Control mechanisms metabolite activation

In allosteric enzymes, the activity of the enzyme is modulated by a non-covalently bound metabolite at a site on a protein other than the catalytic site. Normally, this results in a conformational change, which makes the catalytic site inactive or less active. Covalent modulated enzymes are interconverted between active and inactive forms by the action of other enzymes, some of which are modulated by allosteric-type control. Both of these control mechanisms are responsive to changes in cell conditions and typically the response time in allosteric control is a matter of seconds as compared with minutes in covalent modulation. A third type of control, the control of enzyme synthesis at the transcription stage of protein synthesis (see Appendix 5.6), can take several hours to take effect. 

Phosphoesters are ubiquitous in biochemistry and serve several functions [5L Genetic information is stored in DNA and RNA. In cellular control mechanisms, phosphorylation of proteins is an important mechanism for regulating protein activities161. Phosphorylation can activate metabolites or change solubility properties. Enzyme-catalyzed formation and cleavage of P-O bonds are central to the cellular energy balance171. Biosynthesis depends heavily on phosphorylated intermediates. 

The activity of majority of enzymes is regulated by the concentration of their own substrates in a MichaeUs-Menten fashion. However, such a control may be insufficient for some metabolic purposes. For example, in order to increase the velocity of a simple Michaelian enzyme from o.iV to 0.9V, it is necessary to increase the substrate concentration from Xm/9 to that is, an 81-fold increase. Similarly, an 81-fold increase in inhibitor concentration is required to reduce the velocity from 90% to 10% of the uninhibited value. On one hand, the concentration of metabolites in vivo vary within relatively narrow limits wltile, on the other hand, the activity of specific enzymes must be increased or decreased within very large limits. Consequently, in addition to a simple Michaelian kinetics, nature has a need for additional control mechanisms for the regulation of enzyme activity in vivo. 

Though still scanty and incomplete, the data indicate that the control of synthesis of enzymes participating in secondary biosynthesis is accomplished at the transcription level. Their formation takes place after a reduction in the rate of RNA and protein syntheses. The period of intensive secondary biosynthesis is marked by the continuous replenishment of specific synthases its discontinuation brings about a drop in the production rate. The "late" expression of synthesis of secondary metabolites after the termination of intensive growth is thus probably not the result of a post-translation control (23, 24). So far, we cannot eliminate a number of other control mechanisms at the level of activity of the enzymes formed, yet the rise in synthase levels in the period of active secondary biosynthesis (4, 30, 38) attests to our hypothesis. 

Limited studies indicate that the phosphorylated forms of the en-zjrme are more susceptible to inactivation or inhibition by various negative eflFectors such as palmityl-CoA, avidin, and ATP than the dephosphorylated forms 17,21), whereas the dephosphorylated forms require less citrate for maximum activation 13, 17, 21, 42, 57). For example, the phosphorylated form has a Km for citrate of 2.4 mM whereas the Km of the active dephosphorylated form is 0.2 mAf. Since the citrate concentration in the cell is only about 0.6 mM, and most of this citrate is localized in the mitochondria, it would appear that only the nonphosphorylated form will be active under normal physiological conditions. Thus, the covalent modification mechanism makes the allosteric control mechanism functional at physiological concentrations of cellular metabolites. Such changes in the properties of acetyl-CoA carboxylase also occur under in vivo conditions as reported by Witters etal. 130) using isolated hepatocytes treated with insulin or glucagon. 

The evidence that tamoxifen was able to exert estrogenic effects on several tissues like the bone (Love et al. 1992) opened the door to the concept of SERMs, which is explained in detail in Chaps. 2 and 3 of this book. The mechanisms of action of these substances on ERs is explained in detail in Chap. 3. However, it is pertinent to comment on some special aspects of its action on mammary cancer cells. IGF-1 is a key element in growth control of malignant breast cells through endocrine and paracrine pathways. Tamoxifen and its active metabolite are able to inhibit IGF-l-stimulated growth (Jordan... 

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