Mixing substrates

Microorganisms exhibit nutritional preferences. The enzymes for common substrates such as glucose are usually constitutive, as are the enzymes for common or essential metabohc pathways. Furthermore, the synthesis of enzymes for attack on less common substrates such as lactose is repressed by the presence of appreciable amounts of common substrates or metabolites. This is logical for cells to consei ve their resources for enzyme synthesis as long as their usual substrates are readily available. If presented with mixed substrates, those that are in the main metabolic pathways are consumed first, while the other substrates are consumed later after the common substrates are depleted. This results in diauxic behavior. A diauxic growth cui ve exhibits an intermediate growth plateau while the enzymes needed for the uncommon substrates are synthesized (see Fig. 24-2). There may also be preferences for the less common substrates such that a mixture shows a sequence of each being exhausted before the start of metabolism of the next. 

Fortuitous or co-metabolic biodegradation may account for a significant portion of the removal of xenobiotics in the environment.24 Numerous examples of co-metabolic activity have been described for pure substrates,22 but co-metabolism has been very difficult to demonstrate in mixed-substrate, mixed-culture systems, because products of the co-metabolic reactions of one species may be degraded by another.24 To encourage co-metabolism, easily degradable co-substrates should be included in the leachate prior to biological treatment. Fatty acids, which often occur in landfill leachates, may fulfill this requirement. 

As the calculated values show, mixing substrates does not increase dramatically the overall efficiency (in this example). This seems surprising at first, but... 

Kovarova-Kovar, K. and Egli, T. (1998). Growth kinetics of suspended microbial cells from single-substrate-controlled growth to mixed-substrate kinetics, Microbiol. Mol. Biol. Rev., 62, 646-666. 

P. J. F. Gommers, B. J. van Schie, J. P. van Dijken, and J. G. Kuenen, Biochemical limits to microbial growth yields An analysis of mixed substrate utilization, Biotechnol. Bioeng. 32, 86-94 (1988). 

Figure 3. Kinetics of conq)etitivc inhibition of Clostridium thermohydrosuljur-icum strain 39E purified amylopuUulanase activity with mixed substrates. The solid lines A and C indicate the theoretical plots for competitive inhibition at amylose ccmcentrations of 0.6 and 2.4 mg/ml, respectively. Lines B and D are the theoretical plots for the absence of inhibition at the same respective amylose ccmcentrations. PuUulan was used at concentrations of 0.4, 0.8, 1.2, 1.6, 2.0, 2.4 mg/ml. For clarity, only two sets of data points were used in the above plot. ( ) and (A) are the practical data points obtained at 0.6 and 2.4 mg/ml amylose concentrations. All reaction mixtures contained 5% (v/v) dimethyl sulfoxide for solubility of amylose. [S] = [A] + [P], where S is the total substrate ccmcentration. A and P are the concentrations of amylose and pullulan, respectively. (Reproduced with permissiem from Ref. 13. Copyright 1990 Academic Press, Inc.)...

Mix substrate in water or an appropriate buffer at the desired temperature and pH. 

Porphyrin and a, tt -bipyridine (bipy) groups have been introduced as metal-ion binding units into macropoly cyclic coreceptors (see also [4.38]) containing in addition macrocyclic sites for anchoring NH3+ groups [4.66a]. These receptors yield mixed-substrate supermolecules by simultaneously binding metal ions and diammonium cations as shown in 70. The bis-porphyrin receptor present in 70 binds to... 

Ashby, R. D., Solaiman, D. K. Y., Foglia, T. A., and Liu, C.-K. 2001. Glucose/lipid mixed substrates as a means of controlling the properties of medium chain length poly(hydroxyalkanoates). Biomacromolecules. 2,211-216. 

Goldman, J. C., and Dennett, M. R. (1991). Ammonium regeneration and carbon utilization by marine bacteria grown on mixed substrates. Mar. Biol. 109, 369—378. 

Aldehydes may be converted to ( )-alkenyl halides by the reaction of CrCh with a haloform in THF. The highest overall yields for the conversion were with iodoform, but somewhat higher (E) (Z) ratios were observed with bromoform or chloroform. Other low-valent metals, such as tin, zinc, manganese and vanadium, were ineffective. As the examples in Table 19 indicate, the reaction is selective for the ( )-isomer, except in the case of an a,3-unsaturated aldehyde. In addition, the reaction with ketones is sufficiently slow for chemoselectivity to be observed for mixed substrates. 

Four strains of Rhizopus, two residues and two precursors, mixed substrate combinations... 

The monolayer technique was also used to study the covalent inhibition of lipases using mixed monomolecular films of substrate containing amphiphilic inhibitor molecules and using a zero order trough with a reaction compartment containing the mixed substrate/inhibitor film whereas the reservoir compartment was covered with a film of pure substrate [44, 57]. 

B-Z reaction with mixed substrates Tartaric acid/acetone [Rastogi, R. P., Singh, H. J. and Singh, A. K., Kinetics of Physicochemical Oscillations, Preprints of Submitted Papers. Aachen Discussion Meeting of Deutsche Bunsengesellschaft fur Physicalisehe Chemie, (1979) 98-107], oxalic acid/acetone [Noszticzius, Mag. Kern. Foly 85 (1979) 330]. 

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