Regulation of enzyme amount

The beginning of secondary product formation is often directly coupled with the synthesis of the corresponding enzymes. One example is the biosynthesis of flavonoids in cell cultures of Petroselinum hortense (Fig. 7). Here the regulatory mechanisms were extensively investigated with phenylalanine ammonia-lyase (PAL) (D 22.2.1) and chalcone synthase (D 22.3.3), the key enzymes of the biosynthetic chain. Experiments with inhibitors of transcription and translation showed that the increase of enzyme activity depends on RNA and protein biosynthesis. Labeling experiments demonstrated that it is caused by an accelerated rate of enzyme synthesis. 

Irradiation with UV seems to stimulate RNA synthesis immediately, but to increase the rate of protein synthesis only after the lag period (Fig. 7) which precedes the increase of enzyme activity. In agreement with these results in vitro translation has shown that the amounts of PAL-mRNA and chalcone synthase- 

Group of Substances whose Producer organism Methods 

Polyketides 6-Methylsalicylic acid, patulin (D 3.3.1) Penicillium patulum A, B 

Tetraterpenes Carotenoids, trisporic acids (D 6.5) Blakeslea trispora B 

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Regulation of enzyme amount. Enzyme synthesis is governed by such regulatory mechanisms as induction, repression, and catabolite repression. These mechanisms, which may have drastic effects on cellular composition, have time constants on the order of minutes to hours, and hence result in a much slower adaption than the mechanisms treated before. Drastic... 

In addition to regulation of enzyme amount, secondary metabolism may also be controlled by regulation of the activity of the enzymes involved. Evidence for this comes from the large discrepancies found between the relatively high enzyme activities in vitro and the much lower activities in the living cell. 

Regulation of enzyme activity is achieved in a variety of ways, ranging from controls over the amount of enzyme protein produced by the cell to more rapid, reversible interactions of the enzyme with metabolic inhibitors and activators. Chapter 15 is devoted to discussions of enzyme regulation. Because most enzymes are proteins, we can anticipate that the functional attributes of enzymes are due to the remarkable versatility found in protein structures. 

The rate of synthesis of amino acids depends mainly on the amounts of the biosynthetic enzymes and on their activities. We now consider the control of enzymatic activity. The regulation of enzyme synthesis will be discussed in Chapter 31. 

Figure 1.2 Regulation of hyaluronan amount and chain length by expression of a specific HAS protein. Biochemical characterizations of the vertebrate HAS enzymes expressed in mammalian cell culture have revealed similarities and differences between the respective mammalian hyaluronan synthase enzymes. The differences are depicted in this cartoon. HASl produces small amounts of high-molar-mass hyaluronan. HAS2 produces significantly more high-molar-mass hyaluronan. HAS3 is the most active of the hyaluronan synthases, yet produces low-molar-mass hyaluronan chains. The physiological significance of these differences in enzymatic activity is not yet known [33].

A specific localization of the control mechanism has been found. The enzyme, phosphoribosylpyrophosphate amidotransferase (44S) was inhibited by various purine ribonucleotides ATP and ADP were the most inhibitory (449). The regulation of enzyme activity by purine ribonucleotides offers a controlling mechanism for purine biosynthesis by maintaining a relatively constant amount of nucleotide within the cell. 

Hundreds of metabohc reac tions take place simultaneously in cells. There are branched and parallel pathways, and a single biochemical may participate in sever distinct reactions. Through mass action, concentration changes caused by one reac tion may effect the kinetics and equilibrium concentrations of another. In order to prevent accumulation of too much of a biochemical, the product or an intermediate in the pathway may slow the production of an enzyme or may inhibit the ac tivation of enzymes regulating the pathway. This is termed feedback control and is shown in Fig. 24-1. More complicated examples are known where two biochemicals ac t in concert to inhibit an enzyme. As accumulation of excessive amounts of a certain biochemical may be the key to economic success, creating mutant cultures with defective metabolic controls has great value to the produc tion of a given produc t. 

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