Fig. 3.7. The control of a biosynthetic pathway by the co-operative control by end products D and F.

---50%---»Inhibition of 50% of the activity of the enzyme

Fig. 3.8. The control of a biosynthetic pathway by the cumulative control of products D and F.

branch point to the product. The intermediates which then build up as a result of this control earlier enzymes in the pathway. Thus, in Fig. 3.9, D inhibits the conversion of B to C, and F inhibits the conversion of B to F. The inhibitory action of D, F, or both, would result in an accumulation of B which, in turn, would inhibit the conversion of A to B.

Isoenzyme control. Isoenzymes are enzymes which catalyse the same reaction but differ in their control characteristics. Thus, if a critical control reaction of a pathway is catalysed by more than one isoenzyme, then the different isoenzymes may be controlled by the different end products. Such a control system should be very efficient, provided that control exists immediately after the branch point so that the reduced flow of intermediates is diverted away from the product in excess. An example of the system is shown in Fig. 3.10.

Thus, the levels of microbial metabolites may be controlled by a variety of mechanisms, such that end products are synthesized in amounts not greater than those required for growth. However, the ideal industrial micro-organism should produce amounts far greater than those required for growth and, as sug-

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