Catalytic rate constant enzymes

The overall catalytic rate constant of SNase is (see, for example, Ref. 3) kcat — 95s 1 at T = 297K, corresponding to a total free energy barrier of Ag at = 14.9 kcal/mol. This should be compared to the pseudo-first-order rate constant for nonenzymatic hydrolysis of a phosphodiester bond (with a water molecule as the attacking nucleophile) which is 2 x 10 14 s corresponding to Ag = 36 kcal/mol. The rate increase accomplished by the enzyme is thus 101S-1016, which is quite impressive. 

From these data one can see that the variation in Km is small for the various dipeptide substrates that are amides (-NH2), but that keJKm [proportional to k/(E)] values differ by almost 103. This indicates that the major change in catalysis is in the catalytic rate constant, fcc8t, and that the side chains must bind differently to the enzymes to produce this effect. 

Although it is not known whether the base is an essential feature of a substrate, the nucleoside moiety is necessary since bis- and tris-p-nitrophenyl phosphate esters are not hydrolyzed (61). The nature of the R substitution on the 3 -OH clearly affects the affinity of the substrate or inhibitor for the enzyme, even though it does not affect the maximal catalytic rate constant (Table I). The importance of the 5 - and 3 -phosphate groups in determining the affinity of inhibitors (3, 66) is consistent with the contribution of these groups to substrate affinity (61). These effects result from the phosphoryl groups themselves rather than... 

The rate of reaction 9t0 depends upon the catalytic rate constant and upon the enzyme-substrate concentration. 

Korsvik et al. reported ceria NPs used as catalysts mimicking superoxide dismutase (SOD) (Korsvik et al., 2007). The polycrystalline 3-5 nm sized ceria NPs show excellent catalytic rate constant even exceeding that of enzyme SOD. The mechanism should be further elucidated. 

Affinity labeling agents are intrinsically reactive compounds that initially bind reversibly to the active site of the enzyme then undergo chemical reaction (generally an acylation or alkylation reaction) with a nucleophile on the enzyme (Scheme 8). To differentiate a reversible inhibitor from an irreversible one, often the dissociation constant is written with a capital i, K (65), instead of a small i, K, which is used for reversible inhibitors. The K denotes the concentration of an inactivator that produces half-maximal inactivation. Note that this kinetic Scheme is similar to that for substrate turnover except instead of the catalytic rate constant, kcat for product formation, kmact is used to denote the maximal rate constant for inactivation. 

Catalytic behaviour of the footprint materials followed Michaelis-Menten kinetics, yielding a Michaelis constant and catalytic rate constant A cat for each imprinted matrix. The value of characterises the interaction of the enzyme with the substrate, similar to but not equal to dissociation constants. The constant Acat is the rate at which the active site converts the substrate to product. Together, the ratio kcaJKm is the enzyme selectivity, which describes the active site s efficiency in catalysing a reaction on a particular substrate. 

Stopped-flow fluorescence studies of ES complexes provided a direct comparison of the peptide binding aflBnities of the zinc and cadmium enzymes and, simultaneously, an explanation for the different roles of metals in peptide and ester hydrolysis (48). Cadmium carboxypeptidase binds the peptide Dns-(Gly)3-L-Phe as readily as does [(CPD)Zn] but catalyzes its hydrolysis at a rate that is reduced considerably (Figure 8). Initial rate studies of oligopeptides are in agreement with this observation. For all peptides examined, the catalytic rate constants of the cadmium enzyme are decreased markedly, but the association constants (1) (Km values) of the cadmium enzyme are identical to those of the zinc enzyme (48,51,57). However, in marked contrast, for all esters examined the catalytic rate constants of the cadmium enzyme are nearly the same as those of the zinc enzyme, but the association constants are decreased greatly. 

Michaelis-Menten constant, equivalent to the substrate concentration at which an enzyme or catalytic antibody is 50% saturated Catalytic rate constant, equivalent to the maximal rate of turnover by an enzyme or catalytic antibody when saturated with substrate uncat The rate constant for a non-catalyzed reaction TS transition state... 

Another direct assay method is based on decay kinetics of pulse generated O2 (Takahashi and Asada, 1981). Superoxide was produced within 10 msec by a flash of light through the excitation of flavin mononucleotide in the presence of A,A,A, A -tetramethylethylenedi-amine and oxygen. The kinetics of O2 decay in the presence and absence of enzyme were followed at 240 nm. The catalytic rate constant for bovine erythrocyte copper/zinc superoxide dismutase was found to be 1.75 x... 

Mathematical For the obtained in Question 30, calculate the turnover number (catalytic rate constant) assuming that 1 x 10 mol of enzyme were used. 

The parameter Ka,s is the dissociation constant for the complex of enzyme a and the effector T or F, labeled s, while ra,s is the ratio of the catalytic rate constants for the enzyme with and without effector bound, respectively. This general form models both activators and inhibitors of the enzyme, depending on whether r is greater than or less than 1. The response described is hyperbolic, with the half-maximum effect exerted when e = K, and saturating at the maximum effect r for very high effector concentrations. The cumulative multiplication of modifying factors reflects the assumption that all effectors function at different sites on the enzyme and that the sites do not communicate. 

As opposite to parameter K (or Keq) and kcat, V is not a fundamental property of the enzyme since it depends on its concentration as indicated by Eq. 3.5. This has to be taken into consideration when determining the kinetic parameters. The catalytic rate constant (kcat) is a fundamental property of the enzyme that can be expressed in different ways and in different units, according to how e is expressed (moles gL UL ). If e is expressed in moles L , kcat has dimension of T (known as turnover number). This requires the knowledge of the molecular weight and the specific activity and number of active centers of the enzyme. Sometimes this information is not available so that kcat is expressed in dimensions of M T (mass of substrate converted per unit time and unit of enzyme activity). If U is expressed in international units (lU), then kcat reduces to a dimensionless value of 1, which is to say that it is equivalent to V. 

Specific activity of biocatalyst molar concentration of substrate B (alternatively coefficient in Eq. 5.3) initial molar concentration of substrate B coefficient in Eq. 5.3 concentration of biocatalyst time of a cycle of reactor operation enzyme activity initial enzyme activity molar concentration of enzyme species Eij volumetric activity of enzyme species Ey enzyme volumetric activity initial enzyme volumetric activity bioreactor feed flow-rate total flow-rate to downstream operations initial feed flow-rate to bioreactor i number of half-lives of biocatalyst use film volumetric mass transfer coefficient for substrate Michaelis-Menten constant catalytic rate constant first-order inactivation rate constant transition rate constants... 

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