Biocatalyst mass specific activity

Other very relevant parameter of enzyme immobilization is biocatalyst mass (or volumetric) specific activity, which is simply the amount of enzyme activity expressed per unit mass (or unit volume) of biocatalyst ... 

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... 

Rp protein immobilization yield (mg immobilized enzyme/mg contacted protein) Rp enzyme immobilization yield (units of enzyme activity expressed in the biocattilyst/unit of enzyme activity contacted) SA specific activity of the biocatalyst (units of enzyme activity expressed/unit mass of biocatalyst)... 

In principle, the amount of a biocatalyst may be expressed as a number of elementary entities, an amount of substance, a mass, or by a catalytic effect. In many cases, due to the lack of information or for practical reasons, the later route is used. Then, the generally accepted term for the expression of enzyme activity is the Unit (U) defined in terms of reaction rate. One unit (U), accordingly to the International System of Units, is defined as the amount of enzyme that catalyzes the conversion of 1 pmol of substrate (or the formation of 1 pmol of product) in 1 min under standard conditions. The specific activity is the number of units per milligram of protein. If the relative molecular mass of the enzyme is accurately known it is then possible to express the activity as the molar catalytic activity, defined as the number of units per micromole of enzyme. This is, the number of moles of product formed, or substrate consumed, per mole of enzyme per minute. This may not correspond to the number of moles of substrate converted per enzyme active site since the enzyme may contain more than one active site. If the number of active sites per mole is known, then the activity may be expressed correspondingly as the catalytic center activity. 

Biocatalyst yield. The third metric is the biocatalyst yield (mass product/mass biocatalyst). Clearly, adding more biocatalyst will increase the STY, but may also decrease the biocatalyst yield (if the specific activity is not maintained). It is essential that the biocatalyst yield is sufficiently high, otherwise the cost contribution of the biocatalyst to the final product will be too high. Reasonable values range from 10 g product/g cells to 10 000 g product/g immobilized enzyme. 

The use of immobilized cell reactors have shown improved biocatalyst stability, however, the specific rates of desulfurization have been much lower than for suspended cell (stirred) reactors. Mass transfer limitations have been significant resulting in lower rates. Thus, the activity is sacrificed to achieve stability. Further work in this area and improved immobilization matrices can help improve the activity along with the stability. 

The measure of mass is important with respect to calculating mass balance. However, the elemental composition of biomass is normally ill defined. Another reason for determining biomass is the need for a reference when calculating specific rates (q ) q = r /x. An ideal measure for the biocatalysts in a bioreaction system of interest would be their activity, physiological state, morphology or other classification rather than just their mass. Unfortunately, these are even more difficult to quantify objectively and this is obviously why the biomass concentration is still of the greatest interest. 

The basic concepts related to biocatalytic reactions, in terms of the kinetics and mass transport phenomena involved, have been introduced in order to aid formulation of more detailed mass balance in the systems analysed during the study. Some specific aspects, such as biocatalyst denaturation, concentration polarization and activity decay, have been also described. In order to predict the performance of a membrane bioreactor, a detailed analysis of the effectiveness of the biocatalyst processes has been also presented. 

Enzymes are biocatalysts with an extremely high selectivity. Their molecules are protein molecules with a molecular mass between 10 to 10 Da. Enzymes work under mild conditions, i.e. at room temperature or slightly above and at near-neutral pH. Biosensors with enzymes generally contain a layer of enzyme molecules immobilized at the sensor surface. This layer is able to catalyse just one reaction with a definite biologically active substance. The latter is recognized and determined specifically in this way. 

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