Multienzyme process control

Process control in multienzyme processes, variables such as pH and temperature are often controlled during the process in order to reduce the influence that they produce on the dynamics of other variables and enzymes. For modeling, the controlled variables need to be identified in order to limit the capabilities of the model. In this case, they are included as assumptions of the model. Process control can be divided into two basic control layers [43]. The first is known as the regulatory layer, which controls variables such as pH and temperature. In this case, a simple controller design can be implemented. The second is known as the supervisory layer, which manages variables with more impact on the process such as concentrations of the compounds. In this case, a more detailed controller design is required. For multienzyme processes, this issue is highly relevant especially to achieve process improvements. 

Attempts to unravel the problems posed by cell compartmentation are hampered by the extreme difficulty of determining the exact intracellular location of many enzymes, let alone the distribution of metabolites. Even within a compartment distribution may not be uniform. For example, the existence of multienzyme systems, in which each enzyme is adjacent to the next one in the metabolic pathway, limits the diffusion of metabolites from the site of their production. This inequality of distribution of metabolites within the cell makes it difficult to demonstrate in vitro control processes which may occur in vivo. 

The basic problem of multienzyme metabolic sequences may be put in the following form. The cell carries out a considerable number of metabolic sequences rangmg in complexity from the synthesis of a protein or a steroid, on the one hand, to a relatively simpler process, such as transmethylation, on the other. What provision is made in the cell for these sequences to proceed in an orderly fashion Are all the enzymes concerned in any one sequence grouped together structurally, or are there other control features which permit integration even with all the individual enzymes dispersed in homogenous solution Some partial answers are now possible to these cogent questions, but since most of the complex metabolic sequences have yet to be reconstructed by the enzyme chemist, the available information can speak for only a restricted number of metabolic sequences. 

In general, the process of cell differentiation is a result of (1) the differential activation of genes which spezialized in the syntheses of proteins and enzymes which have structural and functional significance in the specialized tissue and (2) the differential action of gene modifiers controlling the rates of syntheses and degradation of various inhibitors and the formation of multienzyme complexes. 

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