative to that at 65°C, are shown in an Arrhenius plot in figure 4. The experimental results used in the parameter estimation are also shown. The theoretical relationship is shown to be definitely non linear in an Arrhenius plot. 

The bacterium Escherichia coli possesses two different AP endonucleases, exonuclease III and endonuclease IV (Ljungquist and Lindahl, 1977). These enzymes define the two known structural classes of AP endonuclease. The predominant enzyme in E. coli is exonuclease III, which has 3 5 exonuclease activity in addition to the characteristic AP endonuclease activities detailed above (Rogers and Weiss, 1980 Weiss, 1976). Endonuclease IV, whose expression in E. coli is induced in response to oxidative stress (Chan and Weiss, 1987), is homologous to the m or AP endonuclease found in baker s yeast, APNl (Popoff et al, 1990). The m or human AP endonuclease, APEl (also known as Ref-1 or HAPl), is homologous to exonuclease III (Demple et al, 1991). APEl is multifunctional, possessing redox-dependent transcriptional activation (Xanthoudakis et al, 1992) and acetylation-dependent, redox-independent transcriptional repression activities (Bhakat et al, 2003 Okazaki et al, 1994) in addition to AP... 

Add phosphatases are ubiquitously distributed enzymes defined by the pH optimum (usually 4.9-6.0) of their hydrolytic activity toward orthophosphate monoesters. A metal-dependent subclass of these enzymes was first recognized in 1973, when iron-bearing acid phosphatases were isolated from porcine uterine fluid and bovine spleen Because of their intense colors they have become known as the purple add phosphatases. Its source, content of iron, and presumed role in iron transport fix>m pregnant sow to fetal pig have earned the pordne protein the euphonious name of uteroferrin. 

Nemeria NS, Chakraborty S, Balakrishnan A et al (2009) Reaction mechanisms of thiamin diphosphate enzymes defining states of ionization and tautomerization of the cofactor at individual steps. FEES J 276 2432-2446... 

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