Multiple substrate systems growth

In the preceding section, we discussed how the addition of a second substrate. I. to enzyme-catalyzed reactions could deactivate the enzyme and greatly inhibit the reaction. In the present section, we look not only at systems in which the addition of a second substrate is necessary to activate the enzyme, but also at other multiple-enzyme and multiple-substrate systems in which cyclic regeneration of the activated enzyme occurs. Cell growth on multiple substrates is given in the Sunman, Vmes. 

A classic example in which the internal regulatory processes of the cells play a very important role is the phenomenon of diauxic growth discovered by Monod [ 9 ] in multiple substrate systems. In diauxic growth there is preferential utilization of certain substrates over others, although each substrate by Itself... 

In what follows we will present two models, one based on a long term perspective and another based on a short term perspective both addressing the phenomenon of diauxic growth in multiple substrate systems. 

Many mathematical models of situations like the foregoing have been published see (, 18-25). Analyses of these models suggests that pure but not simple competition should often result in coexistence of competitors, even in systems in which spatial homogeneity of the environment is imposed and for which all external influences are time-invariant. Experimental data of Yoon et al. (22) on competition of Bacillus cereus and Candida tropicalis for the substitutable resources glucose and fructose show that coexistence in a chemostat is indeed possible here. Experimental testing of model predictions in situations of elementary but not simple competition is quite important, because the models used are necessarily those for multiple substrate limitation of growth, and all such models have low credibility, in my opinion. 

Multiple substrate limitation frequently occurs in batch growth systems. 

Growth by vapor deposition and oxidation (VDO) of Ce onto a substrate has been used successfully. The simplicity of this approach and its ability to be integrated into UHV systems designed for multiple surface diagnostic methods makes this a common technique for surface studies of chemisorption and surface reaction studies on model catalytic surfaces. Many of the ceria films used in work described below were produced in this way. Ce deposition and oxygen exposure (oxidation) may be performed simultaneously or sequentially. - Single crystal metals (Pt, Cu, Pd, Ni, and Ru ) and oxides, including yttrium-stabilized zirconia (YSZ), and sapphire, have been used as substrates for this approach. Such films have been... 

The whole-cell biocatalysis approach is typically used when a specific biotransformation requires multiple enzymes or when it is difficult to isolate the enzyme. A whole-cell system has an advantage over isolated enzymes in that it is not necessary to recycle the cofactors (nonprotein components involved in enzyme catalysis). In addition, it can carry out selective synthesis using cheap and abundant raw materials such as cornstarches. However, whole-cell systems require expensive equipment and tedious work-up because of large volumes, and have low productivity. More importantly, uncontrolled metabolic processes may result in undesirable side reactions during cell growth. The accumulation of these undesirable products as well as desirable products may be toxic to the cell, and these products can be difficult to separate from the rest of the cell culture. Another drawback to whole-cell systems is that the cell membrane may act as a mass transport barrier between the substrates and the enzymes. 

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