Catalyst activity

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The activity a of a catalyst (also called catalytic activity ) is a measure of how quickly a catalyst converts starting materials into products .

Homogeneous catalysis

From a homogeneous catalysis is used when a chemical reaction of the catalyst and the reactants in the same physical state are present.

Heterogeneous catalysis

Of a heterogeneous catalysis is used when at a chemical reaction of the catalyst and the reacting substances in different aggregate states are present.

Examples:

  • In the case of a catalyzed gas phase reaction, the catalyst is in the form of a solid phase and the reaction mixture is gaseous.
  • In a 3-phase reaction, the gaseous and liquid starting materials are converted with the aid of a solid catalyst.

Biocatalysis

The enzyme activity is defined as the amount of substance metabolized per time. Common units of measurement are:

  • the classic enzyme unit (symbol U or unit ), introduced in 1961 , defined as one micromole per minute (it is persistent with enzymologists: pure enzymes have specific activities that are easy to understand on this scale, between 5 and 500 U / mg),
  • the SI unit katal (symbol kat ) defined in 1972 , defined as one mole per second.

In general, first, the enzyme activity per volume of solution (i.e.,.. Volume activity ) determined; this can be determined as the rate of change of the product or substrate concentration in a conversion reaction ( see below ). The volume activity is a measure of the enzyme concentration because v  =  k cat · c , which is why volume activities are often given when the amount of an enzyme in a sample is actually of interest.

If the mass concentration β of the enzyme in the solution is known (this can best be determined in an unknown solution when the solution is free of other proteins), the activity per mass of enzyme (i.e. the specific activity ) can be derived from the volume activity. calculate. The most meaningful parameters molar activity, also turnover number (engl. Turnover number called), it requires addition knowledge of the molar mass M of the enzyme. The turnover number describes the number of substrate molecules that are turned over by an enzyme molecule per second and ranges between about 0.5 ( lysozyme ) and several million ( catalase ).

definition Connections old units new units
Activity a Δ n / Δ t v · V s · m k cat · n U = µmol / min kat = mol / s
Volume activity v Δ n / (Δ t · V ) a / V s · β k cat · c U / ml kat / l
specific activity s Δ n / (Δ t · m ) a / m v / β k cat / M U / mg cat / g
Turnover number k cat Δ n / (Δ t · n ) a / n v / c s · M U / mmol kat / mol = s −1

Influencing factors

Saturation hyperbola , determined by conversion reactions with different substrate concentrations
  • The reaction can only proceed in the desired direction if the quotient of product concentrations and educt concentrations is below the equilibrium constant for this reaction, and the faster the quotient is from the equilibrium constant, the faster it is.
  • The enzyme activity hyperbolically approaches a maximum with increasing substrate concentration. The substrate concentration at which half the maximum reaction rate is reached is called the Michaelis constant ( K m value) . The state of half-maximum reaction rate is interpreted as the state in which half of the enzyme molecules have bound substrate, so that the K m value represents a measure of the affinity of the enzyme for the substrate.
  • Each enzyme has a pH optimum and a temperature optimum depending on its area of ​​application. The pH optimum comes about when the binding or dissociation of protons changes the tertiary structure via the formation or dissolution of electrostatic bonds . The temperature optimum arises from the fact that, according to the RGT rule , the reaction rate increases with temperature, but enzyme molecules also increasingly denature with increasing temperature .

The measurement of enzyme activities is usually carried out under standardized conditions:

  • no products in solution (i.e. the measurement is carried out immediately after the reaction has started)
  • Excess (i.e. concentrations far beyond the respective K m value) of substrate and possibly cofactors so that the enzyme is saturated , i.e. all enzyme molecules have bound substrate and possibly cofactors at all times.
  • pH optimum. 25 ° C according to standard biochemical conditions

Implementation reaction

The desired volume activity results from the rate of change in the product or substrate concentration over a period that is not too long immediately after the start of the reaction:

In most cases, the measurement of the product formation (P case) is to be carried out more precisely than that of the substrate consumption (S case), because at saturation the latter appears as a small difference of large values. Reaction products are measured by separation processes, chemical detection processes, spectroscopic ( photometric ) or fluorometric measurements. The spectroscopic methods enable a continuous registration of the substrate turnover and are - if applicable - always preferable.

If neither products nor educts can be recorded photometrically, a composite enzymatic test can be used to link to an indicator reaction, in which the product of the original reaction (the so-called measurement reaction ) is immediately converted by another enzyme with the creation or consumption of a photometrically detectable substance .

Catalytic efficiency: the interplay of activity and affinity

The Michaelis-Menten theory describes enzyme catalysis in two steps: 1. Formation of the enzyme-substrate complex, 2. chemical reaction. The reaction rate in this model is v  =  k cat × [ES]. Since the beginning of the reaction is considered for which there is no product, the reverse reaction of the product can be neglected.

[E] tot denotes the total concentration of the enzyme, [ES] the concentration of the enzyme-substrate complex, [E] the concentration of the free enzyme and [S] the concentration of the free substrate. When measuring the enzyme activity under saturation conditions there is so much substrate available that practically all enzyme molecules are present in the ES complex: [ES] = [E] total . The reaction rate is maximum in this case, it is v max  =  k cat × [E] tot and thus depends (in addition to the total enzyme concentration) only on the turnover number.

Under physiological conditions, however, very few enzymes are saturated with substrate, so that the reaction speed depends not only on how fast the enzyme molecules in the ES complex convert their substrate (expressed by the turnover number), but also on how large the proportion of enzyme molecules is, which are actually present in the ES complex. The latter depends on the affinity of the enzyme for its substrate, which is described by the K m value. The reaction speed can be given by the Michaelis-Menten equation:

If one replaces v max with k cat × [E] tot and assumes that [S] is much smaller than K m , so that [S] can be neglected in the denominator, the equation simplifies to:

At substrate concentrations far below the K m value, the enzymatic turnover rate depends on the quotient k cat / K m : the higher the turnover number and the higher the affinity (i.e. the lower the K m value) of an enzyme for its substrate, the more its catalytic efficiency is greater. A graphical estimate of this parameter is obtained by applying a tangent to the origin of the saturation hyperbola, because its slope corresponds to v max / K m , or, with appropriate scaling, k cat / K m .

Upper limit for k cat / K m

If K m is replaced by ( k −1 + k cat ) / k 1 according to the strict definition of the Michaelis constant , it turns out that k cat / K m can never be greater than k 1 :

k 1 is the rate constant for the formation of the ES complex. An ES complex can only arise if the enzyme and substrate accidentally meet in the solution through diffusion. The diffusion velocity limits k 1 (and thus also k cat / K m ) to 10 8 to 10 9 mol −1 · l · s −1 . Enzymes that achieve this catalytic efficiency are called kinetically perfect ; every chance contact with the substrate leads to a reaction. The existence of such enzymes is remarkable in that the active site is only a small part of an enzyme molecule.

In order to be able to increase efficiency beyond that, multifunctional enzymes and multi-enzyme complexes have emerged in the course of evolution , whereby substrates and products are limited to the limited volume of a single protein.

Examples

Catalase (decomposition of the cell toxin hydrogen peroxide), acetylcholinesterase (rapid nerve conduction) and carbonic anhydrase (release of carbon dioxide in the lungs) have the highest catalytic efficiency . Some figures can be found under change number .

Units of measure for enzyme activity in brewing

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